Atomic Structure & Isotopes Cheat Sheet

Subatomic particles, atomic and mass number, isotopes, average atomic mass, shells, and Lewis dots.

Keep going with the free Atomic Structure & Isotopes study guide → (graded practice with a subscription).

Subatomic particles

CORE RULE

Proton: +1, in the nucleus. Neutron: 0 charge, in the nucleus. Electron: −1, outside the nucleus.

Proton ≈ neutron ≈ 1 amu; electron ≈ 0.00055 amu (far lighter).

CORE RULE

In a neutral atom, electrons = protons. Nearly all the mass is in the nucleus.

METHOD

Z (atomic number) = number of protons; it identifies the element.

A (mass number) = protons + neutrons = Z + N.

Neutrons: N = A − Z.

Ion electrons = Z − charge (positive: lost electrons; negative: gained electrons).

CORE RULE

Isotopes: same element (same protons), different neutron count and mass number.

CORE RULE

Symbol notation: mass number A as left superscript, atomic number Z as left subscript (e.g. ¹⁴₆C).

Name notation: element name + hyphen + mass number (carbon-14, uranium-235).

METHOD

Weighted average = Σ (isotopic mass × fractional abundance).

Convert % abundance to decimal first; fractional abundances sum to 1.00.

The more abundant isotope pulls the average toward its mass; that is why atomic masses are not whole numbers.

CORE RULE

Introductory fill pattern through calcium: 2, 8, 8, then up to 2 in shell 4 (not the full physical capacities).

Valence electrons (main group) = electrons in the outermost occupied shell.

CORE RULE

Element symbol surrounded by one dot per valence electron.

One dot on each side before pairing a second on any side. He is drawn with 2 (its first shell is full).

REFERENCE

Group 1 → +1 · Group 2 → +2 · Al → +3 · Group 16 → −2 · Group 17 → −1 · N, P → −3. Introductory patterns, not universal.

CORE RULE

Balance charge with the cross-over rule (each ion's charge magnitude becomes the other's subscript), then reduce to the smallest whole-number ratio.

Binary ionic name: metal + nonmetal root + -ide. Na⁺ + Cl⁻ → NaCl; Mg²⁺ + O²⁻ → MgO; Li⁺ + S²⁻ → Li₂S; Al³⁺ + N³⁻ → AlN.