Everything is made of atoms . An atom is the smallest piece of a substance that can exist . 7 million atoms joined together in a straight line would be about 1mm long . The nucleus contains protons and neutrons .
All atoms have electrons .
For any atom , the number of protons is the same as the number of electrons . If an atom loses or gains electrons , it is called an ion . This is a picture of a sodium atom . Each electron has a charge of -1 . The mass of a neutron and a proton are the same .