Group 7: The Halogens

Introduction

On the right-hand side of the periodic table, one column before the noble gases, lies a family of highly reactive non-metals: fluorine, chlorine, bromine, iodine and astatine. Their group name, halogens , comes from Greek words meaning "salt formers", because they combine with metals to make salts such as sodium chloride. They are the mirror image of the alkali metals: where Group 1 atoms are desperate to lose one electron, Group 7 atoms are eager to gain one.

Core explanation

Seven valence electrons. Every halogen atom has seven electrons in its outer shell , one short of the stable arrangement of eight found in the noble gases.

Element Symbol Electron arrangement --- --- --- Fluorine F 2,7 Chlorine Cl 2,8,7 Bromine Br 2,8,18,7 Iodine I 2,8,18,18,7

This single missing electron controls almost all of their chemistry. (In newer numbering the halogens are Group 17; many school courses still call them Group 7.)

Diatomic molecules. A lone halogen atom is unstable. Two atoms can each reach a full outer shell by sharing one pair of electrons in a single covalent bond. So the elements exist as diatomic molecules : F₂, Cl₂, Br₂ and I₂. When writing equations you must always use these formulas, not Cl or Br on their own.

Typical non-metal properties. Because they are made of small, separate molecules held together only by weak forces between molecules, halogens have low melting and boiling points . They are poor conductors of heat and electricity, and the solids (such as iodine) are brittle. At room temperature fluorine and chlorine are gases, bromine is a liquid and iodine is a solid — a trend described on the next page.

Reactions with metals. A halogen atom reacts by gaining one electron to form a halide ion with a 1− charge:

Cl₂ + 2e⁻ → 2Cl⁻

When a halogen reacts with a metal, the metal transfers electrons to the halogen and an ionic compound called a metal halide forms:

2Na(s) + Cl₂(g) → 2NaCl(s)

Names change ending: chlorine becomes chloride, bromine becomes bromide, iodine becomes iodide, fluorine becomes fluoride. With iron, chlorine forms iron(III) chloride: 2Fe + 3Cl₂ → 2FeCl₃.

Reactions with non-metals. Halogens also form covalent compounds with non-metals. With hydrogen they form hydrogen halides such as hydrogen chloride, HCl, which dissolves in water to form hydrochloric acid.

Hazards. Halogens are toxic and corrosive. Chlorine gas damages the lungs; bromine liquid burns skin and gives off a choking vapour; fluorine is so reactive that it attacks almost everything. In schools, only dilute solutions or tiny amounts are used, in a fume cupboard, with eye protection. The reactivity trend down the group — fluorine most reactive, iodine least — is explained on later pages.

Step-by-step reasoning

To predict the formula of a metal halide:

1. Find the metal's ion charge from its group (Group 1 gives 1+, Group 2 gives 2+). 2. The halide ion is always 1−. 3. Balance the charges: Na⁺ with Br⁻ gives NaBr; Mg²⁺ with two Cl⁻ gives MgCl₂. 4. For the equation, remember the halogen is diatomic and balance atoms.

Visual explanation

Draw two chlorine atoms as rings with seven dots each on the outer shell. Slide them together so their outer shells overlap, and place one dot from each atom in the overlap. Now count the dots around each atom: eight each. The overlapping pair is the covalent bond in Cl₂.

Real-world analogy

Imagine two people each holding seven cards from a set of eight. Neither can complete their set alone, but by agreeing to share one card between them, both can claim a full hand. That shared card is like the electron pair in a halogen molecule.

Real-world example

Chlorine is added to drinking water and swimming pools to kill harmful microbes, and it is used to make PVC plastic and bleach. Fluoride compounds in toothpaste strengthen tooth enamel. Iodine solution is used as an antiseptic, and silver bromide was the light-sensitive chemical in traditional photographic film.

Why?

Why do halogens react so readily with Group 1 metals? The alkali metal needs to lose one electron and the halogen needs to gain one. The transfer gives both a stable noble-gas arrangement and forms strongly attracted ions, releasing a large amount of energy.

Common misconception

"Chlorine and chloride are the same thing." Chlorine, Cl₂, is a toxic, green-yellow gas made of molecules. Chloride, Cl⁻, is an ion with an extra electron, found harmlessly in table salt and seawater. Gaining one electron changes the chemistry completely.

Worked example

Question: Write a balanced equation for potassium reacting with bromine and name the product.

Reasoning: Potassium forms K⁺ and bromine forms Br⁻, so the product is KBr. Bromine is diatomic (Br₂), so two potassium atoms are needed for each bromine molecule.

Answer: 2K + Br₂ → 2KBr; potassium bromide.

Quick check

1. How many electrons does a chlorine atom need to gain to fill its outer shell? Answer: One.

Exam focus

Always write halogens as diatomic molecules in equations (Cl₂, not Cl). Explain their reactions as gaining one electron to form a 1− ion . Use the correct "-ide" ending for compounds, and be able to link seven outer electrons to Group 7 and to a charge of 1−.

Advanced insight

Astatine, at the bottom of the group, is intensely radioactive, and less than a microgram has ever been gathered at one time, so its properties are largely predicted from trends rather than measured. Calculations suggest it may even show some metallic character, which would make it the only halogen that behaves partly like a metal.

Summary

The halogens (fluorine, chlorine, bromine, iodine, astatine) are Group 7 non-metals with seven outer electrons. They form diatomic molecules held together by a shared electron pair, and have low melting and boiling points. They react by gaining one electron to form 1− halide ions, making ionic metal halides with metals. They are toxic and must be handled with care.

Practice questions

1. Explain why chlorine exists as Cl₂ molecules. Answer: Each chlorine atom has seven outer electrons; by sharing one pair, both atoms reach a full outer shell of eight. 2. What is the formula of the ion formed by iodine? Answer: I⁻. 3. Write a balanced equation for sodium reacting with chlorine. Answer: 2Na + Cl₂ → 2NaCl. 4. Give the formula of calcium fluoride and explain it. Answer: CaF₂, because one Ca²⁺ ion balances the charge of two F⁻ ions.