The halogens Later

Group 7 The Halogens

Physical Properties Simple Molecular Structures Diatomic molecules Strong covalent bonds Weak intermolecular forces Low melting and boiling points Poor conductors of heat and electricity Melting point and boiling point increase as you go down the group.

Chemical Properties of the Halogens All have 7 electrons in their outer shell. They all want to gain one more electron for a full outer shell. There are two ways they can do this: Ionic bonding – Accept one electron from a metal atom to become a halide ion with a -1 charge. Covalent bonding – Form one covalent bond with another non-metal.

Examples + - - Na Cl F 2+ Ca - F Ionic Bonding 2Na + Cl2 2NaCl Ca + F2 CaF2 + xo - xo - Na Cl xx xx F xx xx xx 2+ xx Ca xo - F xx xx xx

Examples b) Covalent Bonding HCl Cl2

Reactivity decreases as you go down group 7 Reaction Conditions F2(g) + H2(g) → 2HF(g) Explosive even at -200oC in the dark. Cl2(g) + H2(g) → 2HCl(g) Explosive in sunlight. Slow in the dark. Br2(g) + H2(g) → 2HBr(g) 300oC + Pt catalyst I2(g) + H2(g) → 2HI(g) Very slow at 300oC with Pt catalyst

Group 7 – forming negative ions REACTIVITY INCREASES F Reactivity decreases as you go down the group. In larger atoms the outermost electrons are further from the nucleus. This makes it harder to gain an electron as it will be less strongly attracted to the nucleus Cl

Halogen displacement reactions Mr Sodium Miss Iodide + Miss Bromine Mr Sodium Miss Bromide Miss Iodine Sodium Iodide + Bromine Sodium Bromide + Iodine 2NaI(aq) + Br2(aq) 2NaBr(aq) + I2(aq) Iodide oxidised Bromine reduced

Halogen displacement reactions Miss Potassium Mr Bromide + Mr Chlorine Miss Potassium Mr Chloride Mr Bromine Potassium Bromide + Chlorine Potassium Chloride + Bromine 2KBr(aq) + Cl2(aq) 2KCl(aq) + Br2(aq) Bromide oxidised Chlorine reduced

What if we add fluorine to aqueous potassium chloride? Fluorine is more reactive than chlorine so would displace chloride from it’s salt. However, fluorine is so reactive it would instantly react with the water before it got to the chloride ions!

Cl F