Protons, Neutrons and Atomic Structure

Every atom has a nucleus containing protons and neutrons, surrounded by electrons. The number of protons determines which element an atom belongs to and gives it its chemical identity.

Inside the Atom

Every atom is made of three types of sub-atomic particles:

Particle	Location	Charge	Relative mass
Proton	Nucleus	Positive (+1)	1
Neutron	Nucleus	None (0)	1
Electron	Shells around nucleus	Negative (−1)	~0 (negligible)

The nucleus is incredibly small — if an atom were the size of a sports stadium, the nucleus would be the size of a marble.

Atomic Number and Mass Number

Atomic number (Z) = number of protons in the nucleus. Every element has a unique atomic number. Carbon always has 6 protons; iron always has 26.
Mass number (A) = number of protons + number of neutrons

From these two numbers you can work out the number of neutrons:

$\text{Neutrons} = \text{Mass number} - \text{Atomic number}$

Note

In a neutral atom, the number of electrons equals the number of protons. This balances the charges and gives the atom no overall charge.

Reading Periodic Table Notation

Elements are often written with their mass number above and atomic number below the symbol:

${}^{12}_{6}\text{C}$

This shows carbon with mass number 12 and atomic number 6, so it has 6 protons, 6 neutrons, and 6 electrons.

Why Protons Define the Element

All atoms of the same element have the same number of protons. Change the proton count and you get a completely different element. Chemical reactions change how electrons are shared or transferred between atoms — they do not change the number of protons.

Real world: Nuclear power and proton identity

Uranium (U) has 92 protons — that's what makes it uranium. In a nuclear power station, uranium-235 (92 protons, 143 neutrons) undergoes fission, splitting into smaller nuclei. The proton count changes in these daughter nuclei, creating entirely different elements. In everyday chemistry, proton counts never change — only electron arrangements shift.

Real world: Carbon in living things

Carbon (C, atomic number 6) is the backbone of all organic molecules. Proteins, DNA, sugars — all are carbon-based compounds. Carbon always has 6 protons, but its mass number varies with different isotopes. Carbon-12 (6 protons, 6 neutrons) is the most common form in all living matter, including you.

Tip

Where do you see this in real life? Every material around you is defined by its proton count — iron (26), aluminium (13), gold (79). The periodic table is essentially a catalogue of every known proton count from 1 (hydrogen) to 118 (oganesson).