A Simple Guide to Atoms, Elements, and Compounds

These four words appear in the first chapter of every school chemistry course, and they are the ones students mix up longest. The confusion is understandable: they describe things at different scales, and textbooks often use them in the same paragraph. Once the difference is clear, a large part of early chemistry becomes much easier.

The atom

An atom is the smallest piece of an element that still behaves like that element. It has a small, dense centre — the nucleus, containing protons and neutrons — surrounded by electrons.

• Protons are positively charged and their number decides which element the atom is.
• Neutrons have no charge and add mass.
• Electrons are negatively charged and are what chemistry mostly involves, because they are the part that meets other atoms.

Atoms are extraordinarily small. A single drop of water contains more atoms than there are stars in the observable universe, which is why we describe them with models rather than pictures.

The element

An element is a substance made of only one kind of atom. Iron contains only iron atoms. Oxygen contains only oxygen atoms. The periodic table lists every known element, and there are about 118 of them.

The important idea is only one kind. If you have two different kinds of atom together, you no longer have an element.

Molecules and compounds

A molecule is two or more atoms joined together. The atoms can be the same or different. Oxygen gas in the air is a molecule made of two oxygen atoms, and it is still an element because both atoms are the same.

A compound is a substance made of two or more different elements chemically joined in fixed proportions. Water is a compound: two hydrogen atoms joined to one oxygen atom, always in that ratio.

So a compound is always made of molecules (or a lattice of ions), but a molecule is not always a compound.

Telling them apart

That last row matters. A mixture contains different substances that are not chemically joined, and it can be separated by physical means. A compound cannot be separated that way.

Reading a formula

A chemical formula is a set of instructions:

• The letters are the elements, written with their symbols: H is hydrogen, O is oxygen, C is carbon, Na is sodium.
• A small number after a letter tells you how many atoms of that element are in the formula. H₂O means two hydrogens and one oxygen.
• No number means one atom.
• A number in front of the whole formula multiplies everything: 2H₂O means two water molecules, so four hydrogens and two oxygens in total.
• Brackets group a set of atoms: Ca(OH)₂ means one calcium, and everything in the bracket twice — two oxygens and two hydrogens.

Four mistakes worth avoiding

1. Calling any molecule a compound. O₂ is a molecule but not a compound.
2. Calling a mixture a compound. Air, salty water and sand are mixtures.
3. Confusing the atom with the element. The atom is one particle; the element is the substance those particles make.
4. Reading a subscript as a multiplier of the whole formula. In H₂O the 2 applies only to hydrogen.

Try these

1. How many atoms are in one molecule of H₂SO₄?
2. Is carbon dioxide an element, a compound or a mixture? Explain in one sentence.
3. How many oxygen atoms are in 3CO₂?

Next step

If this topic is giving you trouble at school, our Grade 9 Chemistry and Grade 10 Chemistry courses cover the same ideas with worked examples and marked practice. You can book a free trial class to see whether the pace suits you.