Atomic Structure and the Periodic Table
Atomic Structure and the Periodic Table
This domain tests the structure of atoms, how isotopes and ions differ, electron arrangement, and periodic trends. Questions often use nuclear notation and ask you to predict properties from group and period position.
Core rules
- Protons (positive) define the element and equal atomic number (Z). Neutrons are neutral; mass number (A) = protons + neutrons. Electrons (negative) equal protons in a neutral atom.
- Isotopes have the same Z but different numbers of neutrons. They have nearly identical chemical behavior; the average atomic mass on the periodic table is a weighted average of isotope masses.
- Ions form by gaining or losing electrons, not protons. A cation loses electrons and is positive; an anion gains electrons and is negative.
- Electron configuration fills in order: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p. Valence electrons are in the outermost shell; main-group elements have a number tied to group number (Groups 1, 2, 13–18).
- Periodic table: periods are rows (energy levels), groups are columns (similar valence). Atomic radius increases down a group and decreases across a period. Ionization energy and electronegativity increase across a period and decrease down a group. Metallic character decreases across a period.
- Group 1 alkali metals, Group 2 alkaline earth metals, Group 17 halogens, Group 18 noble gases.
Worked example
An atom is written as ²⁵Mg. Atomic number of Mg is 12, so it has 12 protons and 12 electrons when neutral. Mass number 25 = protons + neutrons, so neutrons = 25 − 12 = 13. If it becomes Mg²⁺, it loses 2 electrons: 10 electrons remain, while protons stay 12, giving a +2 charge.
Traps that cost marks
Do not read the atomic mass (24.31 for Mg) as the mass number of a particular atom. Do not change protons when forming ions. Do not confuse groups and periods. For trends, remember that "increases across a period" applies to ionization energy and electronegativity but not atomic radius. And noble gases are stable because of a full valence shell, not because they have no electrons.