The Periodic Table - Group 7 Halogens

Group 7: The Halogens

Non-metal elements with seven outer-shell electrons

The halogens are a fascinating group of elements that play important roles in our daily lives - from the chlorine that keeps swimming pools clean to the iodine used as an antiseptic. Understanding their properties helps explain why they behave so similarly yet show clear trends.

1. Position in the Periodic Table and Electronic Structure

The halogens are found in Group 7 (sometimes called Group 17) on the right-hand side of the Periodic Table.

Key features:

• Each atom has seven electrons in its outer shell

• They need just one more electron to achieve a stable noble gas configuration

• This makes halogens very reactive non-metals

• All halogens exist as diatomic molecules (molecules containing two atoms)

For example, chlorine exists as Cl2Cl2​ molecules. The two chlorine atoms share a pair of electrons, forming a single covalent bond - like two people sharing an umbrella to stay dry.

Electron configurations

1. Fluorine: 2,72,7

2. Chlorine: 2,8,72,8,7

3. Bromine: 2,8,18,72,8,18,7

4. Iodine: 2,8,18,18,72,8,18,18,7

Notice the pattern - they all end in 7! This common feature of seven outer-shell electrons explains why halogens have such similar chemical behaviour.

2. Physical Trends Down the Group

As you move down Group 7, several clear patterns emerge:

• Relative molecular mass increases - the atoms get heavier

• Melting point and boiling point both rise

• Density increases and colours get darker

Examples:

• Chlorine (Cl2Cl2​): pale green gas, boiling point −34°C−34°C

• Bromine (Br2Br2​): red-brown liquid, boiling point 59°C59°C

• Iodine (I2I2​): grey-black solid, sublimes at 184°C184°C

These trends happen because larger molecules have stronger intermolecular forces (van der Waals forces) between them - like stronger magnets being harder to pull apart.

3. Chemical Reactivity and Displacement

Important trend: Reactivity decreases as you go down the group.

Why? The outer electron shell gets further from the nucleus and becomes more shielded by inner electrons. This means the attraction for gaining an extra electron becomes weaker - like trying to pick up a paperclip with a magnet that's further away.

Displacement reactions

A more reactive halogen can push out (displace) a less reactive halogen from its compound:

Cl2+2KBr→2KCl+Br2Cl2​+2KBr→2KCl+Br2​

Here, chlorine is more reactive than bromine, so it removes the Br−Br− ions from solution.

What you would see:

• Chlorine water + potassium bromide solution: colourless solution turns orange-brown as Br2Br2​ forms

• Bromine water + potassium iodide solution: solution turns brown, then deep black/purple as I2I2​ forms

• Iodine water + any halide solution: no reaction - iodine is the least reactive

4. Compounds with Metals

When halogens react with metals, they gain one electron to form halide ions with a 1−1− charge (X−X−).

Examples:

• Sodium + chlorine: 2Na+Cl2→2NaCl2Na+Cl2​→2NaCl

• Magnesium + bromine: Mg+Br2→MgBr2Mg+Br2​→MgBr2​

These reactions produce ionic salts with typical ionic properties:

• High melting and boiling points

• Conduct electricity when molten or dissolved in water

5. Compounds with Non-metals

When halogens react with other non-metals, they form covalent compounds by sharing electrons.

Example: Formation of hydrogen halides
H2+Cl2→2HClH2​+Cl2​→2HCl

Hydrogen chloride dissolves in water to form hydrochloric acid. This is a strong acid because HClHCl molecules completely split up (ionise) into H+H+ and Cl−Cl− ions.

Thermal stability trend

HClHCl is the most stable when heated, while HIHI decomposes most easily. This happens because the H–X bond gets weaker as the halogen atom gets larger.

6. Explaining and Predicting Properties

The fact that every halogen atom has seven outer electrons allows us to:

1. Explain similarities: They all form 1−1− ions and single covalent bonds

2. Predict properties: We can forecast what astatine (below iodine) should be like:

   • Dark solid with boiling point above 200°C200°C

   • Less reactive than iodine

   • Unable to displace iodide ions from solution

Key terms

Halogen - An element in Group 7 of the Periodic Table with seven outer electrons.

Halide ion - A negatively charged ion (X−X−) formed when a halogen gains one electron.

Diatomic - A molecule containing two atoms bonded together, such as Br2Br2​.

Displacement reaction - A reaction where a more reactive element takes the place of a less reactive element in a compound.

Intermolecular forces - Forces of attraction between molecules that increase with molecular size.

Oxidising agent - A substance that gains electrons from another substance, such as chlorine in displacement reactions.

Worked example

Question: A student mixes 10 cm310cm3 of chlorine water with 10 cm310cm3 of colourless potassium iodide solution.
a) Write the ionic equation for the reaction.
b) Describe the colour change observed.
c) Explain why this reaction occurs.

Solution:
a) Cl2+2I−→2Cl−+I2Cl2​+2I−→2Cl−+I2​

b) The colourless solution turns brown, then may appear black as solid iodine forms.

c) This reaction occurs because:

1. Chlorine is higher up Group 7 than iodine

2. This makes chlorine more reactive than iodine

3. Chlorine can therefore displace iodine from its compounds

4. Chlorine gains electrons from I−I− ions, forming Cl−Cl− ions and I2I2​ molecules

Real-world Application: Water Treatment
Chlorine is added to drinking water and swimming pools because it kills harmful bacteria. The chlorine reacts with organic matter from bacteria, breaking down their cell walls. This is why swimming pools smell of chlorine - it's actually the smell of chlorine compounds formed when chlorine reacts with organic materials in the water.

Photography Connection
Traditional black and white photography used silver halides like AgClAgCl. When exposed to light, the Cl−Cl− ions lose electrons to form chlorine atoms, while Ag+Ag+ ions gain electrons to form silver metal, creating the dark areas of the photograph.

Required Practical: Halogen Displacement Reactions

Aim: To investigate the ability of chlorine, bromine and iodine to displace each other from solution and establish their reactivity order.

Apparatus:

• Test tubes and test tube rack

• Dropper pipettes

• Solutions of KClKCl, KBrKBr, KIKI (all 0.2 mol dm−30.2mol dm−3)

• Chlorine water, bromine water, iodine water

• Safety goggles and gloves

• Fume cupboard

Method:

1. Place 2 cm32cm3 of each potassium halide solution into separate labelled test tubes

2. Add 1 cm31cm3 of chlorine water to each tube and record any colour changes

3. Rinse the dropper thoroughly and repeat using bromine water with fresh halide solutions

4. Repeat again using iodine water with fresh halide solutions

5. Record all observations in a results table

Safety:

• Chlorine gas is toxic - work in a fume cupboard

• Wear safety goggles and gloves throughout

• Bromine water is corrosive to skin and eyes

Expected Results:

Conclusion: The displacement reactions confirm the reactivity order: Cl2>Br2>I2Cl2​>Br2​>I2​. Only more reactive halogens can displace less reactive ones from their compounds.

Comparison table