Halogens and displacement
By UK GCSE Chemistry · independent study · myAudience
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Halogens and displacement
The halogens in GCSE Group 7 have seven outer electrons and commonly form ions with charge −1. Their elemental forms are diatomic molecules. At room temperature chlorine is a pale green gas, bromine a red-brown liquid and iodine a dark solid that can form purple vapour.
Melting and boiling points generally increase down this group, but reactivity decreases. An incoming electron is attracted less effectively as the outer shell becomes farther away and more shielded. This gain-of-electron trend is the opposite of the Group 1 loss-of-electron trend.
A more reactive halogen displaces a less reactive halogen from its halide compound. Chlorine can displace bromine from bromide solution; bromine cannot displace chlorine from chloride. Higher-tier ionic equations show the reacting species without unchanged spectator ions. Halogens are hazardous laboratory substances, not home experiment materials.
A halogen reacting with a metal commonly produces an ionic metal halide, such as sodium chloride. A halogen reacting with a non-metal can produce a molecular covalent compound, such as hydrogen chloride. Hydrogen chloride gas and hydrochloric acid are distinct descriptions: the acid is the aqueous solution. Do not decide bonding merely from the presence of chlorine; the partner element and resulting structure matter.
Worked example: Predict chlorine added to sodium bromide solution.
Chlorine is above bromine, so it is more reactive.
Chlorine gains electrons while bromide loses electrons.
Remove unchanged sodium ions from the full equation.
Answer: Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂
Check your understanding: Can iodine displace bromine from bromide?
Answer: No.
Why: Iodine is below bromine and less reactive; the predicted displacement direction is reversed.