Atomic Structure
Core ideas
An atom has a small, dense nucleus containing protons (relative charge +1, mass 1) and neutrons (charge 0, mass 1), surrounded by electrons (charge −1, negligible mass) in shells. The proton number (atomic number) is the number of protons and identifies the element. The nucleon number (mass number) is protons plus neutrons. Isotopes are atoms of the same element with the same proton number but different numbers of neutrons. Ions form when atoms gain or lose electrons.
Electrons occupy shells in order: 2, 8, 8 for the first three shells in simple models. The outer shell electrons determine chemical properties. Relative atomic mass is the weighted average mass of the isotopes of an element compared with 1/12 the mass of a carbon-12 atom.
Traps to watch
- In a neutral atom, protons equal electrons, but ions have unequal numbers.
- Isotopes have the same chemical properties but may differ in physical properties and mass.
- The mass number is not the relative atomic mass when isotopes exist; you must average.
- Electrons are not in the nucleus.
Rule of thumb
Count from the symbol: protons = atomic number; neutrons = mass number − atomic number; electrons = protons − charge for an ion.
Worked example
An ion of magnesium has symbol ²⁴Mg²⁺. Protons = 12; neutrons = 24 − 12 = 12; electrons = 12 − 2 = 10.