Every type of atom has an exact number of protons. For example every single hydrogen atom must have one proton. If you look at the periodic table, atoms are grouped according to the number of protons in them.
The number in the upper right
hand corner of each box shows the number of protons in an atom. If an atom
has one proton it will be hydrogen. If an atom has three protons it will
not be hydrogen it will be Lithium. The table shows us that an atom with
four protons will be Berylium. If the number of protons in the atom
changes it reacts differently in chemical reactionsA hydrogen atom must have one proton it may however have different numbers of neutrons. A hydrogen atom for example may have one proton and no neutrons, it could also have one proton and one neutron, or it could have one proton and two neutrons. All of these possibilities are shown below.
The label on the picture identifies all of these atoms as Hydrogen. the symbols below the atoms show the atomic number (the number of protons) on the bottom left. All atoms have one proton. The atomic mass numbers are all shown on the right of each symbol. The mass number is the number of protons + neutrons in the atom.
For the first atom 1 proton + 0 neutrons = 1 atomic mass units
For the second atom 1 proton + 1 neutron = 2 atomic mass units
For the third atom 1 proton + 2 neutrons = 3 atomic mass units
All three of these atoms will be hydrogen. They all behave the same in chemical reactions, they are called isotopes of each other.
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