Molecules of Elements

Introduction

Dalton could not believe that two atoms of the same element would stick together. Surely identical atoms would repel? Yet the evidence from reacting gas volumes, and later from many other methods, showed that hydrogen, oxygen and nitrogen gases are made of pairs of atoms. Molecules made from only one kind of atom are called molecules of elements , and they are all around us — in fact, almost all of the air you breathe is made of them.

Core explanation

Definition. A molecule of an element contains two or more atoms of the same element joined by covalent bonds. Because only one element is present, the substance is still an element, not a compound.

The seven diatomic elements. Seven common elements exist as molecules of two atoms (diatomic molecules) at room temperature:

Element Formula State at room temperature Bond between atoms --- --- --- --- Hydrogen H₂ gas single Nitrogen N₂ gas triple Oxygen O₂ gas double Fluorine F₂ gas single Chlorine Cl₂ gas single Bromine Br₂ liquid single Iodine I₂ solid single

A handy memory aid is that these are hydrogen, nitrogen and oxygen plus the four halogens fluorine, chlorine, bromine and iodine. On the periodic table, nitrogen, oxygen, fluorine and chlorine form a neat block, with bromine and iodine below.

Why pairs? A lone hydrogen atom has one electron; by sharing it with another hydrogen atom, each gets a stable pair. A chlorine atom needs one more electron to fill its outer shell, so two chlorine atoms share one pair. Nitrogen atoms share three pairs, forming a very strong triple bond, which is why nitrogen gas is so unreactive.

Larger molecules of elements. Some elements form molecules with more than two atoms:

- Ozone, O₃ — three oxygen atoms; found high in the atmosphere, where it absorbs harmful ultraviolet light. - White phosphorus, P₄ — four phosphorus atoms arranged at the corners of a tetrahedron. - Sulfur, S₈ — eight sulfur atoms joined in a crown-shaped ring. - Buckminsterfullerene, C₆₀ — sixty carbon atoms in a football-shaped cage.

Writing formulae correctly. When you write the formula of hydrogen gas, oxygen gas or chlorine gas, always include the subscript 2: H₂, O₂, Cl₂. The symbol O on its own means a single oxygen atom, which does not normally exist by itself.

Step-by-step reasoning

To write the formula of an element correctly:

1. Find the element's symbol. 2. Check whether it is one of the seven diatomic elements. 3. If so, add the subscript 2 (for example, N₂). 4. If not, check whether it forms a special molecule (O₃, P₄, S₈) or is written as a single symbol (as for most metals and the noble gases).

Visual explanation

Picture two identical green spheres joined by a short stick: that is Cl₂. Two red spheres joined by a double stick make O₂. Two blue spheres joined by three sticks make N₂. The number of sticks shows how many electron pairs are shared, and the matching colours show that only one element is present.

Real-world analogy

Pairs of identical socks come off a washing line together. Each pair contains only one design, just as each diatomic molecule contains only one element. A drawer full of such pairs is still "one kind of sock", just as a jar of O₂ is still pure oxygen.

Real-world example

Air is about 78% nitrogen (N₂) and 21% oxygen (O₂) by volume. Food manufacturers flush bags of crisps with nitrogen gas because the stable N₂ molecules do not react with the food, keeping it fresh and stopping the crisps from being crushed.

Why?

Why is nitrogen gas so unreactive when nitrogen is essential to life? The triple bond in N₂ is one of the strongest bonds known, so a great deal of energy is needed to break the molecule apart before the nitrogen atoms can form new compounds.

Common misconception

"O₂ is a compound because it has two atoms." No. A compound must contain two or more different elements. O₂ contains only oxygen atoms, so it is a molecule of an element.

Worked example

Question: Which of these are molecules of elements: Cl₂, HCl, P₄, CO, O₃?

Reasoning: Check whether every atom in each formula belongs to the same element. Cl₂ — all chlorine. HCl — hydrogen and chlorine. P₄ — all phosphorus. CO — carbon and oxygen. O₃ — all oxygen.

Answer: Cl₂, P₄ and O₃ are molecules of elements; HCl and CO are molecules of compounds.

Quick check

1. Write the formula of bromine as it exists at room temperature. Answer: Br₂, a diatomic liquid.

Exam focus

Memorise the seven diatomic elements and always include the subscript 2 in equations, for example 2H₂ + O₂ → 2H₂O. Losing marks for writing H or O instead of H₂ or O₂ is one of the most common errors in chemistry exams.

Advanced insight

Chlorine, bromine and fluorine are toxic and very reactive, and must only be handled with proper controls such as fume cupboards. Their reactivity comes from the relatively weak single bond between halogen atoms and the strong tendency of each atom to gain an electron. In contrast, N₂ is so stable that industry needs high temperatures, high pressures and a catalyst to make it react with hydrogen.

Summary

Molecules of elements contain atoms of only one element. Seven common elements are diatomic: H₂, N₂, O₂, F₂, Cl₂, Br₂ and I₂. Others form larger molecules, such as O₃, P₄, S₈ and C₆₀. Always write the correct subscript in formulae. A molecule of an element is still an element, not a compound.

Practice questions

1. List the seven diatomic elements. Answer: Hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine and iodine. 2. Why is O₂ an element but CO₂ a compound? Answer: O₂ contains only oxygen atoms, whereas CO₂ contains atoms of two different elements, carbon and oxygen. 3. How many atoms are in one molecule of sulfur, S₈, and of white phosphorus, P₄? Answer: Eight atoms and four atoms respectively. 4. Explain why nitrogen gas is unreactive. Answer: Its two atoms are joined by a very strong triple bond that needs a lot of energy to break.