Proton

Summary: A proton is a positively charged subatomic particle found in the nucleus of an atom, with relative charge +1 and relative mass 1, whose number defines the atomic number and identity of the element. Tags: igcse chemistry Created: 2026-07-18

The Proton is one of the three fundamental subatomic particles that make up an atom, alongside the Neutron and the Electron. It resides in the nucleus and carries a relative positive charge of +1 (equal and opposite in magnitude to the charge of an Electron), with a relative mass of 1 on the atomic mass scale. The number of protons in the nucleus is called the atomic number (Z), and this number is the sole determinant of which Element an atom belongs to — changing the number of protons transforms the atom into a different Element entirely. Because protons are positively charged, they experience electrostatic repulsion from one another within the nucleus, but they are held together by the strong nuclear force, which also acts on neutrons. The identity and chemical behaviour of every Element in the Periodic Table is ultimately determined by its number of protons.

Properties of the Proton

Charge and Mass

The Proton has the following key properties:

The Proton is approximately 1836 times heavier than an Electron. In neutral atoms, the number of protons in the nucleus equals the number of electrons in the shells, giving the atom an overall charge of zero.

Atomic Number and Element Identity

The defining characteristic of any Element is its atomic number (Z), which is the number of protons in the nucleus of each atom of that element. All atoms of a given Element have the same number of protons. For example:

• Hydrogen (H): Z = 1, one Proton
• Carbon (C): Z = 6, six protons
• Oxygen (O): Z = 8, eight protons
• Iron (Fe): Z = 26, twenty-six protons

If the number of protons in an atom changes, the atom becomes a different Element — this is the principle behind nuclear transmutation. The number of protons in an atom never changes during ordinary chemical reactions; it only changes in nuclear reactions.

Role in the Nucleus

Within the nucleus, protons contribute both mass and positive charge. The mass number (A) of an atom is the sum of protons and neutrons (A = Z + N). The positive charge of the protons in the nucleus creates the electrostatic attraction that holds the negatively charged electrons in their electron shells around the nucleus. Without this electrostatic force between nucleus and electrons, atoms could not exist as stable entities.

The protons in the nucleus also participate in the formation of ions. When an atom loses or gains electrons to form an Ion, the number of protons in the nucleus remains unchanged. For instance, a sodium atom (11 protons, 11 electrons) becomes a Na+ ion (still 11 protons, but only 10 electrons).

Proton Number and the Periodic Table

The Periodic Table arranges elements in order of increasing atomic number (proton number). This ordering reflects a fundamental principle: the properties of elements are periodic functions of their atomic numbers. The position of an Element in the Periodic Table — its period and group — is determined by the electron configuration, which in a neutral atom is dictated by the number of protons.

Sources

• Cambridge IGCSE Chemistry 0620 Syllabus (2023-2025) — Section 2.2, Cambridge Assessment International Education

Common Misconceptions