Chemistry (0620) | Group VII properties

🧪 IGCSE Chemistry Study Notes: Group VII Properties (The Halogens)

Hello Chemists! Welcome to the fascinating world of Group VII—the Halogens. This is one of the most exciting groups in the Periodic Table because these elements are extremely reactive and essential for everything from cleaning your swimming pool to keeping your body healthy (iodine!).

Understanding Group VII is crucial for predicting chemical behaviour and recognizing patterns in the Periodic Table. Let's dive in!

1. Introduction to Group VII: The Halogens

The Name and Position

The elements in Group VII are known as the Halogens. The word "halogen" literally means "salt-former" because they react readily with metals (especially Group I) to form salts (like sodium chloride, NaCl).

• The main halogens we study are: Fluorine (F), Chlorine (Cl), Bromine (Br), and Iodine (I).
• They are all found on the right side of the Periodic Table, making them non-metals.

Electronic Structure – The Key to Reactivity

Remember, the Group number tells us the number of outer shell electrons.

• All halogens have 7 electrons in their outermost shell.
• To achieve a stable, full outer shell (like the Noble Gases, Group VIII), they only need to gain 1 electron.
• When they gain this electron, they form negative ions with a charge of 1- (e.g., \(Cl^-\), \(Br^-\), \(I^-\)). These negative ions are called halide ions.

Quick Takeaway: Halogens are desperate to gain one electron, making them powerful oxidising agents and highly reactive non-metals.

2. Physical Properties and Observable Trends

As you move down Group VII, you see distinct changes (trends) in physical appearance and properties.

A. Appearance at Room Temperature (r.t.p.)

This is a core requirement of the syllabus. The state and colour change dramatically as you move down the group:

Element	State at r.t.p.	Appearance/Colour
Chlorine (Cl\(_2\))	Gas	Pale yellow-green gas
Bromine (Br\(_2\))	Liquid	Red-brown liquid
Iodine (I\(_2\))	Solid	Grey-black solid

Don't worry if this seems tricky at first! Just remember the states go Gas (top) → Liquid → Solid (bottom).

Memory Aid (Colour Order): Greenish → Brownish → Blackish.

B. Physical Trends Down the Group

As the atoms get larger moving down Group VII (more electron shells), the forces between the molecules also get stronger. This affects the physical properties:

• Melting Point and Boiling Point: Increases down the group. (It takes more energy to separate the heavier molecules).
• Density: Increases down the group. (Iodine is much denser than Chlorine gas).

Analogy: Imagine trying to heat oil (liquid, like Bromine) versus heating a stone (solid, like Iodine). The stone takes much more heat to melt or boil because its particles are heavier and more strongly bonded together.

3. Chemical Properties and Reactivity Trend

A. Why are Halogens Diatomic?

Halogens exist as diatomic molecules (two atoms bonded together, X\(_2\)).
Since each atom needs one electron to achieve stability, they share a pair of electrons to form a single covalent bond.

Example: Chlorine gas is Cl\(_2\).

B. Reactivity Trend

The most important chemical trend is reactivity:

• Reactivity Decreases down the group.
• This means Fluorine is the most reactive, and Iodine is the least reactive.

C. Explaining the Reactivity Trend (Accessibility Feature)

Why does Chlorine (Cl) react more strongly than Iodine (I)? It’s all about attracting that needed electron:

1. Atomic Size: As you go down the group, the atoms get much bigger because they have more electron shells.
2. Distance: The outer shell where the new electron is trying to enter gets further away from the positively charged nucleus.
3. Shielding: The inner electron shells "shield" or block the attraction of the nucleus for the incoming electron.

Because the incoming electron is less strongly attracted to the large Iodine nucleus than to the smaller Chlorine nucleus, Iodine is less keen to gain the electron, making it less reactive.

4. Displacement Reactions (Halogen Wars!)

The Rule of Displacement

The trend in reactivity is proven by displacement reactions. A more reactive halogen will always displace (kick out) a less reactive halogen from its salt solution (halide ion).

Reactivity Order (Most Reactive to Least Reactive):

Chlorine > Bromine > Iodine

Step-by-Step Examples:

Example 1: Chlorine vs. Potassium Bromide

Chlorine (Cl\(_2\)) is more reactive than Bromine (Br\(_2\)).

If you bubble chlorine gas through a solution of potassium bromide (which contains bromide ions, Br\(\text{-}\)), the chlorine steals the electrons from the bromide ions, forming chlorine ions (Cl\(\text{-}\)) and releasing elemental bromine (Br\(_2\)).

Observation: The colourless solution turns yellow/orange/brown due to the formation of elemental bromine (a red-brown liquid).

Chemical Equation:
\(Cl_{2} (aq) + 2KBr (aq) \rightarrow 2KCl (aq) + Br_{2} (aq)\)

Ionic Equation (showing the reaction clearly):
\(Cl_{2} (aq) + 2Br^{-} (aq) \rightarrow 2Cl^{-} (aq) + Br_{2} (aq)\)

Example 2: Bromine vs. Potassium Iodide

Bromine (Br\(_2\)) is more reactive than Iodine (I\(_2\)).

If you add aqueous bromine to a solution of potassium iodide, the bromine displaces the iodide ions.

Observation: The solution turns brown (or deep purple if iodine precipitates) due to the formation of elemental iodine.

Ionic Equation:
\(Br_{2} (aq) + 2I^{-} (aq) \rightarrow 2Br^{-} (aq) + I_{2} (aq)\)

5. Predicting Properties of Other Halogens (Astatine)

The syllabus requires you to be able to predict the properties of elements you haven't explicitly studied, like Astatine (At), based on the trends observed in Cl, Br, and I.

Astatine is found directly below Iodine in Group VII. We simply extend the trends:

• State at r.t.p.: Since the trend is Gas → Liquid → Solid, Astatine will be a solid, likely darker than iodine (perhaps black).
• Melting/Boiling Point & Density: These properties increase down the group, so Astatine will have a higher melting point and boiling point than iodine and be more dense.
• Reactivity: Reactivity decreases down the group, so Astatine will be the least reactive of the halogens.
• Ion Charge: It will still need 1 electron, so it forms a 1- ion (\(At^-\)).

✅ Quick Review of Group VII

You've successfully tackled Group VII! Remember to practice writing those ionic displacement equations, as they show the core chemical behaviour of these reactive elements. Keep up the great work!