The story of the atom. Mass and charge.

2. Inside the atom

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Structure of atom - interactive graphic

Picture 2.1 A model of the atom with electrons on the edge and a central nucleus made of protons and neutrons.

	Mass and charge

		Atoms have a tiny central nucleus with electrons whizzing around outside it. The electrons are fundamental particles. However, the nucleus is made from collections of two smaller particles: protons and neutrons.

	What are protons and neutrons?

		The protons and neutrons lie in the tiny nucleus. They have very similar masses, but the proton has a positive charge whereas the neutron is not charged (hence the name neutron – neutral). The neutron helps keep the nucleus together. The electron has a negative charge and very little mass (1800 times less than protons and neutrons).

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	relative mass	relative charge
proton	1	+1
neutron	1	0
electron	¹/₁₈₀₀	-1

Table 1. Relative masses and charges of the particles in an atom.

	The nucleus

		Most of the mass of the atom is in the nucleus. However, despite carrying most of the atom’s mass, the diameter of the nucleus is 100,000 times smaller than the diameter of the atom. This means the nucleus is incredibly dense (as if the whole Earth were squashed into a football stadium).