Define Atoms, Elements, Compounds and Mixtures

Define Atoms, Elements, Compounds and Mixtures

How can we work out what things are made of? Imagine cutting something (like a cake) into smaller and smaller pieces. First, we have slices, then crumbs, then little crumbs, until we have pieces so small that they can't be cut any more - not even with a magic microscopic knife! The smallest possible pieces are called atoms, and everything is made of atoms. There are about 120 different types of atom, but that's enough to make everything around us. Atoms are a little like toy blocks. You can use a few types of block to make all sorts of things, depending on how you join them together. In the same way, different things are made of different atoms combined in different patterns. Each atom has a name and a symbol. You need to know the names and symbols for the first twenty. This looks scary, but if you test yourself on a few at a time, you will soon know them all by heart.

The symbols are always either a capital letter or a capital letter followed by a lower case letter. Most of the symbols start with the same letter as the atom's name, but not all - can you see which atoms have symbols that don't match their name?

Because atoms are so small, we need to think about how to combine them to make things. There are three main ways of doing this:

In an element, all the atoms are the same type. In pure gold bars, all the atoms are gold atoms. When a scientist draws a picture of the atoms in a gold bar, all the atoms look the same:

In a compound, different atoms are joined together (chemists say 'bonded'), but in fixed patterns. Water is a compound of hydrogen and oxygen, with two hydrogen atoms for each oxygen atom. In the atomic picture, the two white balls are hydrogen atoms, and the red ball is an oxygen atom.

Table salt is a compound of sodium and chlorine, with one sodium atom for each chlorine atom. In this atomic picture, purple means sodium and green means chlorine.

In a mixture, different elements or compounds are in the same container, but they are not bonded, and they have no pattern. Seawater contains water and dissolved salt, but they don't make a new compound, they are still water and salt.
If you have a mixture, you can usually separate it back into different pure elements and compounds. Some ways of separating a mixture are:

Filtration, using a sieve or filter paper.

Crystallisation, where we remove the liquid by heating it so that it evaporates -  the solid gets left behind.

Simple distillation, where we heat a liquid so that it evaporates. We collect the vapour and cool it back down to collect a pure liquid.

Fractional distillation, which is like simple distillation except that we carefully collect different liquids which condense at different temperatures.

Chromatography separates different substances dissolved in a solution, for example, different colours mixed together to make ink.

You will learn a lot more about these methods in another activity. For now, the important thing to remember is that mixtures are fairly easy to separate into pure elements or pure compounds.

Want a bit more help with this before you begin? Why not watch this short video?