6.2 Atomic Structure, Isotopes, and Reading the Periodic Table
Key Takeaways
- The atomic number is the proton count and defines the element; changing it changes the element, while changing the neutron count produces an isotope and changing the electron count produces an ion.
- Protons and neutrons each have a mass of about 1 atomic mass unit and sit in the nucleus, while an electron has roughly 1/1,836 the mass of a proton, so essentially all of an atom's mass is nuclear and essentially all of its volume is electron cloud.
- The decimal atomic mass printed on the periodic table is a weighted average of an element's naturally occurring isotopes, which is why chlorine is listed as about 35.45 u rather than a whole number.
- Electron shells hold at most 2, 8, and 8 electrons for the first three shells at the 4-8 level, and the valence (outermost) electron count determines how an atom bonds.
- Metals lose valence electrons to form positively charged cations, and nonmetals gain electrons to form negatively charged anions, with the group number predicting the common charge.
Why This Section Precedes Chemical Change
The first descriptive statement of Competency 008 is that the teacher "describes the structure and components of the atom." Every later idea in the competency — bonding, formulas, equations, reaction types — depends on it. This section also matters logistically: the Science 4-8 test supplies an on-screen periodic table, and candidates who know how to extract information from it gain time on every chemistry item.
The Three Subatomic Particles
| Particle | Charge | Approximate mass | Location | Determines |
|---|---|---|---|---|
| Proton | +1 | 1 u (1.67 × 10⁻²⁷ kg) | Nucleus | Element identity (atomic number) |
| Neutron | 0 | 1 u | Nucleus | Isotope; nuclear stability |
| Electron | −1 | ~1/1,836 u | Electron cloud / shells | Chemical behavior; ion charge |
Two structural facts anchor student understanding. First, nearly all of an atom's mass is in the nucleus, because electrons are almost 2,000 times lighter than nucleons. Second, nearly all of an atom's volume is empty space occupied by the electron cloud; Rutherford's gold-foil experiment established this when most alpha particles passed straight through the foil while a few deflected sharply off the tiny dense nucleus.
Atomic Number, Mass Number, and Isotopes
- Atomic number (Z) = number of protons. It defines the element. Every atom with Z = 6 is carbon; an atom with Z = 7 is nitrogen, full stop.
- Mass number (A) = protons + neutrons. It is always a whole number for a specific atom.
- Neutrons = A − Z.
- In a neutral atom, electrons = protons.
Isotopes are atoms of the same element with different neutron counts.
| Isotope | Protons | Neutrons | Mass number | Note |
|---|---|---|---|---|
| Carbon-12 | 6 | 6 | 12 | 98.9% of natural carbon; the mass-unit standard |
| Carbon-13 | 6 | 7 | 13 | Stable; ~1.1% of natural carbon |
| Carbon-14 | 6 | 8 | 14 | Radioactive; half-life ≈ 5,730 years; used to date organic remains up to about 50,000 years old |
Isotopes of an element have identical chemical properties because chemistry depends on electrons, but different physical properties such as mass and nuclear stability. This is exactly why radiocarbon dating works: living tissue takes up carbon-14 in the same chemical reactions as carbon-12, and only after death does the ratio begin to fall.
Average Atomic Mass
The atomic mass printed on the periodic table is a weighted average of naturally occurring isotopes, which is why it is rarely a whole number. Chlorine appears as about 35.45 u because natural chlorine is roughly 75% chlorine-35 and 25% chlorine-37:
(0.75 × 35) + (0.25 × 37) = 26.25 + 9.25 = 35.5 u
Students who expect a whole number are being asked, in effect, to reason about isotope abundance — a well-designed diagnostic question.
Ions
An atom that gains or loses electrons becomes an ion.
- Cation — positively charged, formed by losing electrons. Metals do this. A sodium atom (11 p, 11 e) loses one electron to become Na⁺ (11 p, 10 e).
- Anion — negatively charged, formed by gaining electrons. Nonmetals do this. A chlorine atom (17 p, 17 e) gains one electron to become Cl⁻ (17 p, 18 e).
The mnemonic worth teaching is that a cation is "pawsitive" — and, more usefully, that the charge sign tells you what changed: only electrons move in ordinary chemistry, never protons.
| Group | Valence electrons | Typical ion | Example |
|---|---|---|---|
| 1 (alkali metals) | 1 | 1+ | Na⁺, K⁺ |
| 2 (alkaline earth) | 2 | 2+ | Mg²⁺, Ca²⁺ |
| 16 (chalcogens) | 6 | 2− | O²⁻, S²⁻ |
| 17 (halogens) | 7 | 1− | F⁻, Cl⁻ |
| 18 (noble gases) | 8 (2 for He) | none | Ne, Ar |
Electron Shells, Bohr Diagrams, and Lewis Structures
At grades 4-8 the working model is shells with capacities of 2, 8, and 8 for the first three energy levels. A Bohr diagram draws the nucleus with its proton and neutron counts and arcs for each occupied shell. Magnesium (Z = 12) is drawn 2, 8, 2 — two valence electrons, so it readily forms Mg²⁺.
A Lewis dot structure abbreviates this by showing only valence electrons as dots around the element symbol. Oxygen, with six valence electrons, is drawn with six dots and needs two more to complete an octet, which predicts both the O²⁻ ion and the two bonds oxygen forms in H₂O.
The octet rule — atoms tend toward eight valence electrons, or two for hydrogen and helium — explains why noble gases are unreactive and why Group 1 and Group 17 elements react so vigorously with each other.
Reading the On-Screen Periodic Table
Each cell of the supplied table gives, at minimum, the element symbol, the atomic number (usually above the symbol), and the average atomic mass (usually below). From those three items you can derive everything a 4-8 item needs:
- Protons = atomic number.
- Electrons in a neutral atom = atomic number.
- Neutrons in the most common isotope ≈ round the atomic mass to the nearest whole number, then subtract the atomic number. For iron, 56 − 26 = 30 neutrons.
- Valence electrons = group number for main-group elements (Group 1 → 1, Group 2 → 2, Groups 13-17 → the ones digit, so Group 16 → 6).
- Metal, nonmetal, or metalloid = position relative to the stair-step line.
Practicing this extraction until it is automatic is worth more exam points than memorizing individual elements, because the table is on screen and the derivations are not.
An atom has 15 protons, 16 neutrons, and 15 electrons. Which description is correct?
Natural boron is about 20% boron-10 and 80% boron-11. Which value best matches the average atomic mass listed on a periodic table, and why is it not a whole number?
Using only an on-screen periodic table cell that reads "20 / Ca / 40.08", which conclusion is correct for a neutral calcium atom?
Two atoms have 6 protons and 8 protons respectively. They are: