Halogens: Group 7 diatomic non-metals
Halogens are the Group 7 non-metals; each exists as diatomic molecules (, , ). At room temperature: chlorine is a pale yellow-green gas, bromine a red-brown liquid (brown vapour), iodine a dark grey/black shiny solid (purple vapour). Going down the group two trends run together: the colour gets darker, and the state changes gas → liquid → solid because melting and boiling points rise.
Reactivity: decreases down Group 7
Reactivity decreases down Group 7 (chlorine > bromine > iodine) — the opposite of Group 1. A halogen reacts by gaining one electron to complete its outer shell; further down, the atom is larger and more shielded, so the outer shell is further from the nucleus and holds the extra electron less strongly, making the atom less reactive. Use the order to place other halogens: fluorine (top) most reactive, astatine (bottom) least.
Displacement: more reactive halogen wins
A more reactive halogen displaces a less reactive halogen from a solution of its halide, e.g. , turning the solution orange as bromine forms. These reactions are the evidence for the reactivity order: chlorine displaces both bromine and iodine, and bromine displaces iodine but not the reverse, so the order must be chlorine > bromine > iodine.
Drawn from real examiner reports.
Halogen (element) ≠ halide (ion)
A halogen is a Group 7 element — a neutral diatomic molecule (). A halide is the negative ion made when a halogen gains one electron (), or a compound containing it. In displacement it is the halogen that displaces another halogen from a solution of its halide; "chloride displaces bromine" reverses element and ion and loses the mark.
Halogens vs halide ions confused throughout (Jun23 intro; Q7d)
Reactivity falls DOWN Group 7
Do not carry the Group 1 pattern into Group 7. For the halogens reactivity decreases down the group: chlorine is the most reactive of the three studied, iodine the least. Reversing the direction makes every displacement prediction wrong — e.g. wrongly claiming iodine displaces chlorine. Anchor it: fluorine (top) is the most reactive halogen, astatine (bottom) the least.
Reactivity-trend direction stated the wrong way round (Jun24 1CR Q4cii)
Bromine is a liquid, iodine a solid
Easy recall marks are dropped by a wrong colour or state. Chlorine is a pale yellow-green gas (not just "green"); bromine is a red-brown liquid (not a gas); iodine is a dark grey/black solid giving a purple vapour (not a liquid). Iodine in aqueous solution is red-brown, purple only in an organic solvent. Always state the condition at room temperature.
Iodine: grey solid, purple vapour, red-brown in solution (Jun23 Q7d)
Write halogen elements as diatomic
Halogen elements are diatomic: write , , , never the single atoms Cl, Br, I. Dropping the subscript 2 unbalances the equation and blurs the element/ion distinction. Keep it consistent: the element MOLECULE always carries the subscript 2, while the halide ION is written singly with its charge ().
The colour change is of the solution
In a displacement reaction the mark is for the colour change of the solution, not the colour of the pure element. State that the solution turns orange (bromine displaced) or brown/red-brown (iodine displaced). Describing the grey solid or red-brown liquid of the pure halogen does not answer "what would you observe" for the reaction in solution.
Do not hedge the displacement answer
State the reaction that actually happens. A hedged two-option answer — "if they react then… if they don''t then…" — scores nothing. Decide from the reactivity order whether the added halogen is more reactive than the halide present, then commit to one outcome: a colour change (displacement occurs) or no change (no reaction).
Hedged two-option displacement answers score nothing (Jun23 Q7d)
Deduce the reactivity order
Read the observation table. The added halogen is MORE reactive if the solution changes colour (a reaction happens); no change means it is less reactive than the halide already present. The halogen that displaces the others is the most reactive — rank the rest from there.
Match solution colour to halogen
Match the new solution colour to the halogen released: orange / yellow-brown means bromine formed; brown / red-brown (purple in an organic solvent) means iodine formed. Name the halogen — "it changed colour" alone loses the identification mark.
Balance: diatomic plus a factor of 2
Keep halogens as diatomic molecules and balance the metal and halide. A factor of 2 in front of the halide almost always appears: . Check atoms on both sides; for an ionic equation check the charge balances too.
Predict fluorine and astatine
Extrapolate the trends. Fluorine (above chlorine) is a pale yellow gas and the MOST reactive halogen — it displaces all the others. Astatine (below iodine) is a dark solid and the LEAST reactive — it displaces none. Give the trend reason, not just the prediction.
The halogens are the elements in Group 7 of the periodic table. They are reactive non-metals that exist as diatomic molecules (two atoms joined by a covalent bond, e.g. , , ). For 4SD0 you need chlorine, bromine and iodine, plus the ability to predict the properties of other halogens (fluorine, astatine) from the trends.
Define "halogen".
Chlorine is bubbled through a colourless solution of potassium bromide (). A displacement reaction occurs.
Complete and balance the equation for the reaction:
