3.1.3

• halogens exist as diatomic molecules
• boiling point increases down the halogens as the number of electrons in each molecule increases making the London forces between the molecules stronger
• halogens are usually toxic
• the halogens change state down the group from gas to solid
• chlorine is a pale green gas
• bromine is a brown liquid
• iodine is a purple black solid and a brown red liquid
• halogens are in the p block
• the halogens outer shell consists of s2 p5 that forms 1- ions
• valence electrons = outer shell electrons
• electronegativity decreases down the halogens as electron shielding increases causing a large atomic radius meaning bonding electrons are further away from the bonding nuclei causing reactivity to decrease
• halogens are strong oxidising agents as they only need to gain 1 electron
• disproportionation: oxidation and reduction of the same element
• Cl2 + H20 -> HCL + HCLO
• Chlorine is used in water treatment
• 2NaOH (aq) + Cl2 -> NaCl + NaClO +H2O
• chlorine with cold aqueous sodium hydroxide forms bleach ( NaClO) salt and water
• any halogen can be used to purify water or make bleach
• chlorine is good to clean water as it kills bacteria
• chlorine is toxic in gas form and may form chlorinated hydrocarbons that are used as pesticides
• Halide ion test: dissolve in distilled water, add aqueous sliver nitrate, record, add aqueous ammonia slowly, record
• Cl - test: white precipitate that dissolves in a small amount of ammonia
• Br - test: cream precipitate that dissolves in concentrated ammonia
• I- test: yellow precipitate that does not dissolve in ammonia
• Ag^+ + Cl/Br/I^- -> Ag Cl/Br/I
• something is volatile if it has a low boiling point
• halogen react with cold dilute alkali solutions
• halogen disproportionation: X2 + 2NaOH -> NaXO + NaX + H2O
• halogen displacement table colours

  A) no reaction

  B) no reaction

  C) orange

  D) brown

  
• ionic displacement equation for halogens: halogen added 2 + 2 halogen being displaced- -> 2 new halogen - + displaced halogen 2
• Halogen redox reactions: reactivity decreases down the group due to more shells.
• halogens in cyclohexane

  A) yellow

  B) orange

  C) lilac