Group Trends in Reactivity and Chemical Behaviour
Specification Reference S3.1.4
Quick Notes
- Group 1 (alkali metals):
- Reactivity increases down the group (Li < Na < K < Rb < Cs).
- More metallic character = more vigorous reactions with water.
- Reaction with water produces a metal hydroxide and hydrogen gas.
- Group 17 (halogens):
- Reactivity decreases down the group (F2 > Cl2 > Br2 > I2).
- More reactive halogens can displace less reactive halide ions in solution.
- Displacement reactions are examples of redox reactions.
Full Notes:
Increasing Metallic Character Down Group 1
Elements become more metallic as you move down group 1.
Why?
- Atomic radius increases
- Ionization energy decreases
- Outer electron is more easily lost, meaning the metal atoms more readily form positively charged ions.
Reaction with Water (Group 1 Metals)
General reaction:
Metal (s) + H2O (l) → MetalOH (aq) + H2 (g)
Example Sodium reaction
2Na (s) + 2H2O (l) → 2NaOH (aq) + H2 (g)
| Metal | Observations |
|---|---|
| Li | Gentle fizzing, floats, forms solution slowly |
| Na | Fizzes, melts into a ball, moves on surface |
| K | Sparks, lilac flame, very vigorous reaction |
The reactions become more vigorous down the group due to easier loss of outer electron.
Decreasing Non-Metallic Character Down Group 17
Halogens become less reactive as non-metals down group 17.
Why?
- Larger atomic radius
- More shielding
- Harder to gain an extra electron
Displacement Reactions Between Halogens and Halide Ions
A more reactive halogen will displace a less reactive halide ion:
For ExampleCl2 (aq) + 2KBr (aq) → 2KCl (aq) + Br2 (aq)
(Chlorine displaces bromide)
Colour changes help identify which halogen is displaced:
| Halogen Added | Displaces | Colour in Solution |
|---|---|---|
| Cl2 | Br−, I− | Orange (Br2), brown (I2) |
| Br2 | I− | Brown (I2) |
| I2 | None | No reaction |
These are redox reactions:
- Halogen is reduced (gains electrons)
- Halide ion is oxidised (loses electrons)
Summary
- Group 1 metals show increasing reactivity with water down the group due to decreasing ionization energy
- Group 17 halogens show decreasing reactivity with halide ions down the group due to difficulty gaining electrons
- Both trends reflect underlying changes in atomic structure and electron behaviour