The structure of the atom

All the matter around you — the air, the water, your pen — is made up of atoms, objects so small that a million of them lined up would barely fit across the thickness of a hair.

Two distinct regions

An atom consists of two parts that differ greatly in size:

Part	Contents	Electric charge	Relative size
Nucleus	protons + neutrons	positive (protons)	tiny, contains almost all the mass
Electron cloud	electrons	negative	immense compared to the nucleus

If the nucleus were the size of a marble placed in the centre of a football stadium, the electrons would be orbiting somewhere near the stands. The atom is therefore almost entirely… empty space.

Electrical neutrality

A proton carries a positive charge, an electron carries a negative charge of equal magnitude, and a neutron has no charge. An atom, in its natural state, is electrically neutral: this means it has exactly as many protons as electrons. The positive and negative charges cancel each other out perfectly.

It is the number of protons in the nucleus, known as the atomic number, that defines which chemical element the atom is. All atoms with 8 protons are oxygen atoms; all those with 11 are sodium atoms. Change the number of protons, and you change the element — this is its true identity.

A common misconception to avoid

The nucleus (protons + neutrons) is extremely dense but contains almost all the atom’s mass; the electrons, although they occupy almost the entire volume, weigh about 2,000 times less than a proton. An atom’s volume therefore comes from its electron cloud, but its mass comes almost entirely from its nucleus.