Group 1: The Alkali Metals
Introduction to Group 1
The Group 1 elements are known as the alkali metals. They are: lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs) and francium (Fr).
They are called alkali metals because they react with water to form alkaline solutions (metal hydroxides with pH above 7).
All Group 1 elements have one electron in their outer shell, which explains their similar chemical properties.
Physical Properties
Group 1 metals are unusual compared to most metals:
- Low melting and boiling points (compared to other metals) — these decrease going down the group
- Low density — lithium, sodium and potassium are less dense than water (they float)
- Soft — they can be cut with a knife, revealing a shiny surface that quickly tarnishes in air
- Shiny when freshly cut but tarnish rapidly as they react with oxygen and moisture
| Element | Melting point (°C) | Density (g/cm³) |
|---|---|---|
| Lithium | 181 | 0.53 |
| Sodium | 98 | 0.97 |
| Potassium | 63 | 0.86 |
They are stored under oil to prevent them reacting with oxygen and water in the air.
Reactions with Water
All Group 1 metals react vigorously with water to produce a metal hydroxide and hydrogen gas.
General equation:
metal + water → metal hydroxide + hydrogen
Specific examples:
2Li(s) + 2H₂O(l) → 2LiOH(aq) + H₂(g)
2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)
2K(s) + 2H₂O(l) → 2KOH(aq) + H₂(g)
Observations Going Down the Group
| Element | Observation with water |
|---|---|
| Lithium | Fizzes gently, moves on surface, dissolves slowly |
| Sodium | Fizzes vigorously, melts into a ball, moves rapidly on surface |
| Potassium | Fizzes violently, lilac flame, may ignite hydrogen |
| Rubidium | Explodes on contact |
The reactivity increases going down the group. The hydroxide solutions formed are all alkaline (pH 13-14) and turn universal indicator purple.
Reactions with Oxygen
Group 1 metals tarnish in air and burn with characteristic flame colours:
- Lithium — crimson red flame
- Sodium — yellow/orange flame
- Potassium — lilac flame
4Li(s) + O₂(g) → 2Li₂O(s)
Reactions with Chlorine
Group 1 metals react with chlorine gas to form white metal chlorides:
2Na(s) + Cl₂(g) → 2NaCl(s)
2K(s) + Cl₂(g) → 2KCl(s)
Why Reactivity Increases Down the Group
As you go down Group 1:
1. The atoms get larger — more electron shells
2. The outer electron is further from the nucleus
3. There is more shielding from inner electron shells
4. The outer electron is less strongly attracted to the nucleus
5. Therefore, the outer electron is lost more easily
6. So the element is more reactive
This is the key explanation examiners look for. Potassium is more reactive than sodium because potassium has four electron shells (2.8.8.1) compared to sodium's three (2.8.1), so the outer electron is further from the nucleus, experiences more shielding, and is lost more easily.
Compounds of Group 1
Group 1 metals always form 1+ ions (e.g. Na⁺, K⁺) because they lose their single outer electron. Their compounds are:
- White or colourless solids
- Soluble in water
- Form ionic compounds with non-metals
Exam Tips
- Always explain reactivity trends in terms of atomic radius, shielding and attraction to the nucleus
- Remember that Group 1 metals form ionic compounds with a 1+ charge
- The reaction with water always produces a hydroxide and hydrogen — learn the general word and symbol equations
- Safety: these metals are stored under oil and handled with forceps, never touched