The atomic number of an element was originally just its position on the periodic table. After the discovery of protons, scientists realised that the atomic number of an element is the same as the number of protons in its nucleus.
In the modern periodic table, the elements are arranged according to their atomic number - not their relative atomic mass.
In the periodic table the elements are arranged into:
The metal elements are found on the left hand side of the periodic table, and the non-metal elements are found on the right. Imagine a zig-zag line, starting at B-Al-Si, separating metals from non-metals.
Mendeleev did not know about isotopes, but their existence is an explanation for pair reversals in his table. The positions of iodine and tellurium were reversed in Mendeleev's table because:
The high relative abundance of these tellurium isotopes gives tellurium the greater relative atomic mass. The atomic number of tellurium is 52 and the atomic number of iodine is 53, so these elements are in the correct order in the modern periodic table.