These questions ask you to recognise a family, choose its direction of reactivity, and connect that pattern to products or observations. Name the family first, because group 1, group 17 and group 18 do not share one universal rule.
Classify, rank, then predict
Use three passes. First classify the element or reacting pair. Next apply only that family's pattern. Finally translate the ranking into the requested consequence, such as displacement, reaction products or surface change. For a tiny example, rubidium lies below sodium in group 1, so the general pattern predicts rubidium to be more reactive under comparable conditions. For halogens the arrow reverses: a member higher in group 17 is generally more reactive than one below it.
| Family signal | Method and consequence |
|---|---|
| Group 1 alkali metal | Reactivity generally increases down the group. With water, expect hydrogen gas and a metal hydroxide; rapid reaction with air also explains tarnishing and occurrence mainly in compounds. |
| Group 17 halogen | Reactivity generally decreases down the group. The more reactive halogen can displace the less reactive one from its halide solution. |
| Halogen reacting with a metal or hydrogen | A metal gives an ionic metal halide; hydrogen gives a hydrogen halide. Familiar elemental halogens normally occur as two-atom molecules. |
| Group 18 noble gas | A complete outer shell explains low reactivity. Helium's complete first shell contains two electrons; the other familiar members have eight outer electrons. |
Words such as generally and under suitable conditions matter. Family patterns support broad comparisons, but they are not claims that every reaction has the same speed or that a weakly reactive element can never form a compound. Xenon fluorides show why the absolute claim fails.
Common wrong turns
| Trap | How to reject it |
|---|---|
| Using an oxide or carbon dioxide for a group 1 reaction with water | Water supplies the hydroxide part, while hydrogen gas is released. Keep both product types together. |
| Explaining alkali-metal behaviour as rarity or a change of state | A dull exposed surface and occurrence in minerals both follow from chemical reactivity, not from the metal becoming a gas or merely being hard to find. |
| Copying the group 1 direction onto group 17 | Reset the family before drawing the arrow: the general halogen pattern decreases down the group. |
| Ignoring the reacting partner | A halogen does not always produce the same class of compound. Ask whether its partner is a metal or hydrogen before naming the product. |
| Writing familiar halogens as single atoms or three-atom molecules | Their normal elemental formulas use pairs: F2, Cl2, Br2 and I2. |
| Turning low noble-gas reactivity into an absolute ban | Low reactivity is a comparison, not impossibility. A prepared xenon compound is chemical evidence, unlike a physical mixture or an excited atom. |
Try it
Lithium, sodium, and potassium are compared under similar conditions. Which statement follows the general group 1 reactivity pattern?
- Potassium is generally more reactive than sodium.
- Lithium is generally more reactive than potassium.
- All three have identical reactivity.
- Reactivity generally falls down group 1.
Chlorine is above bromine in group 17 and is more reactive. Which statements correctly apply the halogen-displacement rule? Select all that apply.
- Chlorine can displace bromine from bromide solution.
- Bromine can displace chlorine from chloride solution.
- The more reactive halogen performs the displacement.
- Reactivity ranking is irrelevant to displacement.
In the school shell model, helium has a complete outer shell. Enter the number of electrons in that shell.