Group I: The Alkali Metals

Group I is the most predictable question in the Periodic Table topic: describe what you see when sodium meets water, write the equation, then extend the trend to a metal you have never handled. The marks are lost on observations (“it explodes” scores nothing), on the equation balancing, and, on Extended papers, on the electron-loss explanation, which has a precise required wording.

What makes Group I unusual

Lithium, sodium and potassium are metals, but odd ones. They are soft enough to cut with a knife, the freshly cut surface is shiny but tarnishes within seconds, and lithium, sodium and potassium all float on water, because their densities are lower than typical metals. They are stored under oil because they react with the oxygen and water vapour in air.

Each has one outer-shell electron (Li 2,1; Na 2,8,1; K 2,8,8,1), so each forms a 1+ ion and the chemistry of the whole group is the chemistry of losing that single electron.

The trends down the group

Element	Density (g/cm³)	Melting point (°C)	Reaction with water
Lithium	0.53	181	Fizzes steadily, moves on surface
Sodium	0.97	98	Melts into a ball, fizzes rapidly, darts about
Potassium	0.86	63	Ignites with a lilac flame, very vigorous

Three trends to state, all “down the group”: density increases (potassium is the small exception to a smooth rise, but the overall trend is still increasing), melting point decreases, and reactivity increases. Given rubidium, you predict: lower melting point than potassium, denser, and a more violent reaction with water, likely explosive.

Reactions with water

All Group I metals react with cold water to form the metal hydroxide and hydrogen gas:

• 2Li + 2H2O → 2LiOH + H2
• 2Na + 2H2O → 2NaOH + H2
• 2K + 2H2O → 2KOH + H2

The observations earn separate marks from the equation. For sodium: the metal floats, melts into a silver ball, moves across the surface, fizzes (effervescence), and gets smaller until it disappears. The resulting solution turns universal indicator purple because the hydroxide is a strong alkali, which connects this group to acids, bases and salts. For potassium, add the lilac flame.

Why reactivity increases down the group (Supplement)

The Core answer stops at the trend. The Supplement explanation earns two marks and must contain both ideas:

1. Going down the group, atoms get larger, so the outer electron is further from the nucleus.
2. The outer electron is therefore held less strongly (less strongly attracted) and is lost more easily.

Easier electron loss = faster reaction. Answers that mention “shielding” are fine but not required at 0620; answers that say “potassium has more electrons so it is more reactive” score zero, because total electron count is not the mechanism. Compare this with Group VII, where the same size argument runs in reverse: halogens gain an electron, so larger atoms are less reactive.

Predicting unfamiliar alkali metals

Cambridge regularly asks about rubidium, caesium or francium. Build the answer from the trend plus a comparison: “caesium reacts explosively with cold water, more vigorously than potassium, producing caesium hydroxide and hydrogen: 2Cs + 2H2O → 2CsOH + H2.” The equation transfers directly because every member of the group forms a 1+ ion.

Worked exam question

A small piece of potassium is added to a trough of cold water containing universal indicator. (a) State three observations. [3] (b) Write a balanced symbol equation for the reaction. [2] (c) Supplement: explain why potassium is more reactive than sodium. [2]

Model answer: (a) The potassium floats / moves across the surface (1); it burns with a lilac flame / fizzes rapidly (1); the indicator turns purple/blue (1). (b) 2K + 2H2O → 2KOH + H2: correct formulae (1), correctly balanced (1). (c) The outer electron in potassium is further from the nucleus (1), so it is held less strongly and lost more easily (1).

Mark-by-mark: in (a), “it explodes” or “it reacts” are not observations and score nothing; examiners want what you would see. In (b), KOH2 or unbalanced equations are the standard errors. In (c), both the distance idea and the easier-loss idea are needed; one without the other is one mark.

The mistakes that cost marks

1. “It explodes” as an observation. Observations are things you see: floating, fizzing, a lilac flame, the metal disappearing. Reaction-violence adjectives without a visible detail score zero.
2. Unbalanced water equations. The 2-2-2-1 pattern (2Na + 2H2O → 2NaOH + H2) must be automatic. The most common slip is forgetting the 2 in front of H2O.
3. Explaining reactivity with “more electrons.” The Supplement mark is for distance from the nucleus and weaker attraction on the outer electron, not electron count, not “bigger atom” alone.
4. Melting point trend reversed. Group I melting points decrease down the group. Students mix this up with Group VII, where melting and boiling points increase down the group.

How examiners want it phrased

Student wording	Mark-scheme wording
”Potassium reacts violently"	"Potassium ignites with a lilac flame, fizzes and moves across the water surface"
"It’s more reactive because it’s bigger"	"The outer electron is further from the nucleus, so it is held less strongly and lost more easily"
"Sodium makes an alkali"	"Sodium forms sodium hydroxide solution, which turns universal indicator purple"
"They all act the same"	"All Group I elements have one outer-shell electron, so they have similar chemical properties”

Drill the equation pattern, then the two-step Supplement explanation. If your written answers describe the chemistry correctly but keep missing the mark-scheme wording, a free trial lesson will show you exactly where the gap is in one hour.

Test yourself

Observations first, equation second: write all three answers before checking.

Q1 (3 marks). A small piece of lithium is added to cold water. State two observations and write the word equation for the reaction.

Show answer

• any two of: the lithium floats; fizzes steadily (effervescence); moves on the surface; gets smaller until it disappears [2] • lithium + water → lithium hydroxide + hydrogen [1]

Q2 (2 marks). Rubidium is below potassium in Group I. Predict how the melting point and the reaction with water of rubidium compare with those of potassium.

Show answer

• rubidium has a lower melting point than potassium (melting point decreases down the group) [1] • rubidium reacts more vigorously/violently with water than potassium (reactivity increases down the group) [1]

Q3 (2 marks). Write a balanced symbol equation for the reaction of rubidium (Rb) with water.

Show answer

• 2Rb + 2H2O → 2RbOH + H2: correct formulae (RbOH, H2) [1] • correctly balanced with the 2-2-2-1 pattern [1]