There are more than a hundred different atoms, but each element contains only one kind of atom.
Atoms are what make one element different from another.
So what makes one atom different from another?
The Greeks thought atoms could not be divided into anything smaller but they were wrong! Every atom can be divided into three different components - protons, neutrons and electrons.
The nucleus, or centre of an atom, is made of protons, (which have a positive electrical charge) and neutrons, which are neutral (in other words, they have no charge).
Electrons, which have a negative charge, move around the centre of the atom.
Normally, the number of electrons is equal to the number of protons, which makes atoms electrically neutral.
The number of protons in an atom is the defining feature of an atom. It's what makes one element different from another. The number of protons in an atom is called its atomic number. Oxygen atoms have 8 protons. So the atomic number of oxygen is 8.
All the elements in the universe are arranged according to their atomic number in the periodic table.
Atomic mass
If you add together the number of protons (or atomic number) and the number of neutrons in the nucleus of an atom, you can calculate the mass of the atom. Helium (He), atomic number two, has two protons and two neutrons. So the mass number for Helium is 4.
Isotopes
Atoms with the same number of protons but different numbers of neutrons are called isotopes. Uranium-238 and uranium -235 are two different isotopes of uranium. Uranium- 238 has a mass number of 238. Uranium-235 has a mass number of 235. So uranium-238 has more neutrons than uranium-235. But they are still both atoms of uranium because they have the same number of protons.
