An atom has just a single nucleus which is electrically positive and around it is a cloud of electrons which balance it’s positive charge.
When atoms come close together they stick together to make molecules – we say there is a ‘chemical reaction’, but the ‘sticking’ is just more electrical interaction.
Water is a pure substance made of a single type of molecule, H2O, which is made of two atoms of hydrogen and one of oxygen which are stuck together electrically. If you have a glass of water in front of you what you are looking at is a collection of H2O molecules.
If you added an extra atom of oxygen to H2O to make H2O2 then you would create a completely different substance –in this case a substance called hydrogen per oxide which we use as bleach and to dye hair ‘blonde’ hence the phrase ‘peroxide blonde’