Atomic Structure & Electron Shells

The Atom

An atom is the smallest part of an element that can exist. It has a tiny dense nucleus (protons and neutrons) at its centre, with electrons in shells around it.

Subatomic particles

• Atomic number = number of protons (defines the element).
• Mass number = protons + neutrons.
• Atoms have equal numbers of protons and electrons, so there is no overall charge.

Isotopes

Isotopes are atoms of the same element with different numbers of neutrons – same atomic number, different mass number.

Example: chlorine

• Chlorine-35 (${}_{17}^{35}\text{Cl}$): 17 protons, 18 neutrons.
• Chlorine-37 (${}_{17}^{37}\text{Cl}$): 17 protons, 20 neutrons.

Elements, Compounds and Mixtures

Element

• One type of atom only.
• About 100 known, each with a chemical symbol (for example O, Na, Fe).
• Example: O₂ (oxygen molecule).

Compound

• Two or more elements chemically combined in fixed proportions.
• Can only be separated back into elements by a chemical reaction.
• Example: H₂O (water).

Mixture

• Two or more elements or compounds not chemically joined.
• Substances keep their own properties.
• Separated by physical methods (filtration, crystallisation, distillation, chromatography).
• Example: a mixture of O₂ and N₂.

Electron Shells

Electrons fill shells around the nucleus, lowest energy first.

Shell capacity (GCSE rule): 2, 8, 8

• 1st shell: up to 2 electrons.
• 2nd shell: up to 8 electrons.
• 3rd shell: up to 8 electrons (at GCSE level).

Electron configuration examples

How to draw electron shell diagrams

1. Atomic number = number of electrons.
2. Fill shells from inside out using 2, 8, 8.
3. Outer-shell electrons = the group number (1–7), or 8 for Group 0 (2 for helium).

First 20 Elements

Key Reminder

Atoms are the building blocks. They can join chemically to make compounds, or mix physically without changing. In reactions, atoms are rearranged, not made or destroyed.