Atom (Chemistry)
Summary: An atom is the smallest unit of an element that retains its chemical properties, consisting of a central nucleus containing protons and neutrons, surrounded by electrons arranged in shells. Tags: igcse chemistry Created: 2026-07-18
The atom is the fundamental building block of all matter and the smallest particle of an Element that can take part in a chemical reaction. Every atom consists of a tiny, dense central nucleus containing protons and neutrons, surrounded by electrons that occupy specific electron shells at varying distances from the nucleus. The number of protons in the nucleus — known as the atomic number (Z) — defines which Element the atom belongs to, while the mass number (A) is the total number of protons plus neutrons in the nucleus. In a neutral atom, the number of electrons equals the number of protons, giving the atom an overall charge of zero. The study of the atom is the foundation of Atomic Structure, which underpins all of chemistry including bonding, reactivity, and the periodic table.
Structure of the Atom
An atom is composed of three subatomic particles contained within two distinct regions: the central nucleus and the surrounding electron shells.
| Particle | Relative Charge | Relative Mass | Location |
|---|---|---|---|
| Proton | +1 | 1 | Nucleus |
| Neutron | 0 | 1 | Nucleus |
| Electron | -1 | 1/1836 (~0) | Electron shells |
The Nucleus
The nucleus occupies a tiny fraction of the atom’s total volume but contains nearly all of its mass. It is composed of protons (positively charged) and neutrons (neutral), collectively called nucleons. The strong nuclear force holds these particles together in the nucleus, overcoming the electrostatic repulsion between the positively charged protons. The radius of the nucleus is approximately 10,000 times smaller than the radius of the atom itself — if an atom were scaled to the size of a football stadium, the nucleus would be approximately the size of a pea.
The Electron Shells
Electrons occupy specific electron shells (also called energy levels) surrounding the nucleus. These shells are filled according to fixed rules: the first shell holds a maximum of 2 electrons, the second holds a maximum of 8, and the third holds a maximum of 8 for the first 20 elements. The arrangement of electrons in shells is described by the Electronic Configuration of the atom. The number of electrons in the outermost shell determines an element’s chemical properties and its position in the Periodic Table.
Neutrality
Atoms are electrically neutral because the number of protons (positive charges) equals the number of electrons (negative charges). When an atom gains or loses electrons, it becomes an Ion — a charged particle. The resulting Ion has the same number of protons but a different number of electrons, giving it a net positive or negative charge.
Atomic Number and Mass Number
- Atomic number (Z) = number of protons. This is unique to each Element and determines the element’s identity. All atoms of a given Element have the same atomic number.
- Mass number (A) = number of protons + number of neutrons. The mass number is always a whole number for a specific atom.
- Number of neutrons = mass number (A) - atomic number (Z).
Atoms of the same Element with different numbers of neutrons are called isotopes. For example, carbon has three naturally occurring isotopes: carbon-12 (6 protons, 6 neutrons), carbon-13 (6 protons, 7 neutrons), and carbon-14 (6 protons, 8 neutrons).
Sources
- Cambridge IGCSE Chemistry 0620 Syllabus (2023-2025) — Section 2.2, Cambridge Assessment International Education
Common Misconceptions
| Misconception | Reality |
|---|---|
| ”Atoms are indivisible solid spheres” | Atoms are mostly empty space with a tiny, dense nucleus containing protons and neutrons, surrounded by electrons in shells |
| ”The nucleus takes up most of the atom’s volume” | The nucleus occupies an extremely tiny fraction of the atom’s volume — most of the atom is empty space |
| ”All atoms of an element have the same mass” | Isotopes of the same element have different numbers of neutrons and therefore different masses |
| ”Electrons orbit the nucleus like planets around the Sun” | Electrons exist in shells (energy levels), not in fixed circular orbits — the shell model describes probabilities, not paths |