13. the halogens - wikispaces

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Group 7 – the halogens

The 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


What are the halogens?


Why are they called the ‘halogens’?

Halogens are very reactive non metals.

They are all toxic or harmful because they are so reactive. Before antiseptics, iodine was used to clean wounds as it is harmful to all things, including bacteria.

They are also never found free in nature because of their reactivity – they are found as compounds with metals.

These halogen-metal compounds are salts, which give halogens their name – ‘halo-gen’ means ‘salt-former’.


All halogens have seven electrons in their outer shell.

What is the electron structure of the halogens?

fluorine2,7

chlorine2,8,7

bromine2,8,8,7

They can easily obtain a full outer shell by gaining one electron.

They have similar chemical properties.

They all gain an electron in reactions to form negative ions with a -1 charge.

This means that:


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


poisonous and smelly.

brittle and crumbly when solid

What are the general properties of the halogens?

All the halogens are:

They become darker in colour down the group:

non-metals and so do not conduct electricity

is pale yellow

is green-yellow

is blue-black.

is red-brown


What is the physical state of the halogens?

The melting and boiling points of the halogens increase down the group, as the molecules become bigger.

Halogen Relative size

Melting point (°C)

Boiling point (°C)

State

-220 -118

-101

-7

114

-34

59

184

gas

gas

liquid

solid

What is the state of each halogen at room temperature?


Melting and boiling points of halogens


Halogen vapours

Bromine 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.


True or false?


How do the halogens react with metals?

The reactivity of halogens means that they readily react with most metals.Halogens need to gain electrons for a full electron shell and metals need to lose electrons for a full electron shell.

This means that halogens and metals react to form ionic compounds.

nickel (II) chloride copper (II) chloride

These are metal halides, which are a type of salt.


What are halides?

When halogens react with another substance, they become negative ions, as they are gaining an extra electron.

The name of each of the halogens changes slightly once it has reacted – instead of ending with ‘–ine’, they end with ‘-ide’.

Halogen Halidereaction

fluoride (F-)

chloride (Cl-)

bromide (Br-)

iodide (I-)

(F)

(Cl)

(Br)

(I)

When this happens, they are called halides.


Halogens reacting with iron wool


What is the order of reactivity?


What is the reactivity of the halogens?

The iron wool experiment shows that the reactivity of halogens decreases as you go down the group.

decr

ease

in re

activ

ity

Astatine is the halogen that appears directly below iodine in the periodic table. How do you think astatine would react with iron wool?

Iron wool burns and glows brightly.

Iron wool has a very slight glow.

Iron wool glows but less brightly than with chlorine.

Halogen Reaction with iron wool


Equations of halogens and iron

When a halogen reacts with iron it forms an iron halide:

What would the equation be for the reaction that forms iron (III) bromide?

iron iron (III)chloridechlorine +

3Cl2 (g) 2Fe (s) 2FeCl3 (s)+

halogen + iron iron (III) halide

The word and chemical equations for the reaction between chlorine and iron are:

iron iron (III)bromidebromine +

3Br2 (g) 2Fe (s) 2FeBr3 (s)+


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.

decr

ease

in re

activ

ity

F

Cl

Br


How do the halogens react with non-metals?

Halogens also react with non-metals.For example, halogens react with hydrogen to create hydrogen halides.

All hydrogen halides are gases. They dissolve easily in water and become strong acids.

Unlike their reactions with metals, halogens share electrons with non-metals, and so react to form covalent compounds.

hydrogen

H

chlorine

Cl

hydrogen chloride

H Cl+


Displacement of halogens

If a halogen is added to a solution of a compound containing a less reactive halogen, it will react with the compound and form a new one.

sodiumchloride

sodiumfluoride chlorinefluorine + +

F2 (aq) 2NaCl (aq) 2NaF (aq) Cl2 (aq)++

A more reactive halogen will always displace a less reactive halide from its compounds in solution.

This is called displacement.


Displacement of halogens

Why will a halogen always displace a less reactive halogen?


Cl

-

Displacement theory

If a metal halide is mixed with a more reactive halogen,the extra electron will be transferred from the less reactive to the more reactive halogen.

-

sodium chloride

fluorinefluoride

Na

+

Cl

-

F

chlorine


Displacement reactions of halogens


Displacement reactions: summary

The 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


Is there a displacement reaction?


Reactions of halogens: summary


How many everyday uses of halogens can you see below?

What are the uses of halogens?


Glossarydiatomic – Molecules that exist as two atoms covalently

bonded together.displacement – The reaction when a more reactive

halogen reacts with a compound containing a less reactive halogen.

halide – The name of a halogen when it has reacted withanother substance and gained a full outer electron shell.

halogen – An element that belongs to group 7 of the periodic table.

hydrogen halide – A compound formed from the reaction between hydrogen and a halogen.

metal halide – A compound formed from the reaction between a metal and a halogen.

sublime – To change from a solid to a gas without first becoming a liquid.

volatile – A substance that evaporates or produces vapourat relatively low temperatures.


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13. the halogens - wikispaces

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