2.1Inorganic Chemistry

Periodicity

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Complete PDF with figures and original page layoutPage 1 Oxford AQA International Your notes A Level (IAL) Chemistry Periodicity Contents Classification Physical Properties of Period 3 Elements Page 2 Classification …

Section 1 of 7Structure of the Periodic Table

Structure of the Periodic Table

Order

Order elements are arranged by increasing atomic number (proton number)

Periods

Periods rows, in which atoms have the same number of shells

Groups

Groups columns, in which atoms have similar outer electron configurations

Blocks

Blocks s, p, d and f blocks show which sub-shell holds the outer electron

Periodicity

Periodicity properties show trends that repeat in each period, called periodic trends

Chemical similarity

Chemical similarity elements in the same group show similar chemical properties

Section 2 of 7Atomic radius across Period 3

Definition
- half the distance between the nuclei of two atoms of the same element, called the metallic radius in metals and the covalent radius in non-metals

Element	Na	Mg	Al	Si	P	S	Cl
Radius / nm	0.157	0.136	0.125	0.117	0.110	0.104	0.099

Trend

Trend atomic radius decreases across Period 3

Explanation

Nuclear charge increases by one at each step.
Electrons are added to the same shell, so shielding is about the same.
The nucleus attracts the outer electrons more strongly and pulls them closer.

Misconception

Misconception more electrons do not make the atom larger across the period

Section 3 of 7First ionisation energy across Period 3

First ionisation energy = the energy to remove one mole of electrons from one mole of gaseous atoms to form one mole of 1+ ions,

such as Na(g) → Na⁺(g) + e^-, +496 kJ mol^-1

2Trend

Trend it generally increases across the period

Explanation

Nuclear charge increases and the atomic radius decreases.
The outer electrons are nearer the nucleus, while shielding stays roughly constant.
More energy is needed to remove an electron.

Exceptions

Aluminium is lower than magnesium.
Sulfur is lower than phosphorus.
Both can be explained from the electronic configurations (a 3p electron in Al, and paired 3p electrons in S).

Section 4 of 7Melting point across Period 3

Trend
- melting point generally rises to silicon, which is highest, and then falls sharply

Element	Na	Mg	Al	Si	P	S	Cl	Ar
Melting point / K	371	923	932	1683	317	392	172	84

1Sodium, magnesium and aluminium

Sodium, magnesium and aluminium metallic bonding
Electrostatic attraction exists between positive ions and the sea of delocalised electrons.
Na donates one electron, Mg two and Al three.
The attraction between a 3+ ion and a larger electron sea is much stronger than for Na⁺, so melting point increases.

2Silicon

Silicon giant covalent structure
Each atom is held to four neighbours by strong covalent bonds.
Much energy is needed to break these bonds, so the melting point is highest.

Section 5 of 7Phosphorus to argon: molecular elements

Phosphorus to argon: molecular elements

Structure

Structure simple molecules P₄, S₈, Cl₂ and single Ar atoms

Forces

Forces covalent bonds within molecules are strong, but only weak van der Waals forces act between molecules

Melting

Melting little energy is needed to overcome van der Waals forces, so melting points are low

Order

Order the forces depend on electron number and size, so melting points go S₈ > P₄ > Cl₂ > Ar

Exam point

Exam point on melting, van der Waals forces break, not the covalent bonds

Section 6 of 7Key terms

Term	Meaning
Periodicity	A repeating pattern in properties across each period of the Periodic Table.
Atomic radius	Half the distance between the nuclei of two bonded atoms of the same element.
Nuclear charge	The number of protons in the nucleus.
Giant covalent structure	A huge network of atoms joined by covalent bonds.
Van der Waals forces	Weak forces between molecules.

Section 7 of 7Exam technique

Saying atomic radius increases across Period 3 because of extra electrons — the increased nuclear charge pulls the shell in.

Saying covalent bonds break when phosphorus melts — only van der Waals forces break.

Saying silicon has metallic bonding — it is giant covalent.

Writing the 1+ equation without state symbols — include (g).