Periodic Table Trends

The periodic table is organised by electron structure, and every property trend follows from how electrons are arranged. For 0620, you need to explain trends down Groups 1 and 7, across periods, and for noble gases. Every trend has a cause rooted in atomic structure.

Structure of the periodic table

• Groups (vertical columns): Elements in the same group have the same number of outer-shell electrons and similar chemical properties
• Periods (horizontal rows): Elements in the same period have the same number of electron shells. Properties change progressively across a period

The periodic table is arranged in order of atomic number (number of protons).

Trends across Period 3 (Na to Ar)

Electron configuration

Element	Na	Mg	Al	Si	P	S	Cl	Ar
Configuration	2,8,1	2,8,2	2,8,3	2,8,4	2,8,5	2,8,6	2,8,7	2,8,8
Outer electrons	1	2	3	4	5	6	7	8

Metallic to non-metallic character

Left side (Na, Mg, Al): metals — conduct electricity, form positive ions, basic oxides Right side (Si): metalloid / giant covalent Far right (P, S, Cl, Ar): non-metals — form negative ions (or share electrons), acidic oxides

Oxide character across a period

Element	Na2O	MgO	Al2O3	SiO2	P4O10	SO3	Cl2O7
Type	Basic	Basic	Amphoteric	Acidic	Acidic	Acidic	Acidic

Metal oxides are basic. Non-metal oxides are acidic. Aluminium oxide is amphoteric (reacts with both acids and bases). This is a Supplement point covered in oxides.

Trends down Group 1 (alkali metals)

Group 1 elements: Li, Na, K (and Rb, Cs, Fr beyond IGCSE scope)

All have 1 outer-shell electron. All react by losing this electron to form a 1+ ion.

Physical property trends

Property	Down the group
Melting point	Decreases
Density	Generally increases
Hardness	Decreases (softer)

Chemical property trends

Trend	Down the group	Reason
Reactivity	Increases	Outer electron is further from the nucleus and more shielded by inner shells, so it is lost more easily
Vigour of water reaction	Increases	Li: fizzes. Na: fizzes vigorously, melts into ball. K: fizzes violently, lilac flame

Reactions with water

2M + 2H2O -> 2MOH + H2 (where M = Li, Na, or K)

• Lithium: floats, fizzes steadily, dissolves gradually
• Sodium: floats, fizzes vigorously, melts into a ball, moves on surface
• Potassium: floats, fizzes violently, catches fire with a lilac flame, may explode

All produce an alkaline solution (metal hydroxide) and hydrogen gas. See Group 1 alkali metals.

Why reactivity increases down Group 1

As you go down the group:

1. Atomic radius increases (more electron shells)
2. The outer electron is further from the nucleus
3. Shielding by inner electron shells increases
4. The attraction between the nucleus and the outer electron weakens
5. The outer electron is lost more easily
6. The element is more reactive

Trends down Group 7 (halogens)

Group 7 elements: F, Cl, Br, I (and At beyond IGCSE scope)

All have 7 outer-shell electrons. All react by gaining 1 electron to form a 1- ion.

Physical property trends

Halogen	State at r.t.p.	Colour	Melting point
Fluorine (F2)	Gas	Pale yellow	Very low
Chlorine (Cl2)	Gas	Yellow-green	Low
Bromine (Br2)	Liquid	Red-brown	Higher
Iodine (I2)	Solid	Dark grey/purple vapour	Highest

Boiling/melting points increase down the group because intermolecular forces increase with molecular size.

Chemical property trends

Trend	Down the group	Reason
Reactivity	Decreases	Outer shell is further from nucleus and more shielded; incoming electron is attracted less strongly
Displacement ability	Decreases	More reactive halogens displace less reactive ones

Why reactivity decreases down Group 7

As you go down the group:

1. Atomic radius increases
2. The outer shell is further from the nucleus
3. Shielding by inner electron shells increases
4. The nucleus attracts an incoming electron less strongly
5. It is harder to gain an electron
6. The element is less reactive

This is the opposite trend to Group 1 because the mechanism is different: Group 1 metals lose electrons (easier further down), while Group 7 non-metals gain electrons (harder further down).

Halogen displacement reactions

A more reactive halogen can displace a less reactive one from its salt solution:

Cl2 + 2KBr -> 2KCl + Br2 (chlorine displaces bromine) Cl2 + 2KI -> 2KCl + I2 (chlorine displaces iodine) Br2 + 2KI -> 2KBr + I2 (bromine displaces iodine)

But: Br2 + 2KCl -> no reaction (bromine cannot displace chlorine)

See Group 7 halogens.

Group 0 (noble gases)

Noble gases (He, Ne, Ar, Kr, Xe) have full outer electron shells and are therefore very unreactive (inert). They exist as monatomic gases.

Uses depend on their inertness and specific properties:

• Helium: balloons, airships (low density, non-flammable)
• Neon: advertising signs (glows red-orange in discharge tubes)
• Argon: welding (prevents oxidation), light bulbs (inert atmosphere)

Boiling points increase down the group due to increasing intermolecular forces with larger electron clouds.

Transition elements

Positioned between Groups 2 and 3. Key differences from Group 1/2 metals:

• Form ions with variable charges (Fe2+/Fe3+, Cu+/Cu2+)
• Form coloured compounds
• Act as catalysts (e.g. iron in Haber process, MnO2 for decomposing H2O2)
• Higher melting points and densities than Group 1 metals
• Less reactive than Group 1 metals

See transition elements.

Common exam mistakes

1. “Reactivity increases down Group 7” — it decreases. Group 7 gains electrons; this is harder further down.
2. Confusing the reason for Group 1 and Group 7 trends: Group 1 loses electrons more easily (further from nucleus). Group 7 gains electrons less easily (incoming electron attracted less).
3. “Noble gases have 8 outer electrons” — helium has 2 (full first shell). Neon and argon have 8.
4. Forgetting shielding: The increased distance from the nucleus is not the only factor. Inner electron shells shield the outer electron from the nuclear charge.

Worked exam questions

Explain why potassium is more reactive than sodium. Both are in Group 1. [3 marks]

• Potassium has more electron shells / a larger atomic radius than sodium [1]
• The outer electron is further from the nucleus and more shielded by inner electrons [1]
• So the outer electron is lost more easily / less energy needed to remove it [1]

Chlorine displaces bromine from potassium bromide solution but bromine cannot displace chlorine from potassium chloride solution. Explain why. [2 marks]

• Chlorine is more reactive than bromine [1]
• A more reactive halogen can displace a less reactive halogen from its salt, but not vice versa [1]