SPM Form 4 Chemistry Notes — Chapter 4『4.4』Group 17 Elements

4.4 Group 17 Elements

◆ non-metals and also known as halogens.

◆ In nature, all halogens exist as diatomic molecules. They are written as F₂，Cl₂，Br₂，I₂，At₂ .

◆ The word "Halogens" means "salt formers" because they can form salt easily with metals.

● Most of the halogens exist in the nature as halide salts.

● Halide is the name given to the ion of the halogens, Table below shows the corresponding halides of the halogens.

Halogens	Halide
Fluorine，F2	Fluoride，F-
Chlorine，Cl2	Chloride，Cl-
Bromine，Br2	Bromide，Br-
Iodine，I2	Iodide，I-

Physical Properties

➷ All group 17 elements are non-metals. Therefore they are heat and electricity insulator.

❶ Size of Atom and Density

✖ The atomic size of group 17 elements increases down the group.

✖ This is due to the increase of number of electron shell down the group.

✖ The low density of group 17 elements is also increases down the group.

✖ This is because the rate of increment of the atomic mass is higher than the rate of increment of the volume.

✖ When solid of group 17 elements (Iodine and Astatine) they are brittle and crumbly.

❷ Melting and Boiling Points

❦ Very low melting and boiling points.

❦ The melting and boiling points increase steadily down the group.

❦ The physical state at room temperature also change from gas to liquid and then to solid.

❦ This is because the intermolecular attractive force(van der Waals force) increase with increasing size of the atom or molecule.

❦ More heat is needed to overcome the stronger forces.

❸ Colour and Smell

☛ Physical state and colour changes of halogens.

Name	Physical state at room conditions	Colour
Fluorine，F2	Gas	Pale yellow
Chlorine，Cl2	Gas	Yellowish-green
Bromine，Br2	Liquid	Dark red(Brown liquid or vapour)
Iodine，I2	Solid	Black(Brown liquid or purple vapour)

✎ Have pungent smell and are poisonous.

✎ All halogens are coloured non-metallic elements.

✎ The colour of the halogen gets darker down the group.

✲✲ 温馨小提示：

提到关于颜色，我常常遇到一些读者把颜色给混乱了，所以我就在这里解释一下，免得大家再次犯错了哦！^.^

首先呢！你要记得的是Group 17 elements很可爱，它们到底在什么时候才会呈现颜色呢？那又在什么时候是透明的？在这里，我会一一解释的，放心！

① Group 17 elements 是Halogens的时候才会有颜色！！

▶ Halogens 和 Halide是不同的东西，Halogens是molecules，而Halide则是Ions，所以清楚了吗？

打个比方：Chlorine，Cl₂是molecule，而Chloride，Cl^-就是Ion. Molecules 是share electron的，而Ions则是donate or gains electrons！Donate electron的atom就会变成positive ions（好人~XD），然而gains electron的就是negative ions了！

② Halide 是无色的！！！

▶ 对，你没看错，Halide 是无色的，无色的，无色的！【因为很重要所以重复三次】我这么一说，那你是否有一个疑问了？ 为什么有些Halide有颜色？就如 Iron(II) chloride 是绿色，Iron(III) chloride是褐色的？

▶ 其实你是被halide骗了，首先，Halide绝对是无色的，那为什么iron chloride有颜色呢？那是因为iron本身是transition element，所以你所看到的颜色并不是Halide所呈现的，而是transition elements.

▶ 总结来说，如果group 17 element和无色的metal结合，就是无色的；如果它们和transition elements结合就会有颜色，但是所呈现的颜色是transition elements的颜色，不是Halide本身，因为Halide是无色的，我都重复了那么多次都该记得了啦吼！（我真是啰嗦）

Examples：

 Molecules：F2，Cl2，Br2，I2

 React with metals(except transition metals)

White solid，Colourless soloution

 React with transition metals

Coloured solid，Coloured soloution（Depends on colour of transition metals）

Chemical Properties

☢ Group 17 elements are very reactive non-metals.

☢ The atoms all have 7 valence electrons, makes them have similar chemical properties.

☢ During chemical reaction, the atom gain one electron to form an negatively charged ion to achieve the stable electron arrangement of noble gas.

Explaining the Reactivity Trend/Electronegativity of the Group 17 Elements

✍ The reactivity/electronegativity of group 17 elements decreases when down the group.

✍ This is because force of attraction between the nucleus and the valence electrons become weaker when the atomic size increases.『妈妈顾不到离家最远的孩子~XD』

✍ The ability to attract electron to fill the outermost shell and form negative ion decreases.

Reaction of group 17 elements：

⑴ React with water

▷ Chlorine + Water → Hydrochloric acid + Hypochlorus(I) acid

    Cl₂ + H₂O → HCl + HOCl

✲ Hypochlorus acid is a strong bleaching agents. It decolourises the colour of litmus paper.

▷ Bromine + Water → Hydrobromic acid + Hypobromous(I) acid

    Br₂ + H₂O → HBr + HOBr

✲ Hypobromous(I) acid is a weak bleaching agent. Blue litmus paper turns red and then bleached slowly.

▷  Iodine + Water → Hydroiodic acid + Hypoiodous(I) acid

     I₂ + H₂O → HI + HOI

✲  Hypoiodous(I) acid is a weak bleaching agent. Blue litmus paper turns red but it is not bleached.

⑵ React with Iron

ஐ Reaction with iron, Fe to produce brown iron(III) halide.

▷ Chlorine + Iron → Iron(III) chloride

    3Cl₂ + 2Fe → 2FeCl₃

▷ Bromine + Iron → Iron(III) bromide

    3Br₂ + 2Fe → 2FeBr₃

▷ Iodine + Iron → Iron(III) iodide

    3I₂ + 2Fe → 2FeI₃

ஐ Halogen gas is poisonous. Excess halogen gas is absorbed by the soda lime(sodium hydroxide,NaOH) so that it does not escape to surrounding.

⑶ React with Sodium Hydroxide, NaOH solution

☬ Chlorine + Sodium hydroxide → Sodium chloride + Sodium chlorate(I) + Water

    Cl₂ + 2NaOH → NaCl + NaOCl + H₂O

☬ Bromine + Sodium hydroxide → Sodium bromide + Sodium bromate(I) + Water

    Br₂ + 2NaOH → NaBr + NaOBr + H₂O

Safety Precaution

☀ Fluorine, chlorine and bromine gases are poisonous『有毒』.

☀ Do not inhale halogen gas or vapour.

☀ Do not touch halogens with bare hands(wear gloves).

☀ Wear safety goggles『佩戴安全护目镜』when handling halogens.

☀ Perform the experiments in a fume chamber.

☀ The experiments involve fluorine are not done in school. This is because fluorine is so reactive that it will react with most of the substances it comes into contact with.

Uses of Halogens

Halogen	Uses
Fluorine，F2	Coolant in refrigerator
Chlorine，Cl2	Bleaching agents and disinfectants(to clean floors and bed sheets)
Bromine，Br2	Pesticides and fire-extinguishing agents
Iodine，I2	Antiseptics and colouring agents. Silver iodide is used in photography paper