Halogens: Trends and Oxidising Ability

Learning Objectives (OCR Spec 3.1.3)

By the end of this lesson you should be able to:

• State the trends in appearance, boiling point, electronegativity and oxidising ability of the halogens (F → I)
• Write balanced equations for halogen-halide displacement reactions
• Interpret displacement reactions as evidence for the trend in oxidising ability down Group 7
• Describe observations in cyclohexane extraction of the halogens
• Explain the trends in terms of atomic radius, shielding and effective nuclear charge

The Halogens (Group 7)

The halogens are the Group 7 non-metals: F, Cl, Br, I, At (astatine is radioactive and rarely studied). All have outer configuration ns2 np5 and form 1- ions (halides) by gaining one electron to complete the p sub-shell.

Element	Z	Outer config	State at 25 degC	Colour
F2	9	2s2 2p5	Pale yellow gas	Pale yellow
Cl2	17	3s2 3p5	Green gas	Green
Br2	35	4s2 4p5	Dark red liquid	Red-brown
I2	53	5s2 5p5	Grey-black solid	Black (violet vapour)
At2	85	6s2 6p5	Black solid (radioactive)	-

Trend in Boiling Point

graph LR
  A[F2 -188 degC] --> B[Cl2 -34 degC]
  B --> C[Br2 59 degC]
  C --> D[I2 184 degC]
  E[Boiling point INCREASES down the group] --> A

Down Group 7, boiling points increase because:

• Halogen molecules (X2) are simple molecular with only London dispersion forces between them
• The number of electrons per molecule increases down the group (F2 = 18, I2 = 106)
• More electrons → more polarisable electron cloud → stronger London forces
• Stronger London forces → more energy needed to separate molecules → higher b.p.

This is why F2 and Cl2 are gases, Br2 is a liquid and I2 is a solid at room temperature.

Trend in Electronegativity

Electronegativity is the ability of an atom to attract the bonding pair of electrons in a covalent bond. Halogens are among the most electronegative elements in the Periodic Table.

Element	Pauling electronegativity
F	4.0 (highest of any element)
Cl	3.0
Br	2.8
I	2.5

Electronegativity decreases down Group 7 because:

• Atomic radius increases down the group → bonding electrons are further from the nucleus
• Shielding by inner electrons increases
• Although nuclear charge increases, the effects of distance and shielding dominate
• The nucleus therefore has a weaker pull on the bonding pair

Trend in Oxidising Ability

An oxidising agent gains electrons (is itself reduced). For a halogen:

X2 + 2e- → 2X- (reduction)

The halogen is the oxidising agent and the halide ion (X-) is its reduced form. Oxidising ability decreases down Group 7 (F2 > Cl2 > Br2 > I2) because:

• Down the group, the outer shell of the halogen atom is further from the nucleus (larger radius)
• Shielding by inner shells increases
• An incoming electron is therefore held less strongly (less released energy on electron capture)
• The halogen is less ready to gain an electron → weaker oxidising agent

Fluorine is the strongest oxidising agent in Group 7 (and one of the strongest of any element). Iodine is the weakest halogen oxidising agent you will meet at A-Level.

Halogen-Halide Displacement Reactions

The key evidence for the oxidising ability trend is displacement:

A halogen will displace a less reactive (lower) halide from solution because it is the stronger oxidising agent.