← All Guides

Chemistry · June 2026

Atomic Structure Explained

Everything around you — air, water, your desk, your own body — is made of atoms. Understanding what atoms are made of and how they are organised is the foundation of all chemistry. This guide walks you through sub-atomic particles, atomic notation, electron arrangement, isotopes, and ions.

The Three Sub-Atomic Particles

An atom has a tiny, dense nucleus at its centre surrounded by a cloud of fast-moving electrons. The nucleus contains two types of particle:

  • Protons — positively charged (+1), relative mass 1. The number of protons in an atom's nucleus is what makes it a particular element. Change the proton count and you have a different element entirely.
  • Neutrons — no charge (neutral), relative mass 1. Neutrons contribute to the mass of the nucleus and help hold it together by counteracting the repulsion between positively charged protons.
  • Electrons — negatively charged (−1), relative mass approximately 1/1836 (so small it is treated as negligible). Electrons occupy shells (energy levels) around the nucleus and are responsible for chemical bonding and reactions.

In a neutral atom, the number of electrons always equals the number of protons, so the positive and negative charges cancel out exactly.

Atomic Number and Mass Number

Two key numbers describe any atom:

  • The atomic number (Z) is the number of protons in the nucleus. It uniquely identifies the element. Carbon always has 6 protons; oxygen always has 8.
  • The mass number (A) is the total number of protons plus neutrons. Electrons are excluded because their mass is negligible.

From these two numbers you can work out everything else. If a carbon atom has A = 12 and Z = 6, then: number of neutrons = 12 − 6 = 6; number of electrons (in a neutral atom) = 6.

Reading Atomic Notation

Elements are often written in the form AZX, where X is the chemical symbol. For example, 2311Na is sodium: mass number 23, atomic number 11. Protons = 11, neutrons = 23 − 11 = 12, electrons = 11 (neutral atom).

Electron Shells

Electrons do not drift randomly around the nucleus. They occupy shells (also called energy levels), each of which can hold only a fixed maximum number of electrons:

  • Shell 1 (closest to nucleus): maximum 2 electrons
  • Shell 2: maximum 8 electrons
  • Shell 3: maximum 8 electrons (at GCSE level; more detail appears at A-level)

Electrons fill the innermost shell first before moving outward. A sodium atom (Z = 11) has the electron configuration 2, 8, 1: two electrons in the first shell, eight in the second, and one in the third. This outer-shell (valence) electron is the one sodium loses when it forms a positive ion.

The number of electrons in the outer shell determines how an element behaves chemically. Elements in the same group of the periodic table have the same number of outer-shell electrons, which is why they share similar chemical properties.

Isotopes

Atoms of the same element always have the same number of protons (by definition), but they can have different numbers of neutrons. Atoms of the same element with different neutron counts are called isotopes.

Carbon, for example, has three naturally occurring isotopes: carbon-12 (12C, 6 neutrons), carbon-13 (13C, 7 neutrons), and carbon-14 (14C, 8 neutrons). All three are carbon and behave almost identically chemically, because chemical behaviour is determined by electrons, not neutrons. However, they have slightly different masses, and some isotopes (like carbon-14) are unstable and radioactive.

The relative atomic mass (Ar) shown on the periodic table is a weighted average of all naturally occurring isotopes of that element, taking into account how abundant each isotope is. This is why many Ar values are not whole numbers: chlorine's Ar is 35.5 because it is roughly 75% chlorine-35 and 25% chlorine-37.

Ions

A neutral atom has equal numbers of protons and electrons. When an atom gains or loses electrons, it becomes an electrically charged particle called an ion.

  • Cations are positive ions, formed when an atom loses one or more electrons. Metals typically form cations: sodium loses its single outer electron to become Na+; magnesium loses two to become Mg2+.
  • Anions are negative ions, formed when an atom gains electrons. Non-metals typically form anions: oxygen gains two electrons to become O2−; chlorine gains one to become Cl−.

The charge on an ion tells you the electron imbalance: Na+ has 11 protons but only 10 electrons; Cl− has 17 protons but 18 electrons. Proton count never changes in ordinary chemical reactions — only electrons are transferred.

The Scale of an Atom

The atom is astonishingly small. A hydrogen atom is about 1 × 10−10 m (0.1 nanometres) across. The nucleus is roughly 100,000 times smaller still — if the atom were the size of a football stadium, the nucleus would be a grain of sand on the centre spot. The rest is the space through which electrons move. This mostly-empty structure is why high-speed particles can pass straight through thin sheets of material — the discovery that led Rutherford to propose the nuclear model of the atom in 1911.

Summary

Atoms contain protons (positive) and neutrons (neutral) in the nucleus, with electrons (negative) in shells around it. The atomic number gives the proton count and identifies the element; the mass number is protons plus neutrons. Electrons fill shells from the inside out: 2, 8, 8. Isotopes are atoms of the same element with different neutron counts; their weighted average gives the relative atomic mass. Ions form when atoms gain or lose electrons, producing a net positive or negative charge. Mastering these basics makes every subsequent chemistry topic — bonding, reactions, the periodic table — far easier to understand.