Elements That Exist as Molecules

Introduction

When we say that the air contains oxygen, we do not mean that it contains loose oxygen atoms floating on their own. Almost all the oxygen in air is made of pairs of oxygen atoms joined together. Each pair is a molecule , written O₂. Several other elements behave in the same way. Knowing which elements exist as molecules is essential for writing correct formulae and, later, correct chemical equations.

Core explanation

Atoms and molecules. An atom is the smallest particle of an element. A molecule is a particle made of two or more atoms held together by chemical bonds. A molecule can contain atoms of the same element or of different elements. On this page we look at molecules made of just one element.

Diatomic elements. Seven elements exist as molecules made of two identical atoms. They are called diatomic elements:

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

The subscript 2 shows that each molecule contains two atoms. Whenever you write one of these elements as a substance on its own, you must use the diatomic formula: oxygen gas is O₂, not O.

Monatomic elements. The noble gases — helium, neon, argon, krypton, xenon and radon — exist as single, unbonded atoms. They are called monatomic , and their formulae are simply their symbols: He, Ne, Ar.

Polyatomic elements. Some non-metals form larger molecules. White phosphorus is made of P₄ molecules, and common sulfur is made of ring-shaped S₈ molecules. Oxygen can also form O₃ molecules, called ozone.

Metals and giant structures. Metals, and non-metals such as carbon (as diamond or graphite) and silicon, do not form small molecules. Instead, they form giant structures containing enormous numbers of atoms bonded together. Their formulae are written as the symbol alone, such as Fe, Cu or C.

Step-by-step reasoning

To write the formula of an element as it normally exists:

1. Check whether it is one of the seven diatomic elements. If so, write the symbol with a subscript 2. 2. If it is a noble gas, write the symbol alone. 3. If it is phosphorus or sulfur, P₄ and S₈ may be used, though P and S are often accepted in simple equations. 4. For metals and giant-structure elements, write the symbol alone.

Visual explanation

Picture a box of oxygen gas as a crowd of tiny dumbbells, each made of two spheres joined by a short bar, drifting and tumbling. Now picture a box of helium as a crowd of single marbles. The dumbbells are diatomic molecules; the marbles are monatomic atoms.

Real-world analogy

Diatomic elements are like dancers who are never seen alone on the dance floor: they always move as couples. Noble gases are like solo dancers who never take a partner. Metals are more like a huge crowd packed shoulder to shoulder with no separate groups at all.

Real-world example

Nitrogen gas, N₂, makes up about 78% of the air. The two nitrogen atoms in each molecule are held together by a very strong triple bond, which is why nitrogen gas is so unreactive. Food packets are often filled with nitrogen to keep crisps fresh, because N₂ does not react with the food as oxygen would.

Why?

Why do hydrogen, oxygen and the halogens pair up while helium does not? A single atom of hydrogen or chlorine is unstable and readily forms a bond. Sharing electrons with another identical atom gives each atom a more stable arrangement, so the atoms pair up. Noble gas atoms already have a stable electron arrangement, so they have no tendency to bond.

Common misconception

"Diatomic means a compound of two elements." Diatomic means two atoms per molecule. O₂ is diatomic but is an element, because both atoms are oxygen. Carbon monoxide, CO, is also diatomic, but it is a compound because the atoms are different.

Worked example

Question: Write the formulae of the following as they normally exist: chlorine, argon, magnesium and iodine.

Reasoning: Chlorine and iodine are halogens and are diatomic. Argon is a noble gas and monatomic. Magnesium is a metal with a giant structure.

Answer: Cl₂, Ar, Mg, I₂.

Quick check

1. What is the formula of a molecule of nitrogen gas? Answer: N₂.

Exam focus

Learn the seven diatomic elements; a popular memory aid is that they are hydrogen plus the elements forming a "7" shape on the periodic table (nitrogen, oxygen, fluorine, then down through chlorine, bromine and iodine). A common error in equations is writing O or H for the gases instead of O₂ and H₂.

Advanced insight

The strength of the bond in a diatomic molecule varies greatly. The triple bond in N₂ is one of the strongest known, which is why nitrogen from the air is hard for living things to use. The single bond in F₂ is unusually weak, which helps make fluorine the most reactive of all the elements.

Summary

A molecule is two or more atoms bonded together. Seven elements are diatomic: H₂, N₂, O₂, F₂, Cl₂, Br₂ and I₂. The noble gases are monatomic, phosphorus and sulfur form larger molecules such as P₄ and S₈, and metals and carbon form giant structures written with the symbol alone. Diatomic describes the number of atoms, not whether a substance is an element or compound.

Practice questions

1. Name the seven diatomic elements. Answer: Hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine and iodine. 2. Why is the formula of neon written as Ne and not Ne₂? Answer: Neon is a noble gas whose atoms do not bond to each other, so it exists as single atoms. 3. Is HCl diatomic? Is it an element? Explain. Answer: It is diatomic because each molecule has two atoms, but it is a compound, not an element, because the atoms are of different elements. 4. How many atoms are there in one molecule of common sulfur, S₈? Answer: Eight.