0.2 The particles that substances are made of

Introduction: the atom as the building block of matter

We have now seen that different materials have different properties. Some materials are metals and some are non-metals; someare electrical or thermal conductors, while others are not. Depending on the properties of these materials, they can be used in lots of useful applications.But what is it exactly that makes up these materials? In other words, if we were to break down a material into the parts that make it up, what would we find? Andhow is it that a material's microscopic structure (the small parts that make up the material) is able to give it all these different properties?

The answer lies in the smallest building block of matter: the atom . It is the type of atoms, and the way in which they are arranged in a material, that affects the properties of that substance. This is similar to building materials. We can use bricks, steel, cement, wood, straw (thatch), mud and many other things to build structures from. In the same way that the choice of building material affects the properties of the structure, so the atoms that make up matter affect the properties of matter.

It is not often that substances are found in atomic form (just as you seldom find a building or structure made from one building material). Normally, atoms are bonded (joined) to other atoms to form compounds or molecules . It is only in the noble gases (e.g. helium, neon and argon) that atoms are found individually and are not bonded to other atoms. We looked at some of thereasons for this in earlier chapters.

Compounds

Compound

A compound is a group of two or more different atoms that are attracted to each other by relatively strong forces or bonds.

Almost everything around us is made up of molecules. The only substances that are not made of molecules, but instead are individual atoms are the noble gases. Water is made up of molecules, each of which has two hydrogen atoms joined to one oxygen atom. Oxygen is a molecule that is made up of two oxygen atoms that are joined to one another. Even the food that we eat is made up ofmolecules that contain atoms of elements such as carbon, hydrogen and oxygen that are joined to one another in different ways. All of these are known as small molecules because there are only a few atoms in each molecule. Giant molecules are those where there may be millions of atoms per molecule. Examples of giantmolecules are diamonds , which are made up of millions of carbon atoms bonded to each other and metals , which are made up of millions of metal atoms bonded to each other.

As we learnt in [link] atoms can share electrons to form covalent bonds or exchange electrons to form ionic bonds. Covalently bonded substances are known as molecular compounds. Ionically bonded substances are known as ionic compounds. We also learnt about metallic bonding. In a metal the atoms lose their outermost electrons to form positively charged ions that are arranged in a lattice, while the outermost electrons are free to move amongst the spaces of the lattice.