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# Structure of the atom

## Atoms, protons, neutrons and electrons

The atom is the most basic particle of a chemical element that exhibits the properties of the element. Atoms are made of three subatomic particles: protons, neutrons and electrons.

Protons are positively charged, neutrons are uncharged (neutral) and electrons are negatively charged.

Protons and neutrons are also called nucleons, since they make up the atomic nucleus.

A proton and a neutron each have approximately the same mass. An electron has a much smaller mass ($\displaystyle{\frac{1}{1837}}$ of a proton).

The image shows the model of a helium atom ($\ce{He}$). At the centre is the nucleus, with two neutrons and two protons (represented by green and orange circles).

There are two electrons (shown as blue circles) in the electron shell.

## Atomic nucleus

An atom is made up of a tiny central region, the nucleus, containing all of the protons and neutrons of the atom. This means that the nucleus has a net positive charge.

The nucleus is surrounded by electron shells, which are orbits where electrons are found.

Electrons are kept in the electron shells by attractive forces between the positively charged protons and the negatively charged electrons.

Most of the mass of an atom (over 99.94%) is concentrated in the nucleus. The mass of an electron is negligible because its mass is $\displaystyle{\frac{1}{1837}}$ of a proton's.

The relative charges and masses of subatomic particles are obtained by dividing their individual charges/masses by the charge/mass of a proton.

The relative mass of a neutron is 1 as the mass of a proton is approximately equal to that of a neutron.

Summary of charges and masses of subatomic particles
Subatomic particle Relative charge Relative mass
Proton 1 1
Neutron 0 1
Electron $-1$ $\displaystyle{\frac{1}{1837}}$

## Relative charges and masses of subatomic particles

The relative charges and masses of subatomic particles are obtained by dividing their individual charges/masses by the charge/mass of a proton.

The relative mass of a neutron is 1 as the mass of a proton is approximately equal to that of a neutron.

Subatomic particle Relative charge Relative mass
Proton 1 1
Neutron 0 1
Electron -1 1/1837