Group 1 Alkali Metals Exam Questions

• The sodium floats on the surface [1]
• It melts into a shiny ball and fizzes / moves rapidly across the surface / effervescence [1]
• The sodium gets smaller and eventually disappears [1]
• Products: sodium hydroxide and hydrogen gas [1]

Examiner note: Key observations are: floats (low density), melts (exothermic reaction), fizzes (hydrogen produced), disappears (sodium reacts completely). The solution left behind is alkaline — it turns universal indicator blue/purple.

2K(s) + 2H2O(l) → 2KOH(aq) + H2(g) [1 for correct formulae, 1 for balancing, 1 for state symbols]

Examiner note: This equation applies to all Group 1 metals — replace K with Li or Na. The general equation is: 2M + 2H2O → 2MOH + H2. All Group 1 hydroxides are soluble (they are alkalis), hence (aq).

• All Group 1 metals react by losing their one outer electron to form a 1+ ion [1]
• Going down the group, the atomic radius increases / the outer electron is further from the nucleus [1]
• There are more inner electron shells that shield the outer electron from the nuclear attraction [1]
• Therefore the outer electron is lost more easily, so the metal is more reactive [1]

Examiner note: The key factors are: increased distance from the nucleus AND increased shielding by inner electrons. Both make it easier to remove the outer electron. “The atom gets bigger” alone is not sufficient — you must link size to ease of electron loss.

Any three from:

• Low density (Li, Na, and K float on water) [1]
• Low melting point (compared to most metals) [1]
• Soft — can be cut with a knife [1]
• Shiny when freshly cut but tarnish rapidly in air [1]

Examiner note: Group 1 metals are untypical metals — they are soft, have low melting points, and low densities. They must be stored under oil to prevent reaction with air and moisture.

• Rubidium would react more vigorously than potassium — possibly with a flame/explosion [1]
• It would fizz vigorously / move very rapidly on the water / melt into a ball [1]
• Explanation: rubidium is below potassium in Group 1, so it is more reactive because its outer electron is further from the nucleus and more easily lost [1]

Examiner note: This is a prediction question — you must use the Group 1 trend (reactivity increases down the group) to predict behaviour of an element you have not observed. Examiners want to see the reasoning, not just the prediction.

• Group 1 metals are very reactive and react rapidly with oxygen in the air (tarnishing) and with water/moisture [1]
• Oil prevents contact with air and water, stopping these reactions [1]

Examiner note: The oil acts as a barrier. Lithium, sodium, and potassium would all oxidise rapidly if exposed to air, and would react dangerously with any water present.

(a) Melting point decreases down the group [1]

(b) Atomic radius increases down the group [1]

(c) All three have densities less than 1.0 g/cm3 (the density of water), so they float [1]

(d) Each element has one more electron shell than the one above, so the outermost electron is further from the nucleus [1]

Examiner note: Melting point decreases because the metallic bond weakens down the group — larger ions with the same charge (+1) are held less strongly by the sea of delocalised electrons.