All substances are made up of small particles. These particles can be:
• Atoms
• Molecules
• Ions
Atoms
An atom is the smallest part of an element that behaves like bulk sample. Each atom is ~10-10m in diameter. E.g. this scanning tunnelling microscope image clearly shows the individual atoms that make up this sheet of gold (Au) atoms.
Molecules
A few elements exist as individual atoms e.g. helium, neon, argon. Most exist as molecules. A molecule is a group of atoms bound together so they behave as a single particle. Molecules can consist of just two atoms (diatomic) or many atoms (polyatomic).
Giant molecules - Many elements exist as giant 3D arrangements of atoms.
e.g. metals, carbon (as diamond or graphite).
Small molecules - Some elements exist as small molecules i.e. entities containing a few atoms only. e.g. hydrogen, oxygen, nitrogen, phosphorus and sulphur. The molecules of a particular element always contain the same number of atoms.
Molecule
Compounds formed between atoms of different non-metals generally exist as molecules e.g. carbon dioxide, methane, aspirin (C9H8O4) and caffeine (C8H10N4O2). A molecule is the smallest particle of such a compound that behaves like bulk sample.
