2.2Atoms, Elements and Compounds

Atomic structure and the Periodic Table — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

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📚 Cambridge IGCSE Chemistry 0620 Hub / Topical Revision

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2.2 Atomic structure and the Periodic Table — Notes — IGCSE Chemistry notes: Describe the structure of the atom, give the relative charges and masses of its particles, and write electronic configurations that link to the Periodic Table.Atom: the smallest particle of an element, n…

Proton number
Mass number
The electrons move very fast around the nucleus
Shells are energy levels, and shells further from the nucleus have more energy
The mass of the electron is negligible, so almost all of the atom's mass is in the nucleus
Rutherford's alpha scattering gave evidence for the nucleus.Most particles passed straight through the foil

Part 1 of 6Structure of the atom

Atom: the smallest particle of an element, neutral overall, with a central nucleus containing protons and neutrons surrounded by electrons in shells.

The electrons move very fast around the nucleus.

Shells are energy levels, and shells further from the nucleus have more energy.

The mass of the electron is negligible, so almost all of the atom's mass is in the nucleus.

Rutherford's alpha scattering gave evidence for the nucleus.

Most particles passed straight through the foil.
A few were deflected sharply and very few bounced back.
So atoms are mostly empty space and the nucleus is small, dense and positive.

Part 2 of 6Subatomic particles

Particle	Relative mass	Relative charge	Location
Proton	1	+1	Nucleus
Neutron	1	0	Nucleus
Electron	1/1840 (negligible)	−1	Shells around the nucleus

Subatomic particles

Saying the electron's mass is negligible is enough.

Relative masses are compared with 1/12 of the mass of a carbon-12 atom and have no units.

Part 3 of 6Proton number and mass number

Proton (atomic) number Z: the number of protons in the nucleus.

It equals the number of electrons in a neutral atom.
It identifies the element and fixes its position in the Periodic Table.

Mass (nucleon) number A: the total number of protons and neutrons, which are collectively nucleons.

Neutrons = A − Z.

Notation: the mass number is written at the top left and the proton number at the bottom left of the symbol,

so sodium-23 has 11 protons, 11 electrons and 12 neutrons.

On the Periodic Table the mass number is the bigger of the two numbers (MASS = MASSIVE), though some tables put them the other way round.

The relative atomic mass can be used as the mass number, except for chlorine.

Part 4 of 6Particles in atoms and ions

Protons = Z, and in a neutral atom electrons = protons.

For an ion the protons and neutrons do not change.

Electrons = Z − charge for a positive ion.
Electrons = Z + charge for a negative ion.

Al³⁺ (mass number 27): 13 protons, 14 neutrons, 10 electrons, configuration 2,8.

S²⁻ (mass number 32): 16 protons, 16 neutrons, 18 electrons, configuration 2,8,8.

Part 5 of 6Electronic configuration

Electrons fill the shell nearest the nucleus first, and a full shell sends extra electrons to the next.

The first shell holds 2, the second 8 and the third 8 (it can hold 18, but the simplified model of 8 is used), then the fourth shell fills.

For potassium and calcium the extra electrons go into the fourth shell first.

Configuration: the number of electrons in each shell written with commas (full stops are accepted), such as carbon 2,4 and sodium 2,8,1.

Ions have configurations too, such as Na⁺ 2,8, and they can be written for atoms and ions up to proton number 20.

Valence (outer) shell: the outermost shell, which makes the atom stable when full.

Method

Find the proton number Z, which equals the electrons in a neutral atom.
Place 2 electrons in shell 1.
Place up to 8 in shell 2, then up to 8 in shell 3.
Put any remaining electrons in shell 4.

Element	Z	Configuration	Group	Period
Hydrogen	1	1	I (special)	1
Helium	2	2	VIII	1
Carbon	6	2,4	IV	2
Oxygen	8	2,6	VI	2
Neon	10	2,8	VIII	2
Sodium	11	2,8,1	I	3
Magnesium	12	2,8,2	II	3
Chlorine	17	2,8,7	VII	3
Argon	18	2,8,8	VIII	3
Calcium	20	2,8,8,2	II	4

Part 6 of 6Link to the Periodic Table

Order: elements are arranged in order of increasing proton number.

Period: the number of occupied shells, which is the number of numbers in the configuration.

Group: for Groups I to VII, the number of outer electrons, which is the last number in the configuration.

Elements in a group have the same number of outer electrons, so they react in similar ways.

Chlorine 2,8,7 has 3 shells and 7 outer electrons, so it is in Period 3, Group VII.

Metals are on the left and centre of the table and non-metals on the right.

Noble gases (Group VIII or 0): full outer shells (2 for helium, 8 for the rest), so they are stable and unreactive.

Most atoms have outer shells that are not full,

so they react to gain a full shell, sometimes by emptying the outer shell so the one below becomes full.

Mendeleev ordered elements by their properties and left gaps for undiscovered elements, correctly predicting their properties.

Key terms

Term	Meaning
Proton number	the number of protons in the nucleus of an atom.
Mass number	the total number of protons and neutrons in the nucleus.
Electronic configuration	the number of electrons in each shell, written with commas.
Shell	an energy level around the nucleus that holds a limited number of electrons.
Period	a row of the Periodic Table, equal to the number of occupied shells.
Group	a column of the Periodic Table, equal to the number of outer electrons for Groups I to VII.

Avoid these mistakes

Group number equals the number of shells: group is the outer electrons and period is the number of shells.

The first shell holds eight electronsit holds two.

Mass number equals the number of neutrons: it is protons plus neutrons.

Atomic number and mass number are the same: atomic number is protons and mass number is protons plus neutrons.

Electrons have no massthey have a relative mass of 1/1840, which is negligible.

Last updated: 13 October 2026