GCSE Chemistry - Atoms & Ions | Protons, Neutrons, Electrons (2026/27 exams)

Atoms are the tiny particles that make up everything and consist of a central nucleus surrounded by electrons orbiting in shells. The nucleus is composed of protons and neutrons.

Protons and neutrons have a relative mass of one. Electrons are much smaller, with a mass 2,000 times less. Protons have a +1 charge, neutrons have no charge (neutral), and electrons have a -1 charge.

The size of an atom varies by element, with a typical radius of 0.1 nanometers. Most of an atom is empty space. The nucleus is extremely small compared to the atom, about 10,000 times smaller.

An atom is typically neutral with an equal number of protons and electrons. If an atom loses or gains electrons, it becomes charged and is then called an ion. Gaining electrons creates a negative ion, while losing electrons creates a positive ion.

The periodic table contains nuclear symbols for each element. The elemental symbol (e.g., O for Oxygen) identifies the element. The atomic number (bottom left) indicates the number of protons and electrons. The mass number (top left) is the sum of protons and neutrons. The number of protons determines the element.