Proton Number – IGCSE Chemistry Definition and Key Facts

The proton number is the most fundamental property of an atom in the Cambridge 0620 syllabus. It defines which element an atom is, determines the electron configuration, and fixes the element’s position in the periodic table.

The 0620 definition

The proton number (symbol Z), also called the atomic number, is the number of protons in the nucleus of an atom. Every atom of a given element has the same proton number. If the proton number changes, it becomes a different element.

Why the proton number defines the element

All carbon atoms have 6 protons. All nitrogen atoms have 7 protons. If an atom has 6 protons, it is carbon — no matter how many neutrons or electrons it has. This is the rule that makes the proton number the identity of an element.

Proton number	Element	Symbol
1	Hydrogen	H
6	Carbon	C
8	Oxygen	O
11	Sodium	Na
17	Chlorine	Cl
26	Iron	Fe

Where to find the proton number

On the periodic table, the proton number is the smaller whole number shown with each element. Elements are arranged in order of increasing proton number — hydrogen (1) is first, helium (2) is second, and so on.

Proton number, electrons, and ions

In a neutral atom, the number of electrons equals the proton number. When an atom forms an ion:

• Positive ions (cations): the atom loses electrons. The proton number does not change. A sodium atom (11 protons, 11 electrons) becomes Na⁺ (11 protons, 10 electrons).
• Negative ions (anions): the atom gains electrons. A chlorine atom (17 protons, 17 electrons) becomes Cl⁻ (17 protons, 18 electrons).

The proton number never changes in chemical reactions. Only nuclear reactions (not tested at IGCSE) can change the proton number.

Proton number and electron configuration

The proton number tells you how many electrons a neutral atom has, which determines the electron configuration:

Element	Proton number	Electrons	Configuration
Carbon	6	6	2, 4
Sodium	11	11	2, 8, 1
Chlorine	17	17	2, 8, 7
Calcium	20	20	2, 8, 8, 2

The electron configuration determines the element’s chemical properties and its position in the periodic table (group and period).

Proton number and isotopes

Isotopes are atoms of the same element with the same proton number but different mass numbers. They have different numbers of neutrons but the same number of protons.

Carbon-12 and carbon-14 both have proton number 6. They differ only in neutron count (6 vs 8).

Proton number vs mass number

Feature	Proton number (Z)	Mass number (A)
What it counts	Protons only	Protons + neutrons
Defines the element?	Yes	No
Same for all isotopes of an element?	Yes	No
Found on the periodic table?	Yes (smaller number)	No
Changes in ions?	No	No

Worked exam question

An atom is represented as ⁴⁰₂₀X. (a) State the proton number. [1] (b) Name the element. [1] (c) Calculate the number of neutrons. [1] (d) If this atom forms an ion X²⁺, state the number of electrons in the ion. [1]

(a) 20 [1]

(b) Calcium [1]

(c) 40 - 20 = 20 neutrons [1]

(d) 20 - 2 = 18 electrons [1]

Common exam mistakes

1. Confusing proton number with mass number. The proton number is the smaller number; the mass number is the larger.
2. Thinking the proton number changes when an ion forms. Only the electron count changes — protons are in the nucleus and do not change in chemical reactions.
3. Writing the proton number as a superscript instead of a subscript. Mass number goes on top (superscript), proton number goes on the bottom (subscript).
4. Confusing proton number with number of neutrons. Neutrons are not counted in the proton number — they contribute only to the mass number.