4.7 Halogens

This topic explains the halogens — the elements in Group 17 of the periodic table — including their appearance, electronic configuration, density, and reactivity.

Definition

The elements in Group 17 (or Group VII-A) are called halogens.

• "halous" meaning salt
• "gen" meaning former

So halogen means "salt former".

Simple explanation: Halogens are the salt-forming elements on the right side of the periodic table.

Main Idea

• Fluorine (F)
• Chlorine (Cl)
• Bromine (Br)
• Iodine (I)
• Astatine (At) — radioactive
• Tennessine (Ts) — radioactive

Astatine and tennessine are radioactive elements. Little is known about their properties.

All halogens are reactive non-metals and exist as diatomic molecules.

Simple explanation: Halogens are reactive non-metals that form molecules of two atoms (like Cl₂, F₂).

Core Concept

Appearance of Halogens

They all exist as diatomic coloured molecular substances. The colour of halogens becomes darker as you go down the group.

At room temperature and pressure (RTP):

Iodine easily turns into dark purple vapours on warming.

Simple explanation: Halogens get darker and change state (gas → liquid → solid) as you go down the group.

Electronic Configuration

Halogens possess 7 electrons in their valence shell. They have the general electronic configuration ns² np⁵.

They need only one electron to complete their valence shell. Consequently, they tend to gain one electron to form univalent negative ions:

F^-, Cl^-, Br^-, I^-

Simple explanation: Halogens have 7 valence electrons, so they want 1 more to complete their octet. They gain 1 electron to become −1 ions.

Detailed Explanation

Density of Halogens

• The number of electrons and protons increases.
• The size of the atom increases.
• The volume increases.
• However, the increase in mass exceeds the increase in volume.
• So the density (mass per unit volume) increases in general.

• Fluorine and chlorine are gases.
• Bromine is a liquid.
• Iodine is a solid.

So the forces of attraction between molecules increase down the group.

Solid iodine has molecules that are highly attracted and tightly packed together than bromine. Therefore, as you go down the group of halogens, the forces of attraction between molecules increase and the density of the halogen increases.

Densities of halogens:

Simple explanation: Halogens get denser down the group because their molecules attract each other more strongly.

Reactivity of Halogens

The reactivity of halogens is directly related to their ability to gain an electron and form a halide ion when they react with other elements.

Trend: Fluorine has the greatest tendency to gain electrons and form a halide ion, making it the most reactive halogen. As you move down the group, the electronegativity of the halogens decreases. This leads to a decrease in reactivity.

Question from the book: Which halogen is the least reactive? Bromine or iodine? Answer: Iodine is less reactive than bromine.

Because halogens have a strong tendency to gain electrons, they have a strong oxidizing power, and this power decreases down the group.

Thus, the order of decreasing oxidizing power is:

F₂ > Cl₂ > Br₂ > I₂

Simple explanation: Fluorine is the most reactive and strongest oxidizing agent; iodine is the least reactive and weakest oxidizing agent.

Displacement Reactions of Halogens

Oxidizing power of F₂ is the highest and that of I₂ is the lowest. Due to the relative strength as oxidizing agent, it is possible for a free halogen to oxidize or displace the halogen next to it in the group from their aqueous solutions.

This means F₂ can oxidize and displace all the halide ions to free halogen.

For example:

F₂ + 2KCl → 2KF + Cl₂

F₂ + 2KBr → 2KF + Br₂

F₂ + 2NaI → 2NaF + I₂

Similarly, Cl₂ can oxidize Br⁻ and I⁻ ions. But I₂ cannot oxidize any halide ion.

Simple explanation: A more reactive halogen can kick out a less reactive halogen from its compound. Fluorine can displace all; chlorine can displace Br and I; iodine cannot displace any.

Hydrogen Halides and Their Thermal Stabilities

Halogens react with hydrogen to form hydrogen halides:

H₂ + X₂ → 2HX

Where X = F₂, Cl₂, Br₂, I₂.

The strength of the hydrogen–halogen bond is related to the electronegativity difference between the hydrogen and halogen atoms. A larger electronegativity difference results in a stronger bond.

As we move from HF to HI, the electronegativity difference between the hydrogen and halogen atoms decreases, resulting in weaker bonds in HCl, HBr, and HI.

So, the relative thermal stability of hydrogen halides gradually decreases from HF to HI.

Consequently, the energy needed to break H–X decreases in the following order:

HF > HCl > HBr > HI

Simple explanation: HF is the most stable hydrogen halide; HI is the least stable.

Prediction of Properties of Elements in Group VIIA or Group 17

The elements present in Group 17 or VIIA are called halogens. They are poisonous non-metals that have low melting and boiling points that increase down the group.

• Fluorine and chlorine (1st and 2nd halogens) are gases.
• Bromine (3rd halogen) is a liquid.
• Iodine (4th halogen) is a solid.

The colours of halogens also get darker from top to bottom.

Activity 4.6 (from the book)

Predict the properties of other elements in Group VII from the given data.

Answer: From this data you can predict how the halogens will behave up and down the group. Astatine, the fifth halogen, will have high melting and boiling points so will be solid at room temperature, and will have a very dark colour.

Concept Assessment Exercise 4.9

Which of the following displacement reactions will occur?

• Cl₂ + 2NaF → 2NaCl + F₂
• Br₂ + 2KI → 2KBr + I₂
• I₂ + 2KBr → 2KI + Br₂
• Cl₂ + 2KBr → 2KCl + Br₂
• Cl₂ + 2NaI → 2NaCl + I₂

• Reaction 1: No — Cl₂ cannot displace F₂ (F is more reactive than Cl).
• Reaction 2: Yes — Br₂ can displace I₂ (Br is more reactive than I).
• Reaction 3: No — I₂ cannot displace Br₂ (I is less reactive than Br).
• Reaction 4: Yes — Cl₂ can displace Br₂ (Cl is more reactive than Br).
• Reaction 5: Yes — Cl₂ can displace I₂ (Cl is more reactive than I).

Intermediate Level

Why Halogens Are Reactive

Halogens have 7 valence electrons. They need only 1 more electron to complete their octet. So they have a strong tendency to gain an electron and form a −1 ion.

• Atomic size increases.
• Electronegativity decreases.
• The nucleus has a weaker pull on incoming electrons.

So fluorine is the most reactive halogen, and iodine is the least reactive (among the common halogens).

Why Density Increases Down the Group

• Mass increases more than volume.
• Intermolecular forces increase (gases → liquid → solid).
• So density increases.

Why Thermal Stability of Hydrogen Halides Decreases

The H–X bond strength depends on the electronegativity difference between H and X.

• HF: largest electronegativity difference → strongest bond → most stable.
• HI: smallest electronegativity difference → weakest bond → least stable.

So thermal stability decreases: HF > HCl > HBr > HI.

Displacement Reactions

A more reactive halogen can displace a less reactive halogen from its compound.

Simple explanation: The stronger oxidizing agent (higher up in the group) displaces the weaker one (lower down).

Important Points

• - Halogens: Group 17 elements (F, Cl, Br, I, At, Ts).
• - Name: from Greek "halous" (salt) + "gen" (former) = salt former.
• - Valence electrons: 7; general configuration ns² np⁵.
• - Ion formed: −1 (gain one electron).
• - Diatomic molecules: F₂, Cl₂, Br₂, I₂.
• - State at RTP: F₂ and Cl₂ gases; Br₂ liquid; I₂ solid.
• - Colours: F₂ pale yellow; Cl₂ yellow-green; Br₂ red-brown; I₂ grey-black.
• - Density increases down the group.
• - Reactivity decreases down the group.
• - Oxidizing power: F₂ > Cl₂ > Br₂ > I₂.
• - Displacement: more reactive halogen displaces less reactive one.
• - Thermal stability of HX: HF > HCl > HBr > HI.

Remember

• - **Halogens** = Group 17 = F, Cl, Br, I, At, Ts.
• - 7 valence electrons → gain 1 → −1 ion.
• - Diatomic molecules: F₂, Cl₂, Br₂, I₂.
• - F₂ pale yellow gas; Cl₂ yellow-green gas; Br₂ red-brown liquid; I₂ grey-black solid.
• - Density increases down the group.
• - Reactivity decreases down the group.
• - Oxidizing power: F₂ > Cl₂ > Br₂ > I₂.
• - Thermal stability: HF > HCl > HBr > HI.

Common Mistakes

• 1. Thinking halogens are metals. They are non-metals.
• 2. Thinking halogens have 8 valence electrons. They have 7.
• 3. Thinking reactivity increases down the group. It decreases for halogens.
• 4. Confusing the states of halogens. F₂ and Cl₂ are gases; Br₂ is liquid; I₂ is solid.
• 5. Thinking iodine is more reactive than bromine. Bromine is more reactive than iodine.
• 6. Forgetting the displacement rule. A more reactive halogen can displace a less reactive one.

Quick Revision

• Halogens: Group 17 elements — F, Cl, Br, I, At, Ts.
• Name: from Greek "halous" (salt) + "gen" (former).
• Valence electrons: 7; general configuration ns² np⁵.
• Ion formed: −1 (univalent negative ion).
• Diatomic molecules: F₂, Cl₂, Br₂, I₂.
• State at RTP: F₂ pale yellow gas; Cl₂ yellow-green gas; Br₂ red-brown liquid; I₂ grey-black solid.
• Density increases down the group.
• Reactivity decreases down the group.
• Oxidizing power: F₂ > Cl₂ > Br₂ > I₂.
• Displacement reactions: more reactive halogen displaces less reactive one.
• Thermal stability of hydrogen halides: HF > HCl > HBr > HI.

60-Second Revision

• **Halogens** = Group 17 = F, Cl, Br, I.
• 7 valence electrons → gain 1 → −1 ion.
• Diatomic: F₂, Cl₂, Br₂, I₂.
• F₂ pale yellow gas; Cl₂ yellow-green gas; Br₂ red-brown liquid; I₂ grey-black solid.
• Reactivity decreases down the group.
• Oxidizing power: F₂ > Cl₂ > Br₂ > I₂.
• Thermal stability: HF > HCl > HBr > HI.
• Warning: halogens are non-metals; reactivity decreases down the group, not increases.