Chapter Summary
- Group 1 (alkali metals) have 1 outer electron; Group 2 (alkaline earth metals) have 2 outer electrons. Both lose these electrons to form ions with noble gas configuration, acting as reducing agents.
- Going down either group: ionisation energy decreases → reactivity increases (bigger atoms, more shielding, weaker nuclear pull on the outer electron).
- Group 1 and 2 metals react with oxygen (→ oxide), chlorine (→ chloride), and water (→ hydroxide + hydrogen) — reactions get more vigorous down the group.
- Metal oxides + water → hydroxides (alkaline solutions). Metal oxides/hydroxides + acid → salt + water (neutralisation).
- Group 2 hydroxides get more soluble down the group (more alkaline solutions); Group 2 sulfates get less soluble down the group.
- Thermal stability of carbonates and nitrates increases down the group — smaller, more charge-dense cations polarise (distort) the big anion more, weakening it so it decomposes more easily at lower temperatures.
- Flame tests identify Group 1/2 metal ions by characteristic flame colours (e.g. lilac = K⁺, brick red = Ca²⁺).
- Qualitative test-tube tests exist for ammonium (NH₄⁺), carbonate/hydrogencarbonate (CO₃²⁻/HCO₃⁻), and sulfate (SO₄²⁻) ions.
1. Ionisation Energy — Groups 1 & 2
Think of an atom's outer electron like a satellite orbiting a planet. The "pull" that keeps it in orbit is the attraction between the negative electron and the positive nucleus. Ionisation energy is just a measure of how hard you'd have to yank to remove that electron completely.
Every Group 1 atom has exactly 1 electron in its outermost shell, and every Group 2 atom has exactly 2 electrons in its outermost shell. When these metals react, they don't want to gain electrons to fill the shell (that would take way too much energy) — instead, it's much easier to lose those 1 or 2 outer electrons entirely. Once they're gone, the shell underneath — which is already completely full — becomes the new outer shell. That's a stable, noble-gas-like electron arrangement, and it's the "reward" that makes losing electrons energetically worthwhile.
Key Definitions
First ionisation energy = the energy needed to remove one mole of electrons from one mole of gaseous atoms, forming 1+ ions.
Second ionisation energy = the energy needed to remove a second mole of electrons from the 1+ ions already formed, giving 2+ ions.
📉Why ionisation energy falls going down the group
Here's the part students often get backwards: nuclear charge increases going down the group (more protons = more positive charge in the nucleus), so you might expect the electron to be held on tighter. But three things overpower that increased pull:
| Factor | What happens going down the group |
|---|
| Number of shells |
| Shielding effect |
| Nuclear attraction felt by outer electron |