Group 1 Alkali Metals
Overview
The Group 1 metals (lithium, sodium, potassium, rubidium, caesium, francium) are known as the alkali metals. They form alkaline solutions when they react with water.
All alkali metals share similar characteristic properties because they each have one electron in their outer shell.
Physical properties of alkali metals:
- Soft metals that can be cut with a knife
- Relatively low density (Li, Na and K are less dense than water)
- Relatively low melting points for metals
- Shiny when freshly cut, but quickly tarnish in air
Reactions with Water
The reaction of Group 1 metals with water provides evidence for classifying them as a family.
General equation: 2M(s) + 2H2O(l) → 2MOH(aq) + H2(g)
where M = Li, Na, K, Rb or Cs.
The hydroxide formed is an alkaline solution (pH > 7).
| Metal | Equation | Observations |
|---|---|---|
| Lithium | 2Li(s) + 2H2O(l) → 2LiOH(aq) + H2(g) | Fizzes steadily; moves on surface; relatively slow |
| Sodium | 2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g) | More vigorous fizzing; melts into a ball; moves rapidly; dissolves quickly |
| Potassium | 2K(s) + 2H2O(l) → 2KOH(aq) + H2(g) | Very vigorous; burns with a lilac flame; moves very rapidly; dissolves very quickly |
The reactions become more vigorous going down the group, providing evidence for an increasing trend in reactivity.
Reactions with Oxygen (in Air)
All alkali metals react with oxygen in the air to form metal oxides. This causes the freshly cut surface to tarnish rapidly.
| Metal | Equation |
|---|---|
| Lithium | 4Li(s) + O2(g) → 2Li2O(s) |
| Sodium | 4Na(s) + O2(g) → 2Na2O(s) |
| Potassium | 4K(s) + O2(g) → 2K2O(s) |
Metals tarnish more rapidly going down the group.
Trend in Reactivity
Reactivity increases going down Group 1. Lithium is the least reactive; francium would be the most reactive.
Evidence for this trend:
- More vigorous reaction with water (more bubbles, more violent)
- Potassium burns with a lilac flame; lithium does not
- Metals lower in the group tarnish more quickly in air
Predicting Properties of Other Alkali Metals
Using the trends, we can predict that rubidium and caesium would:
- React even more vigorously with water than potassium (possibly explosively)
- Tarnish almost instantly in air
- Be softer and have lower melting points than potassium
Higher Tier
Explaining the trend in reactivity using electronic configurations:
When Group 1 metals react, their atoms lose the single outer electron to form 1+ ions.
Going down Group 1, the number of electron shells increases. This means:
- The outer electron is further from the nucleus
- There is more electron shielding (inner shells of electrons reduce the attraction between the nucleus and the outer electron)
- The force of attraction between the nucleus and the outer electron is weaker
- Less energy is required to remove the outer electron
Therefore, the outer electron is lost more easily as you go down the group, so reactivity increases.
Exam Questions
Q: Describe what you would see when potassium is added to water.
Potassium moves very rapidly on the surface of the water, fizzing vigorously. It burns with a lilac flame. The potassium dissolves very quickly. The solution formed turns universal indicator blue/purple, showing it is alkaline.
Q: Write a balanced equation for the reaction of sodium with water, including state symbols.
2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)
Q: Explain why the reactivity of Group 1 metals increases going down the group.
Going down the group, the outer electron is further from the nucleus and there is greater electron shielding from inner shells. The attraction between the nucleus and the outer electron decreases, so it is easier to lose the outer electron, making the metal more reactive.
Q: Predict two properties of rubidium (below potassium in Group 1).
Rubidium would be more reactive than potassium, reacting more vigorously (possibly explosively) with water. It would have a lower melting point than potassium and be a softer metal.
Exam Tip
The Group 1 metals all react with water to produce hydrogen gas and a metal hydroxide. The solution formed is always alkaline. The reactivity increases down the group, so you should describe increasingly vigorous reactions as you go from Li to K.