Rule
Reactivity of alkali metals increases going down the group — the single outer electron is progressively farther from the nucleus and easier to lose, so the metal reacts more readily.
Remember
Alkali metals get softer going down the group because the metallic bond (holding the atoms together) weakens as atomic size increases and the outer electron is held less tightly — this is why it becomes easier to cut them lower in the group.
Lithium behaves somewhat differently from the rest of the alkali metals because its very small size gives it unusually strong bonding for its group, making some of its reactions and compounds resemble Group II (magnesium) more closely.
Rule
Reactivity of halogens decreases going down the group — the single electron needed to complete the outer shell is progressively farther from the nucleus, so it's harder to attract.
Remember
This opposite trend (alkali metals more reactive going down, halogens less reactive going down) makes sense together: alkali metals react by losing an electron (easier when it's far from the nucleus), halogens react by gaining one (easier when the nucleus's pull is still strong).
Example 1 — comparing halogen reactivity
Compare the reactivity of chlorine and bromine as oxidising agents.
- Chlorine is above bromine in the group, so its outer shell is closer to the nucleus.
- A closer, stronger nuclear pull makes chlorine gain electrons (oxidise other substances) more readily.
Answer: Chlorine is the stronger oxidising agent
Rule
Metals: generally tough, strong, malleable/ductile, good conductors — because of the metallic bond's sea of free electrons and layered ion structure.
Non-metals: generally brittle (if solid) or exist as gases/liquids, poor conductors — their covalent bonding doesn't give the same free-electron mobility.
Remember
Transition elements differ from main-group elements because their outer electron configuration can vary — they commonly form multiple different ions/oxidation states, and their compounds are often coloured, unlike most main-group element compounds.
Quick Recap
- ●Alkali metals: reactivity increases down the group (electron easier to lose); get softer going down
- ●Halogens: reactivity decreases down the group (harder to gain an electron farther from the nucleus)
- ●Metals: tough, conduct well, malleable (free-electron sea). Non-metals: brittle/gas, poor conductors
- ●Transition elements: variable oxidation states, often coloured compounds
