The atomic number (bottom) is the number of protons. The mass number (top) is protons + neutrons.

1. Sodium-23: atomic number 11, mass number 23
2. protons 11, electrons 11, neutrons 23 − 11 = 12

Isotopes are atoms of the same element with different numbers of neutrons: chlorine-35 and chlorine-37. Ions have gained or lost electrons: Na⁺ has 10 electrons, Cl⁻ has 18.

An element's relative atomic mass is the mean mass of its atoms, weighted by how common each isotope is.

1. Chlorine: 75% chlorine-35, 25% chlorine-37
2. Ar = (35 × 75 + 37 × 25) ÷ 100
3. Ar = 35.5

Electrons fill shells from the nucleus outwards: up to 2 in the first, then up to 8, then up to 8 (for the first 20 elements).

1. Magnesium, 12 electrons: 2,8,2
2. Chlorine, 17 electrons: 2,8,7
3. Potassium, 19 electrons: 2,8,8,1

The outer shell decides how an element reacts: that is why elements in a group behave alike.