Potassium

Potassium (K) is the most reactive Group I alkali metal that students study at IGCSE. Its violent reaction with water illustrates the trend in reactivity down Group I.

Where it appears in 0620

Potassium is the “top of the group” reference point:

• Group I alkali metals: it is the most reactive of the lithium, sodium, potassium trio you must compare, and the basis for predicting rubidium and caesium.
• Reactivity series: potassium sits at the very top, above sodium and calcium.
• Flame tests: the lilac flame is one of the standard cation colours in the identifying-unknowns tests.
• Fertilisers: potassium is one of the three essential elements (N, P, K), supplied as KNO3 or KCl.

Position in the periodic table

• Symbol: K (from Latin kalium)
• Atomic number: 19
• Group 1 (I), Period 4
• Electron configuration: 2, 8, 8, 1
• Forms K+ ions by losing 1 electron

Physical properties

Property	Detail
Appearance	Silvery, tarnishes to grey almost instantly
Melting point	63 C (lower than Na or Li)
Density	0.86 g/cm3 (floats on water)
Hardness	Very soft, easily cut with a knife

Down Group I the melting points fall and the metals get softer — both trends fit potassium being softer and lower-melting than lithium or sodium.

Reaction with water

2K(s) + 2H2O(l) -> 2KOH(aq) + H2(g)

Observations:

• Potassium floats (low density)
• Melts instantly into a ball
• Whizzes rapidly across the surface
• Burns with a lilac flame (the hydrogen ignites)
• May spit or explode with a larger piece
• Vigorous fizzing/effervescence
• The solution becomes strongly alkaline (KOH forms)

This is markedly more vigorous than sodium with water, which in turn beats lithium — the pattern that reactivity increases down Group I.

Explaining the trend

1. K has 4 electron shells (2,8,8,1); Na has 3 (2,8,1); Li has 2 (2,1)
2. The outer electron in K is furthest from the nucleus
3. There is more shielding from inner shells
4. The attraction between nucleus and outer electron is weakest
5. The outer electron is lost most easily, so K reacts fastest

Flame test

Potassium gives a lilac flame. Because sodium’s yellow flame is bright and persistent, it can mask the lilac, so a blue cobalt glass is used to filter out the yellow. See flame tests.

Key compounds

Compound	Formula	Use
Potassium hydroxide	KOH	Strong alkali
Potassium chloride	KCl	Fertiliser
Potassium nitrate	KNO3	Fertiliser, oxidiser
Potassium manganate(VII)	KMnO4	Oxidising agent, titrations

Every common potassium salt is soluble in water.

Comparison of Group I metals

Property	Li	Na	K
Electron config	2,1	2,8,1	2,8,8,1
Melting point	181 C	98 C	63 C
Water reaction	Gentle fizzing	Vigorous, melts	Very vigorous, lilac flame
Flame colour	Red	Yellow	Lilac

Key facts at a glance

Fact	Detail
Symbol	K
Proton number	19
Group / Period	1 (I) / 4
Ion formed	K+
Flame colour	Lilac
Storage	Under oil (reacts with air and water)

Common exam mistakes

• Wrong flame colour. Potassium is lilac — not the red of lithium or the yellow of sodium.
• Not balancing the water equation. It is 2K + 2H2O -> 2KOH + H2, not “K + H2O -> KOH + H2”.
• Explaining the trend by nuclear charge. Nuclear charge does rise down the group, but reactivity still increases — because the outer electron is further away and more shielded, so it is lost more easily. Quoting only “greater nuclear charge” gives the wrong prediction.
• Storing potassium in water. It is stored under oil; it reacts violently with water and quickly with air.

Exam-style questions

Rubidium is directly below potassium in Group I. Predict how vigorously rubidium reacts with water compared with potassium, and explain your prediction. (3 marks)

Mark scheme

• rubidium reacts more vigorously than potassium [1]
• a rubidium atom has an extra electron shell, so its outer electron is further from the nucleus and more shielded [1]
• the outer electron is lost even more easily, so the reaction is faster/more violent [1]

Examiner note: this is a “predict and explain” question — the prediction alone earns only one mark; the shielding/distance reasoning is essential.

A student observes lithium, sodium, and potassium each reacting with water. State the trend in reactivity and explain it in terms of electronic structure. (4 marks)

Mark scheme

• reactivity increases from lithium to potassium / potassium is the most reactive [1]
• going down the group the atoms gain more electron shells [1]
• the outer electron is further from the nucleus and more shielded [1]
• so it is lost more easily and the metal reacts more vigorously [1]

Examiner note: link the electronic structure to losing the outer electron more easily — that is what “more reactive” means for a Group I metal.