Atomic Structure

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Cambridge International AS & A Level Chemistry (9701)

Atomic Structure

The big idea: An atom is a tiny, dense, positive nucleus surrounded by electrons arranged in shells, sub-shells and orbitals. How strongly the nucleus holds its outer electrons (nuclear charge, distance and shielding) explains atomic radius, electron configuration and ionisation energy.

Summary — What This Chapter Covers

Atoms are mostly empty space: a very small, dense nucleus of protons and neutrons, with electrons in shells around it.
Proton: relative charge +1, relative mass 1. Neutron: 0, 1. Electron: −1, 1/1836.
Isotopes have the same number of protons but different numbers of neutrons; same chemical properties, different mass and density.
Electrons fill orbitals in order of increasing energy: 1s 2s 2p 3s 3p 4s 3d 4p.
s sub-shell: 1 orbital (2 e⁻); p: 3 orbitals (6 e⁻); d: 5 orbitals (10 e⁻). Each orbital holds two electrons of opposite spin.
First ionisation energy is the energy needed to remove one electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ ions.
Ionisation energy depends on nuclear charge, distance from the nucleus, shielding and spin-pair repulsion.
Successive ionisation energies reveal the shell structure and the group of an element.

1. Particles in the Atom and Atomic Radius

Inside the atom

A tiny nucleus in empty space

An atom is mostly empty space. Almost all of its mass is in a very small, dense nucleus that contains protons and neutrons. The electrons are found in shells in the empty space around the nucleus. The nucleus is about 10⁻¹⁵ m across; the whole atom is about 10⁻¹⁰ m across.

Particle	Relative charge	Relative mass	Location
Proton	+1	1	Nucleus
Neutron	0	1	Nucleus
Electron	−1	1/1836 (≈ 0.0005)	Shells around the nucleus

Distribution of mass and charge: the mass is concentrated in the nucleus (protons and neutrons). The nucleus carries all the positive charge; the negative charge is spread through the much larger volume around it, carried by the electrons. In a neutral atom the number of electrons equals the number of protons.

Atomic number and mass number

Atomic (proton) number, Z: the number of protons in the nucleus of an atom.
Mass (nucleon) number, A: the total number of protons and neutrons in the nucleus.
Number of neutrons = A − Z. Number of electrons = Z − (charge on the ion).

Counting particles neutrons = A − Z electrons = Z − ionic charge

Worked Example

Find the numbers of protons, neutrons and electrons in ²⁷Al³⁺ (Z = 13) and in ³²S²⁻ (Z = 16).

Practice Question

A species has 20 protons, 20 neutrons and 18 electrons. Identify it, with its mass number and charge.

Beams in an electric field

How protons, neutrons and electrons are deflected

Beams of protons, neutrons and electrons moving at the same velocity pass between charged plates:

Protons are deflected towards the negative plate.
Electrons are deflected towards the positive plate, and much more strongly (about 1836 times less mass, same size of charge).
Neutrons are uncharged and are not deflected.

Atomic and ionic radius

Trends across a period and down a group

Atomic radius

Across a period

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Also in the full note

Same element, different neutrons
The vocabulary
Why electrons go where they go
s is a sphere, p is a dumbbell
First and successive ionisation energies
Across a period and down a group
Reading the shell structure

What's inside

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Atomic Structure