Atoms & the periodic table · Topic 3 of 15

Atomic Structure, Isotopes & Ions

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Chemistry · atoms

Atomic Structure, Isotopes & Ions

Protons set the element (atomic number), protons plus neutrons give the mass number, extra neutrons make isotopes, and gained or lost electrons make ions.

01 · The particlesProtons + neutrons in nucleus; electrons outside

02 · IdentityAtomic number = number of protons

03 · Find neutronsNeutrons = mass number − atomic number

What it is

Atomic number = protons (= electrons in a neutral atom) must know

Isotopes: same protons, different neutrons (different mass numbers) must know

Lose e⁻ → cation (+) · gain e⁻ → anion (−); protons stay the same must know

Carbon-14 = ¹⁴₆C: mass number upper left, atomic number lower left

Periodic-table mass = weighted average of the isotopes (amu), so it has decimals

Valence electrons = outer-shell electrons; they do the bonding

Electron configuration by subshell: Na = 1s² 2s² 2p⁶ 3s¹

Parts & jobs

Proton +1 and neutron 0 in the nucleus (~1 amu each); electron −1 outside must know

Nucleus: tiny but nearly all the mass · electrons: nearly all the volume

Electron shells: the 1st holds up to 2, the 2nd up to 8

Know by heart

Mass number = protons + neutrons, so neutrons = mass number − atomic number must know

The rules

Ion charge = protons − electrons (Mg²⁺: 12 − 10 = 2+)

Valence e⁻ by group: 1 → 1, 2 → 2, 14 → 4, 17 → 7; group 18 full

Octet rule: atoms lose, gain or share e⁻ to reach a full outer shell of 8

Metals lose e⁻ → cations (Na⁺) · nonmetals gain e⁻ → anions (Cl⁻)

Worked examples

³⁴S²⁻: 16 protons · 34 − 16 = 18 neutrons · 16 + 2 = 18 electrons

Li-6 vs Li-7: 3 protons each; 3 neutrons vs 4 neutrons

Isotope or ion?

Commonly confused

	Isotope	Ion
What differs	Number of neutrons	Number of electrons
Protons	Same as the element	Same as the element
Charge	Stays neutral	Positive (lost e⁻) or negative (gained e⁻)
Mass number	Changes	Unchanged
Example	Li-6 vs Li-7	Mg → Mg²⁺

More detail

Show 4 more facts

Electron ≈ 1/1800 of a proton's mass, so it is left out of mass number

Pauli: ≤ 2 e⁻ per orbital, opposite spins · Hund: fill singly before pairing

Rutherford's gold-foil experiment found a tiny, dense, positive nucleus

1 amu = 1/12 the mass of a carbon-12 atom

Pearl

Protons pick the element, neutrons pick the isotope, electrons pick the charge.

Sources · OpenStax Chemistry 2e §2.2 Evolution of Atomic Theory · OpenStax Chemistry 2e §2.3 Atomic Structure and Symbolism · OpenStax Chemistry 2e §6.4 Electronic Structure of Atoms (Electron Configurations) · OpenStax Chemistry 2e §7.1 Ionic Bonding (cations and anions) · OpenStax Biology 2e §2.1 Atoms, Isotopes, Ions, and Molecules (electron shells, octet rule) · OpenStax Chemistry 2e §7.3 Lewis Symbols and Structures (octet rule) · Union Test Prep — HESI Chemistry study guide, page 1 (atoms, ions, atomic number, mass number, isotopes) · NurseHub — HESI A2 Chemistry review course (unit: Atomic Structure and The Periodic Table) · Built from the verified NurseSavvy HESI A2 fact map (facts reviewed 2026-10-16). HESI is a trademark of Elsevier; NurseSavvy is not affiliated with Elsevier.

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