a isotope is define: Nuclei containing the same number of protons but different numbers of neutrons.
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but in the perodic table loads of element have a different number of neutrons to protons:
You have misunderstood the definition.
The tree most important isotopes of H, as Borek has already said, are protium, deuterium and tritium
Protium ("normal" hydrogen): 1 proton, 0 neutrons
Deuterium: 1 proton, 1 neutron
Tritium: 1 proton, 2 neutrons.
Now, all three have the same number of protons in the nucleus (1) but the number of neutrons vary, so protium, deuterium and tritium are said to be isotopes of hydrogen.
In the same way,
35Cl and
37Cl (the superscript number is the atomic mass) are isotopes of Cl. Both have 17 protons, but the number of neutrons differ. These are the main isotopes of Cl, and exist in a 3:1 ratio of
35Cl to
37Cl (approximately), which is why the atomic mass of Cl is quoted on periodic tables as about 35.5.