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If you cannot explain why carbon-12 and carbon-14 are the same element but behave differently in a PET scanner, why sodium loses an electron while chlorine gains one, or why helium never forms bonds, you will struggle with every chemical reaction in biology. Concept 2.2 is the engine room of chemistry for AP Biology, A-Level, IB HL, College Biology and USABO: it shows that an element's identity comes from protons, its mass variants come from neutrons, and its chemical personality comes entirely from electrons. In this definitive guide you will master subatomic particles, atomic number versus mass number, isotopes and radioisotopes, radioactive tracers and radiometric dating, energy levels and potential energy, electron shells, valence electrons, periodicity, and the true 3D shape of electron orbitals. Master this and bonding, water, and macromolecules become intuitive.
- An atom is the smallest unit retaining an element's properties and consists of protons, neutrons, and electrons.
- Atomic number equals proton number and defines the element, mass number equals protons plus neutrons.
- Isotopes of an element have the same protons but different neutrons, some radioactive.
- Electrons exist only at fixed energy levels or shells, gaining or losing energy in quantized steps.
- Chemical behavior depends on valence electrons and unpaired electrons in orbitals, not on the nucleus.
An element's properties depend on the structure of its atoms
Each element consists of a certain type of atom that is different from the atoms of any other element. An atom is the smallest unit of matter that still retains the properties of an element. Atoms are extraordinarily small; it would take about a million of them to stretch across the period at the end of this sentence. We symbolize atoms with the same abbreviation used for the element that is made up of those atoms. For example, the symbol C stands for both the element carbon and a single carbon atom.
The key insight for all of biology is that elements differ because their atoms differ internally. While nuclei never touch during a chemical reaction, the arrangement of electrons around the nucleus determines whether an atom will form a covalent bond, become an ion, or remain inert. This is why knowledge of internal atomic structure is not physics trivia; it is the explanation for why CHON forms life and why trace elements like iron carry oxygen.
Subatomic Particles
Although the atom is the smallest unit having the properties of an element, these tiny bits of matter are composed of even smaller parts called subatomic particles. Using high-energy collisions, physicists have produced more than 100 types of particles from the atom, but only three kinds are relevant for biology: neutrons, protons, and electrons.
Protons and electrons are electrically charged. Each proton has one unit of positive charge, and each electron has one unit of negative charge. A neutron, as its name implies, is electrically neutral.
Protons and neutrons are packed together tightly in a dense core, or atomic nucleus, at the center of an atom; protons give the nucleus a positive charge. The rapidly moving electrons form a cloud of negative charge around the nucleus, and it is the attraction between opposite charges that keeps the electrons in the vicinity of the nucleus.
Two commonly used models illustrate a helium atom. One represents the two electrons as a cloud of negative charge, a result of their rapid motion around the nucleus. In the more simplified Bohr model, the electrons are shown as two small spheres on a circle around the nucleus. Both models are not to scale; they greatly overestimate the size of the nucleus in relation to the electron cloud. If a helium atom were the size of a football stadium, its nucleus would be the size of a pencil eraser in the center, and its electrons would be like two gnats buzzing around.
The neutron and proton are almost identical in mass, each about 1.7 × 10-24 gram. Grams are not useful for objects so minuscule. For atoms and subatomic particles, and molecules too, we use a unit called the dalton, in honor of John Dalton, the British scientist who helped develop atomic theory around 1800. The dalton is the same as the atomic mass unit, or amu. Neutrons and protons have masses close to 1 dalton. Because the mass of an electron is only about 1/2000 that of a neutron or proton, we can ignore electrons when computing the total mass of an atom. Almost all of an atom's mass is concentrated in its nucleus.
| Subatomic Particle | Charge | Mass (daltons) | Location | Role in Biology |
|---|---|---|---|---|
| Proton | +1 | ∼1 | Nucleus | Determines atomic number and element identity |
| Neutron | 0 | ∼1 | Nucleus | Determines isotope and mass number, stabilizes nucleus |
| Electron | -1 | ∼0.0005 (1/2000) | Electron cloud / shells around nucleus | Determines chemical bonding and reactivity |
Atomic Number and Atomic Mass
Atoms of the various elements differ in their number of subatomic particles. All atoms of a particular element have the same number of protons in their nuclei. This number of protons, which is unique to that element, is called the atomic number and is written as a subscript to the left of the symbol for the element. For example, the abbreviation \(_{2}He\) tells us that an atom of helium has 2 protons in its nucleus.
Unless otherwise indicated, an atom is neutral in electrical charge, which means its protons must be balanced by an equal number of electrons. Therefore, the atomic number tells us the number of protons and also the number of electrons in an electrically neutral atom.
We can deduce the number of neutrons from a second quantity, the mass number, which is the total number of protons and neutrons in the nucleus. The mass number is written as a superscript to the left of the element's symbol. For example, we can use shorthand to write an atom of helium as \(^{4}_{2}He\). Because the atomic number indicates how many protons there are, we can determine the number of neutrons by subtracting the atomic number from the mass number.
For sodium:
Mass number = number of protons + neutrons = 23 for sodium
Atomic number = number of protons = number of electrons in a neutral atom = 11 for sodium
Number of neutrons = mass number - atomic number = 23 - 11 = 12 for sodium
The simplest atom is hydrogen \(^{1}_{1}H\), which has no neutrons; it consists of a single proton with a single electron.
Because the contribution of electrons to mass is negligible, almost all of an atom's mass is concentrated in its nucleus. Neutrons and protons each have a mass very close to 1 dalton, so the mass number is close to, but slightly different from, the total mass of an atom, called its atomic mass. For example, the mass number of sodium (\(^{23}_{11}Na\)) is 23, but its atomic mass is 22.9898 daltons. The difference is due to nuclear binding energy and the weighted average of isotopes explained below.
| Concept | Symbol Position | Definition | Example |
|---|---|---|---|
| Atomic Number | Subscript left: 11Na | Number of protons = number of electrons in neutral atom, defines element | Sodium atomic number 11 means 11 protons, 11 electrons if neutral |
| Mass Number | Superscript left: 23Na | Total protons + neutrons in that specific atom | Sodium-23 has 12 neutrons |
| Atomic Mass (Atomic Weight) | Usually under symbol in periodic table | Weighted average mass of all naturally occurring isotopes in daltons | Sodium 22.99 daltons, Carbon 12.01 daltons |
Isotopes
All atoms of a given element have the same number of protons, but some atoms have more neutrons than other atoms of the same element and therefore have greater mass. These different atomic forms of the same element are called isotopes of the element. In nature, an element may occur as a mixture of its isotopes.
As an example, the element carbon, which has atomic number 6, has three naturally occurring isotopes. The most common isotope is carbon-12, \(^{12}C\), which accounts for about 99% of the carbon in nature. \(^{12}C\) has 6 neutrons. Most of the remaining 1% of carbon consists of atoms of the isotope carbon-13, \(^{13}C\), with 7 neutrons. A third, even rarer isotope, carbon-14, \(^{14}C\), has 8 neutrons. Notice that all three isotopes of carbon have 6 protons; otherwise, they would not be carbon. Although the isotopes of an element have slightly different masses, they behave identically in chemical reactions because chemical behavior depends on electrons, not neutrons.
For an element with more than one naturally occurring isotope, the atomic mass is an average of those isotopes, weighted by their abundance. Thus, carbon has an atomic mass of 12.01 daltons, slightly above 12 because of the small amount of heavier isotopes.
Both \(^{12}C\) and \(^{13}C\) are stable isotopes, meaning that their nuclei do not have a tendency to lose subatomic particles, a process called decay. The isotope \(^{14}C\), however, is unstable, or radioactive. A radioactive isotope is one in which the nucleus decays spontaneously, giving off particles and energy. When the radioactive decay leads to a change in the number of protons, it transforms the atom to an atom of a different element. For example, when a carbon-14 (\(^{14}C\)) atom decays, a neutron decays into a proton, transforming the atom into a nitrogen (\(^{14}N\)) atom. Radioactive isotopes have many useful applications in biology.
Radioactive Tracers
Radioactive isotopes are often used as diagnostic tools in medicine. Cells can use radioactive atoms just as they would use nonradioactive isotopes of the same element. The radioactive isotopes are incorporated into biologically active molecules, which are then used as tracers to track atoms during metabolism, the chemical processes of an organism. For example, certain kidney disorders are diagnosed by injecting small doses of radioactively labeled substances into the blood and then analyzing the tracer molecules excreted in the urine. Radioactive tracers are also used in combination with sophisticated imaging instruments, such as PET scanners that can monitor growth and metabolism of cancers in the body. A PET scan, a medical use for radioactive isotopes, is an acronym for positron-emission tomography, and it detects locations of intense chemical activity in the body. The bright yellow spot marks an area with an elevated level of radioactively labeled glucose, which in turn indicates high metabolic activity, a hallmark of cancerous tissue.
Although radioactive isotopes are very useful in biological research and medicine, radiation from decaying isotopes also poses a hazard to life by damaging cellular molecules. The severity of damage depends on the type and amount of radiation an organism absorbs. One of the most serious environmental threats is radioactive fallout from nuclear accidents. The doses of most isotopes used in medical diagnosis, however, are relatively safe.
Radiometric Dating
EVOLUTION Researchers measure radioactive decay in fossils to date these relics of past life. Fossils provide a large body of evidence for evolution, documenting differences between organisms from the past and those living at present and giving us insight into species that have disappeared over time. While the layering of fossil beds establishes that deeper fossils are older than more shallow ones, the actual age in years of the fossils in each layer cannot be determined by position alone.
A parent isotope decays into its daughter isotope at a fixed rate, expressed as the half-life of the isotope, the time it takes for 50% of the parent isotope to decay. Each radioactive isotope has a characteristic half-life that is not affected by temperature, pressure, or any other environmental variable. Using a process called radiometric dating, scientists measure the ratio of different isotopes and calculate how many half-lives in years have passed since an organism was fossilized or a rock was formed.
Half-life values range from very short for some isotopes, measured in seconds or days, to extremely long, uranium-238 has a half-life of 4.5 billion years! Each isotope can best measure a particular range of years: uranium-238 is used to determine that moon rocks are approximately 4.5 billion years old, similar to the estimated age of Earth. Carbon-14 with a half-life of 5,730 years is used to date fossils up to about 75,000 years old. Fossils older than that contain too little \(^{14}C\) to be detected. For dating dinosaur bones that are 65.5 million years old, carbon-14 cannot be used; uranium-235 with a half-life of 704 million years can be incorporated into surrounding rocks, however.
| Isotope | Protons | Neutrons | Stability | Half-Life | Use |
|---|---|---|---|---|---|
| Carbon-12 12C | 6 | 6 | Stable | - | 99% of natural carbon, reference standard |
| Carbon-13 13C | 6 | 7 | Stable | - | ∼1% natural, tracer in metabolic studies |
| Carbon-14 14C | 6 | 8 | Radioactive | 5,730 years | Radiometric dating up to 75k years, fossil dating, photosynthesis experiments |
| Nitrogen-14 14N | 7 | 7 | Stable | - | Daughter product of 14C decay |
| Uranium-238 238U | 92 | 146 | Radioactive | 4.5 billion years | Dating moon rocks, Earth age |
| Uranium-235 235U | 92 | 143 | Radioactive | 704 million years | Dating ancient rocks, dinosaur era |
The Energy Levels of Electrons
The simplified models of the atom greatly exaggerate the size of the nucleus relative to that of the whole atom. Atoms are mostly empty space. When two atoms approach each other during a chemical reaction, their nuclei do not come close enough to interact. Of the three subatomic particles we have discussed, only electrons are directly involved in chemical reactions.
An atom's electrons vary in the amount of energy they possess. Energy is defined as the capacity to cause change, for instance, by doing work. Potential energy is the energy that matter possesses because of its location or structure. For example, water in a reservoir on a hill has potential energy because of its altitude. When the gates of the reservoir's dam are opened and the water runs downhill, the energy can be used to do work, such as moving the blades of turbines to generate electricity. Because energy has been expended, the water has less energy at the bottom of the hill than it did in the reservoir. Matter has a natural tendency to move toward the lowest possible state of potential energy; in our example, the water runs downhill. To restore the potential energy of a reservoir, work must be done to elevate the water against gravity.
The electrons of an atom have potential energy due to their distance from the nucleus. The negatively charged electrons are attracted to the positively charged nucleus.
Electrons exist only at fixed levels of potential energy called electron shells.
A ball bouncing down a flight of stairs can come to rest only on each step, not between steps. Similarly, an electron can exist only at certain energy levels, not between levels.
Third shell highest energy level in this model, second shell higher energy level, first shell lowest energy level, atomic nucleus, energy absorbed, energy lost.
An electron's energy level is correlated with its average distance from the nucleus. Electrons are found in different electron shells, each with a characteristic average distance and energy level. In diagrams, shells can be represented by concentric circles. The first shell is closest to the nucleus, and electrons in this shell have the lowest potential energy. Electrons in the second shell have more energy, and electrons in the third shell even more energy.
An electron can move from one shell to another, but only by absorbing or losing an amount of energy equal to the difference in potential energy between its position in the old shell and that in the new shell. When an electron absorbs energy, it moves to a shell farther out from the nucleus. For example, light energy can excite an electron to a higher energy level. Indeed, this is the first step taken when plants harness the energy of sunlight for photosynthesis, the process that produces food from carbon dioxide and water. When an electron loses energy, it falls back to a shell closer to the nucleus, and the lost energy is usually released to the environment as visible light or ultraviolet radiation.
It takes work to move a given electron farther away from the nucleus, so the more distant an electron is from the nucleus, the greater its potential energy. Unlike the continuous flow of water downhill, changes in the potential energy of electrons can occur only in steps of fixed amounts. An electron having a certain amount of energy is something like a ball on a staircase. The ball can have different amounts of potential energy, depending on which step it is on, but it cannot spend much time between steps. Similarly, an electron's potential energy is determined by its energy level, not between them.
Electron Distribution and Chemical Properties
The chemical behavior of an atom is determined by the distribution of electrons in the atom's electron shells. Beginning with hydrogen, the simplest atom, we can imagine building the atoms of the other elements by adding 1 proton and 1 electron at a time along with an appropriate number of neutrons. The periodic table of the elements shows this distribution of electrons for the first 18 elements, from hydrogen (\(_{1}H\)) to argon (\(_{18}Ar\)). The elements are arranged in three rows, or periods, corresponding to the number of electron shells in their atoms. The left-to-right sequence of elements in each row corresponds to the sequential addition of electrons and protons.
Hydrogen's 1 electron and helium's 2 electrons are located in the first shell. Electrons, like all matter, tend to exist in the lowest available state of potential energy. In an atom, this state is in the first shell. However, the first shell can hold no more than 2 electrons; thus, hydrogen and helium are the only elements in the first row of the table. In an atom with more than 2 electrons, the additional electrons must occupy higher shells because the first shell is full. The next element, lithium, has 3 electrons. Two of these electrons fill the first shell, while the third occupies the second shell. The second shell holds a maximum of 8 electrons. Neon, at the end of the second row, has 8 electrons in the second shell, giving it a total of 10 electrons.
The chemical behavior of an atom depends mostly on the number of electrons in its outermost shell. We call those outer electrons valence electrons and the outermost electron shell the valence shell. In the case of lithium, there is only 1 valence electron, and the second shell is the valence shell. Atoms with the same number of electrons in their valence shells exhibit similar chemical behavior.
For example, fluorine (F) and chlorine (Cl) both have 7 valence electrons, and both form compounds when combined with the element sodium (Na): Sodium fluoride (NaF) is commonly added to toothpaste to prevent tooth decay, and, as described earlier, NaCl is table salt.
An atom with a completed valence shell is unreactive; that is, it will not interact readily with other atoms. At the far right of the periodic table are helium, neon, and argon, the only three elements shown in the first 18 that have full valence shells. These elements are said to be inert, meaning chemically unreactive. All the other atoms are chemically reactive because they have incomplete valence shells.
| Shell | Max Electrons | Elements Using This As Valence | Valence Electron Trend |
|---|---|---|---|
| First shell | 2 | H (1), He (2) | H reactive (1 valence), He inert (2 full) |
| Second shell | 8 | Li to Ne (3 to 10) | Li 1 valence -> highly reactive, to Ne 8 valence -> inert |
| Third shell | 8 for first 18 elements, later 18 with d orbitals | Na to Ar (11 to 18) | Na 1 valence -> reactive metal, Cl 7 valence -> reactive nonmetal, Ar 8 valence -> inert |
Electron Orbitals
In the early 1900s, the electron shells of an atom were visualized as concentric paths of electrons orbiting the nucleus, somewhat like planets orbiting the sun. It is still convenient to use two-dimensional concentric-circle diagrams to symbolize three-dimensional electron shells. However, you must remember that each concentric circle represents only the average distance between an electron in that shell and the nucleus. Accordingly, the concentric-circle diagrams do not give a real picture of an atom. In reality, we can never know the exact location of an electron. What we can instead do is describe the space in which an electron spends most of its time. The three-dimensional space where an electron is found 90% of the time is called an orbital.
Each electron shell contains electrons at a particular energy level, distributed among a specific number of orbitals of distinctive shapes and orientations. The orbitals of neon are a good example, with its electron distribution diagram for reference. You can think of an orbital as a component of an electron shell. The first electron shell has one spherical s orbital called 1s, but the second shell has four orbitals: one large spherical s orbital called 2s and three dumbbell-shaped p orbitals called 2p orbitals. The third shell and other higher electron shells also have s and p orbitals, as well as orbitals of more complex shapes.
No more than 2 electrons can occupy a single orbital. The first electron shell therefore accommodates up to 2 electrons in its 1s orbital. The lone electron of a hydrogen atom occupies the 1s orbital, as do the 2 electrons of a helium atom. The four orbitals of the second electron shell can hold up to 8 electrons, 2 in each orbital. Electrons in each of the four orbitals in the second shell have nearly the same energy, but they move in different volumes of space.
The reactivity of an atom arises from the presence of unpaired electrons in one or more orbitals of its valence shell. Atoms interact in a way that completes their valence shells. When they do so, it is the unpaired electrons that are involved.
| Shell | Orbital Types | Number of Orbitals | Max Electrons | Shape |
|---|---|---|---|---|
| 1st | 1s | 1 | 2 | Spherical |
| 2nd | 2s + 2p | 1 + 3 = 4 | 8 | s = spherical, p = dumbbell along x,y,z |
| 3rd | 3s + 3p + 3d (for full periodic table) | 1 + 3 + 5 = 9 | 18 (but first 8 for elements up to Ar) | s spherical, p dumbbell, d complex |
Practice Questions
MCQs (Multiple Choice Questions)
01. Which particles are in the nucleus?
- Protons and electrons
- Protons and neutrons
- Neutrons and electrons
- Only protons
Correct: B
Explanation: Nucleus contains protons positive and neutrons neutral. Electrons are in cloud/shells around. Nucleus dense core.
02. An atom has 3 protons and 4 neutrons. What is its mass number?
- 3
- 4
- 7
- 12
Correct: C
Explanation: Mass number = protons + neutrons = 3+4=7. This is lithium-7, most common lithium isotope. Atomic number 3 defines element lithium.
03. A radioactive isotope of nitrogen has 7 protons and 8 neutrons. How is it written?
- \(^{7}_{15}N\)
- \(^{15}_{7}N\)
- \(^{7}_{8}N\)
- \(^{8}_{7}N\)
Correct: B
Explanation: Atomic number = protons =7 subscript, mass number = protons+neutrons =15 superscript. So \(^{15}_{7}N\). This isotope decays via beta emission.
04. How many electrons does fluorine have in a neutral atom? How many valence electrons?
- 9 total, 7 valence
- 7 total, 9 valence
- 9 total, 9 valence
- 19 total, 7 valence
Correct: A
Explanation: Fluorine atomic number 9 = 9 protons = 9 electrons neutral. Electron configuration 2 in first shell, 7 in second valence. Needs 1 to fill.
05. Which statement about isotopes is true?
- Isotopes have different numbers of protons
- Isotopes behave very differently chemically
- Carbon-12 and Carbon-14 have same protons but different neutrons
- Atomic mass equals mass number of most common isotope
Correct: C
Explanation: Isotopes same element same protons, different neutrons. Carbon-12 has 6 neutrons, carbon-14 has 8. Chemical behavior identical. Atomic mass is weighted average, not equal to one mass number.
06. What happens when carbon-14 decays?
- Proton turns into neutron
- Neutron decays into proton, atom becomes nitrogen-14
- Electron becomes proton
- Mass number decreases
Correct: B
Explanation: Beta decay: neutron -> proton + electron emitted. So 6 protons becomes 7 protons, element changes from C to N, mass number stays 14.
07. Which property makes radioactive isotopes useful as tracers?
- They are chemically different from stable isotopes
- They behave identically chemically but can be detected by radiation
- They have different valence electrons
- They have no neutrons
Correct: B
Explanation: Cells use radioactive isotopes same as stable. Difference only nuclear, not chemical. Allows tracking metabolism while detecting radiation. Used for PET scans.
08. An electron moves from second shell to first shell. What happens?
- Energy is absorbed
- Energy is released as light
- Proton number changes
- Atom becomes different element
Correct: B
Explanation: Moving closer to nucleus = lower potential energy, excess energy released as photon (visible or UV). Absorbing energy moves outward. This is quantized.
09. In the periodic table, elements in same column have what in common?
- Same number of electron shells
- Same mass number
- Same number of valence electrons and similar chemical behavior
- Same number of neutrons
Correct: C
Explanation: Column = group = same valence electrons. Row = period = same number of shells. Fluorine and chlorine both group 17, 7 valence, both form salts with sodium.
10. How many orbitals and max electrons in second electron shell?
- 2 orbitals, 4 electrons
- 4 orbitals (1s+3p), 8 electrons
- 8 orbitals, 16 electrons
- 1 orbital, 2 electrons
Correct: B
Explanation: Second shell has one 2s spherical and three 2p dumbbell orbitals oriented x,y,z. Each orbital max 2 electrons, so 4 orbitals x2 =8 electrons max.
FRQs (Free Response Questions)
FRQ 1: (Subatomic + Atomic Number)
A sample of sodium has mass number 23 and atomic number 11. Calculate protons, neutrons, electrons in neutral atom. Explain why atomic mass is 22.99 not 23.
Rubric:
- 11 protons (atomic number = protons) (1 pt)
- 11 electrons neutral, 12 neutrons (23-11) (1 pt)
- Atomic mass is weighted average of isotopes Na-23 plus tiny Na-22 etc, not mass number of single isotope (1 pt)
- Electron mass negligible, almost all mass in nucleus, mass number close but not equal to atomic mass due to binding energy (1 pt)
- Dalton definition: mass close to 1 dalton per proton/neutron (1 pt)
FRQ 2: (Isotopes + Radioactivity)
Carbon has isotopes 12C, 13C, 14C. Compare protons, neutrons, stability, abundance, and explain why 14C is used for dating fossils up to 75,000 years while uranium-238 is used for moon rocks.
Rubric:
- All have 6 protons, neutrons 6,7,8 respectively, 12C 99%, 13C ∼1%, 14C rare (1 pt)
- 12C 13C stable, 14C radioactive half-life 5730 years beta decay to 14N (1 pt)
- Chemical behavior identical, so organisms incorporate 14C while alive at atmospheric ratio (1 pt)
- After death no new intake, 14C decays, ratio 14C/12C measures time since death, half-life limits to ∼75k years (too little left after) (1 pt)
- U-238 half-life 4.5 billion years suitable for billion-year-old moon rocks, C-14 too short, would have decayed completely (1 pt)
FRQ 3: (Tracers + PET)
Describe how radioactive tracers are used to diagnose kidney disorders and detect cancer via PET scan. Include why cells can use them and safety considerations.
Rubric:
- Tracers are biologically active molecules labeled with radioisotope (e.g., glucose with F-18 or C-14) (1 pt)
- Cells use them identically chemically to stable isotopes, incorporated into metabolism (1 pt)
- Kidney example: inject labeled substance into blood, analyze urine excretion to assess kidney function (1 pt)
- PET example: cancer cells high metabolism take up more labeled glucose, bright spot indicates elevated activity (1 pt)
- Safety: doses low relatively safe, but radiation damages molecules, severity depends on type and amount absorbed, risk vs benefit (1 pt)
FRQ 4: (Energy Levels + Photosynthesis)
Explain potential energy of electrons, staircase analogy, what happens when electron absorbs vs releases energy, and link to photosynthesis.
Rubric:
- Potential energy due to distance from nucleus, attraction negative to positive nucleus, farther = higher potential (1 pt)
- Natural tendency to move to lowest potential, but electrons only exist at fixed shells/levels not between (quantized), like ball on staircase steps (1 pt)
- Absorbing exact energy equal to difference moves electron outward to higher shell excitation (1 pt)
- Losing energy falls inward, releases photon as light/UV, energy lost to environment (1 pt)
- Photosynthesis: light excites chlorophyll electron to higher level, first step harnessing solar energy to produce food from CO2 and water (1 pt)
FRQ 5: (Electron Distribution + Orbitals + Reactivity)
Fluorine atomic number 9. Draw electron distribution, state shells, orbitals occupied, valence electrons, unpaired electrons, and predict reactivity compared to neon. Explain inertness of neon.
Rubric:
- Total 9 electrons, first shell 2 (1s2), second shell 7 (2s2 2p5), 2 shells (1 pt)
- Orbitals: 1s full (2), 2s full (2), 2p three orbitals: two full (4), one with 1 unpaired = total 5 orbitals occupied (1 pt)
- Valence electrons 7, needs 1 to fill octet, 1 unpaired causes high reactivity, forms ionic or covalent bonds (1 pt)
- Neon atomic number 10, configuration 1s2 2s2 2p6, 2 shells, valence 8 full, all orbitals paired, completed valence shell (1 pt)
- Neon inert chemically unreactive because no unpaired electrons, no tendency to gain/lose/share, stable low potential energy (1 pt)
Frequently Asked Questions
What are subatomic particles in biology?
The three relevant subatomic particles are protons (positive, mass ∼1 dalton, in nucleus), neutrons (neutral, mass ∼1 dalton, in nucleus), and electrons (negative, mass ∼1/2000 dalton, in cloud/shells around nucleus). Only electrons participate in chemical reactions.
What is atomic number vs mass number?
Atomic number is number of protons, written as subscript, defines element and equals electrons in neutral atom. Mass number is protons plus neutrons, written as superscript, defines isotope. Neutrons = mass number minus atomic number.
What are isotopes and why do they behave same chemically?
Isotopes are atomic forms of same element with same protons but different neutrons, e.g., carbon-12 (6 neutrons) and carbon-14 (8 neutrons). They behave identically chemically because chemical behavior depends on electrons, not neutrons.
What is a radioactive isotope?
A radioactive isotope has unstable nucleus that decays spontaneously, emitting particles and energy. If proton number changes, it becomes different element, e.g., carbon-14 decays to nitrogen-14 when neutron becomes proton.
How do radioactive tracers work in medicine?
Radioactive tracers are biologically active molecules labeled with radioisotopes. Cells use them like stable isotopes, so they track metabolism. PET scans detect labeled glucose uptake; kidney tests analyze labeled molecules excreted in urine.
What is radiometric dating and half-life?
Half-life is time for 50% of parent isotope to decay to daughter, unaffected by environment. Radiometric dating measures parent/daughter isotope ratio to calculate age. Carbon-14 half-life 5,730 years dates fossils up to 75k years; uranium-238 half-life 4.5 billion years dates moon rocks.
What are electron shells and energy levels?
Electrons exist only at fixed energy levels called shells, like steps on staircase, not between. First shell lowest energy holds 2 electrons, second holds 8, third holds 8 for first 18 elements. Farther from nucleus means higher potential energy.
Why are valence electrons important?
Valence electrons are outermost shell electrons that determine chemical behavior. Atoms with same valence electrons have similar reactivity. Full valence shell (8, or 2 for first shell) makes atom inert like helium, neon, argon; incomplete valence makes atom reactive.
What is an electron orbital?
An orbital is 3D space where electron found 90% of time. First shell has one spherical 1s orbital. Second shell has four: one spherical 2s and three dumbbell 2p oriented along x, y, z axes. Each orbital holds max 2 electrons. Reactivity comes from unpaired electrons.
Exam Mapping
| Concept from 2.2 | A-Level Biology (AQA, OCR, Edexcel) | AP Biology (College Board) | IB Biology HL | College Bio 1 & 2 | USABO / IBO |
|---|---|---|---|---|---|
| Subatomic particles, nucleus, electron cloud | 3.1.1 Atomic structure, protons neutrons electrons | Big Idea 1: Structure of atoms determines properties | 2.1 Atoms, atomic structure, dalton | Chapter 2: atom smallest unit retaining properties | Atomic composition, mass negligible electrons |
| Atomic number and mass number, notation \(^{A}_{Z}X\) | Calculations protons neutrons electrons, isotope notation | Atomic number defines element, mass number protons+neutrons | Isotope notation, neutrons = A-Z, ions | Sodium example 23-11=12 neutrons, hydrogen no neutron | Quick calculations, ion charge vs electrons |
| Isotopes, stable vs radioactive, carbon isotopes | Isotopes same protons different neutrons, chemical similarity | Isotopes identical chemical behavior, carbon-12 13 14 | Nature mixture of isotopes, weighted average atomic mass 12.01 | Carbon isotopes 99% 12C, 1% 13C, rare 14C | Weighted average atomic mass calculations |
| Radioactive tracers, PET scan, medical uses | Use of radioisotopes in medicine, tracers | Tracers track metabolism, glucose uptake | Medical diagnosis, PET scanner cancer detection | Kidney disorders diagnosis, tracers in metabolism | Explain why tracers work chemically identical |
| Radiometric dating, half-life, evolution evidence | Half-life not affected by environment, dating fossils/rocks | Radiometric dating fossils, uranium-238 moon rocks 4.5B years | Half-life definition 50% decay, carbon-14 5730y limit 75k y | Fossils deeper older but need isotope ratio for years | Calculation remaining fraction (1/2)n, choose isotope by timescale |
| Energy levels, potential energy, electron shells, quantized | Potential energy vs distance, absorption emission, light excites electron | Energy levels quantized, electrons only at fixed levels, photosynthesis excitation | Staircase analogy, energy absorbed moves out, lost emits photon | Reservoir analogy potential energy, tendency lowest energy | Explain discrete spectra, why nuclei don't interact |
| Electron distribution, periodic table first 18, valence electrons, inert gases | Periods = shells, groups = valence similar behavior, valence shell full = inert He Ne Ar | Valence electrons determine reactivity, fluorine 7 chlorine 7 both form Na compounds | Periodic trends, inert vs reactive due to full vs incomplete valence | Building atoms adding proton+electron+neutron, lithium 2+1, neon 2+8 | Predict reactivity Na 1 valence reactive metal, Cl 7 reactive nonmetal |
| Electron orbitals, s spherical, p dumbbell, 2 electrons per orbital, unpaired reactivity | Orbital 90% probability, shapes 1s 2s 2p x y z, max 2 per orbital | Orbitals as components of shells, reactivity from unpaired electrons | s and p orbitals, second shell 4 orbitals max 8 electrons | Neon example 1s 2s three 2p, superimposed orbitals | Draw orbital occupancy fluorine 1s2 2s2 2p5 one unpaired need 1 |
Conclusion
You now have the complete atomic blueprint that underpins all biology. You can calculate protons, neutrons, and electrons from \(^{23}_{11}Na\), explain why \(^{12}C\), \(^{13}C\), and \(^{14}C\) are same element but only \(^{14}C\) can date a fossil, describe why PET scans glow where cancer grows, why half-life is immune to temperature and pressure, why electrons sit on staircase steps and why light can kick them up to power photosynthesis, why fluorine and chlorine both make salt with sodium, why helium and neon never bond, and why a spherical s orbital and three dumbbell p orbitals holding two electrons each explain chemical reactivity. This is the language of chemistry that AP, A-Level, IB HL, and Olympiad examiners expect you to speak fluently before you ever discuss bonding or water.
References
- Campbell Biology, 12th Edition, Reece et al., Unit 1 The Chemistry of Life, Chapter 2 The Chemical Context of Life, Concept 2.2 An element's properties depend on the structure of its atoms: Subatomic Particles, Atomic Number and Atomic Mass, Isotopes, Radioactive Tracers, Radiometric Dating, The Energy Levels of Electrons, Electron Distribution and Chemical Properties, Electron Orbitals.
- Additional: IUPAC periodic table, atomic mass unit dalton definition, PET imaging principles, carbon-14 calibration data.
