The Atom and Subatomic Particles

Structure of an atom

A tiny central nucleus (protons + neutrons) surrounded by electrons in shells. Radius ≈ 1 × 10⁻¹⁰ m; the nucleus is ~1/10000 of that.

Particle	Relative charge	Relative mass	Location
Proton	+1	1	Nucleus
Neutron	0	1	Nucleus
Electron	−1	~1/1836 (negligible)	Shells

Numbers to know

Atomic number = number of protons   (defines the element)
Mass number   = protons + neutrons
Neutrons      = mass number − atomic number

Atoms are neutral, so protons = electrons.

Worked example

An atom of ²³Na (sodium): atomic number 11, mass number 23.

protons = 11,  electrons = 11,  neutrons = 23 − 11 = 12

Isotopes

Atoms of the same element (same protons) with different numbers of neutrons.

• Relative atomic mass (Ar) = the average mass allowing for the abundance of each isotope.

Electron shells (configuration)

Fill from the inside out: 2, 8, 8 … Sodium (11) = 2, 8, 1.

Exam tip

"Why is an atom neutral?" → the number of positive protons equals the number of negative electrons. Isotopes differ only in neutrons.