The word atom comes from atomos , an ancient Greek word meaning indivisible . The
Greek philosopher Demokritos (460-370 BCE) maintained that all matter could be divided and sub-divided into smaller and smaller units, and eventually there would be a tiny particle that could not be divided any further - an atom.
The understanding of atoms didn’t progress much beyond Demokritos’ theory until the
English chemist John Dalton (1766 - 1844) started to look at it in the 1800s. His most important conclusions are summarised below: all matter is made of atoms, and atoms are indestructible and cannot be broken down into pieces all the atoms of a particular element are identical to each other and different from the atoms of other elements atoms are rearranged in a chemical reaction compounds are formed when two or more different kinds of atoms join together
Although the word 'atom' comes from the Greek for indivisible, we now know that atoms are not the smallest particles of matter. Instead, they have a small central nucleus surrounded by even smaller particles called electrons.
All substances are made from atoms. The atoms of any element are different from the atoms of any other element. So iron contains a different sort of atoms from those of sulfur, and the atoms in carbon are different from those of oxygen.
There are more than 100 different elements. The periodic table is a chart showing all the elements arranged in a particular way. The vertical columns in the periodic table are called groups. Each group contains elements that have similar properties. The periodic table has eighteen main groups. For example, group 1 contains very reactive metals such as sodium - Na
- while group 7 contains very reactive non-metals such as chlorine - Cl .
New substances are formed by chemical reactions. When elements react together to form compounds their atoms join to other atoms using chemical bonds.
Chemical bonds involve electrons from the reacting atoms. Bonds can form when:

electrons are transferred from one atom to another, so that one atom gives electrons and the other takes electrons, or electrons are shared between two atoms.
The chemical formula of a compound shows how many of each type of atom join together to make the units that make the compound up.
When elements are joined to cause a chemical reaction, no atoms are made or lost during the process - but at the end of it they are joined differently from the way they were at the start. This means that the mass of the substances at the start - the reactants - is the same as the mass of the substances at the end - the products .
We use balanced equations to show what happens to the different atoms in reactions, for example, copper and oxygen react together to make copper oxide.
Take a look at the word equation for the reaction, here: copper + oxygen → copper oxide
You can see that copper and oxygen are the reactants, and copper oxide is the product.