Atomic Structure and the Periodic Table
Review subatomic particles, isotopes, ions, valence electrons, and periodic trends.
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Questions Covered in This Set
11 cards to master
Charge, mass, and location of a proton
Charge +1, mass ≈1 amu, located in the nucleus.
What does the atomic number (Z) tell you?
The number of protons, which defines the element (6 protons = carbon, always). In a neutral atom it also equals the number of electrons.
Mass number (A) formula
A = protons + neutrons.
What are isotopes?
Atoms of the same element with the same number of protons but different numbers of neutrons — e.g., C-12 (6n) and C-14 (8n).
Why is chlorine's atomic mass 35.45 instead of a whole number?
It's a weighted average of natural isotopes: (0.758)(35) + (0.242)(37) ≈ 35.5 amu.
How do ions form?
By gaining or losing electrons, never protons. Na loses 1 e⁻ → Na⁺ (11 p, 10 e⁻); Cl gains 1 e⁻ → Cl⁻ (17 p, 18 e⁻).
What are valence electrons and why do they matter?
Electrons in the outermost shell; they determine an element's chemical behavior. Elements in the same group have the same number of valence electrons.
Group vs. period on the periodic table
A group is a vertical column (same valence electrons, similar behavior); a period is a horizontal row (number of shells in use).
Trend in atomic radius
Decreases left→right across a period (more protons, same shell) and increases down a group (new shells added). Cs is huge, F is tiny.
Trend in ionization energy
Increases left→right across a period and decreases down a group — the opposite of atomic radius, since closely held electrons are harder to remove.
What did Rutherford's gold foil experiment show?
Atoms are mostly empty space with a tiny, dense, positively charged nucleus at the center.
