Periodic Trends

This lesson covers the trends across periods and down groups in the periodic table — as required by the Edexcel GCSE Chemistry specification (1CH0), Topic 1: Key Concepts in Chemistry. You need to be able to describe and explain trends in reactivity, atomic radius, and character (metallic/non-metallic) in terms of electron structure.

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Trends Down a Group

When you move down a group in the periodic table, several properties change in a predictable way. These trends are caused by changes in electron structure — specifically, the number of electron shells increases.

Atomic Radius Increases Down a Group

• As you go down a group, atoms have more electron shells.
• Each additional shell is further from the nucleus, making the atom larger.
• For example, lithium (2, 1) has 2 shells, sodium (2, 8, 1) has 3 shells, and potassium (2, 8, 8, 1) has 4 shells.

Element	Group	Electron Configuration	Number of Shells	Atomic Radius (pm)
Li	1	2, 1	2	152
Na	1	2, 8, 1	3	186
K	1	2, 8, 8, 1	4	227

Reactivity Trends Down Groups

The trend in reactivity depends on whether the element is a metal or a non-metal:

Group 1 (Alkali Metals) — reactivity INCREASES down the group:

• Group 1 metals react by losing their outer electron to form a +1 ion.
• As you go down the group, the outer electron is in a shell that is further from the nucleus.
• There are more inner shells of electrons between the outer electron and the nucleus — these provide shielding (they repel the outer electron).
• The outer electron experiences less attraction from the positive nucleus, so it is easier to lose.
• Therefore, the element is more reactive.

Element	Electron Config.	Outer Electron Shell	Ease of Losing Outer Electron	Reactivity
Lithium	2, 1	2nd shell	Hardest (closest to nucleus)	Least reactive
Sodium	2, 8, 1	3rd shell	Easier	More reactive
Potassium	2, 8, 8, 1	4th shell	Easiest (furthest from nucleus)	Most reactive

Exam Tip: When explaining reactivity trends in metals, always mention THREE things: (1) the outer electron is further from the nucleus, (2) there is more shielding from inner electrons, and (3) therefore there is less attraction between the nucleus and the outer electron, making it easier to lose.

Group 7 (Halogens) — reactivity DECREASES down the group:

• Group 7 elements react by gaining one electron to form a −1 ion (to achieve a full outer shell).
• As you go down the group, the outer shell is further from the nucleus and there is more shielding.
• The incoming electron is less strongly attracted to the nucleus.
• It is therefore harder to gain an electron, so the element is less reactive.

Element	Electron Config.	Outer Shell	Ease of Gaining Electron	Reactivity
Fluorine	2, 7	2nd shell	Easiest (closest to nucleus)	Most reactive
Chlorine	2, 8, 7	3rd shell	Moderate	Moderate
Bromine	2, 8, 18, 7	4th shell	Harder	Less reactive
Iodine	2, 8, 18, 18, 7	5th shell	Hardest	Least reactive

Exam Tip: The reactivity trends for metals and non-metals go in opposite directions. Metals become MORE reactive down a group (easier to lose electrons), while non-metals become LESS reactive down a group (harder to gain electrons). This is a very common source of confusion — make sure you are clear about which direction the trend goes for each type.

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Melting and Boiling Points Down a Group

Group 1 (Alkali Metals)

Melting and boiling points decrease down Group 1:

Element	Melting Point (°C)	Boiling Point (°C)
Lithium	181	1342
Sodium	98	883
Potassium	63	759

This is because the metallic bonding becomes weaker as the atoms get larger. The delocalised electrons are further from the positive metal ions, so the electrostatic attraction is weaker.

Group 7 (Halogens)

Melting and boiling points increase down Group 7:

Element	State at Room Temp	Melting Point (°C)	Boiling Point (°C)
Fluorine	Gas	−220	−188
Chlorine	Gas	−101	−34
Bromine	Liquid	−7	59
Iodine	Solid	114	184

This is because the halogens are simple molecular substances held together by intermolecular forces (London dispersion forces). Larger molecules have stronger intermolecular forces, so more energy is needed to overcome them.

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Trends Across a Period

When you move across a period (from left to right), the properties of elements change from metallic to non-metallic.

Period 3 as an Example

Element	Na	Mg	Al	Si	P	S	Cl	Ar
Type	Metal	Metal	Metal	Metalloid	Non-metal	Non-metal	Non-metal	Noble gas
Outer electrons	1	2	3	4	5	6	7	8

Key Trends Across a Period

1. Change from metallic to non-metallic character:

• Elements on the left of a period are metals (Na, Mg, Al).
• Elements on the right are non-metals (P, S, Cl, Ar).
• Silicon sits on the boundary as a metalloid.

2. Atomic radius decreases across a period:

• As you move left to right, the number of protons increases (higher nuclear charge).
• Electrons are added to the same shell (no additional shielding).
• The increased nuclear charge pulls the electrons closer to the nucleus.
• Result: the atom gets smaller across the period.