Explain, in terms of electrons, why potassium reacts more violently than sodium. (3 marks)

• bigger atom or outer shell electron further from nucleus or more shells (1)

• less attraction to nucleus or more shielding (1)

• outer electron more easily lost (1)

Group 7 – The halogens

L.O.:

Predict the outcome of a reaction between a halogen and halide salts.

Explain the trend in reactivity of group 7 in terms of electronic structure.

Homework: workbook pages 105-107

Group 7 – the halogensThe elements in group 7 of the periodic table, on the right, are called the halogens.

fluorine

chlorine

bromine

iodine

astatine

I

Br

Cl

F

At

Halogens – what do they look like?

Chlorine Bromine

Iodine

Halogen vapoursBromine and iodine are not gaseous, but have low boiling points. This means that they produce vapour at relatively low temperature. They are volatile.

Bromine produces some red-brown vapour, seen here above the liquid bromine in the jar.

When iodine is heated gently, it changes directly from a solid to a gas without first becoming a liquid.This is called sublimation.

How does electron structure affect reactivity?The reactivity of alkali metals decreases going down the group. What is the reason for this? The atoms of each element get

larger going down the group. This means that the outer shell gets

further away from the nucleus and is shielded by more electron shells.

The further the outer shell is from the positive attraction of the nucleus, the harder it is to attract another electron to complete the outer shell.

This is why the reactivity of the halogens decreases going down group 7.

dec

reas

e in

rea

ctiv

ity

F

Cl

How do halogen molecules exist?

All halogen atoms require one more electron to obtain a full outer shell and become stable.

Each atom can achieve this by sharing one electron with another atom to form a single covalent bond.

This means that all halogens exist as diatomic molecules:F2, Cl2, Br2 and I2.

+ F F F F

Reactions of the group 7 elements

fluorine

chlorine

bromine

iodine

astatine

I

Br

Cl

F

At

• KBr + Cl2

clip

Displacement Equations

Potassium Bromide and Chlorine:

• 2KBr + Cl2 2KCl + Br2

Potassium Iodide and Chlorine:

• 2KI + Cl2 2KCl + I2

Potassium Iodide and Bromine:

• 2KI + Br2 2KBr + I2

Displacement reactions: summaryThe reactions between solutions of halogens and metal halides (salts) can be summarised in a table:

2KCl + I2

2KBr + I2

halogen

chlorine

bromine

iodine

salt (aq)potassiumchloride

potassiumiodide

potassiumbromide

2KCl + Br2

no reactionno reaction

no reaction

Q1) What is the product of the reaction between iron and iodine?

Q2) Which gas is more reactive, chlorine or iodine?

Tasks:

1)Workbook page 108.

2)Halogens worksheet.