Moseley's Periodic Table
Mendeleev's table was nine tenths of the way there, but needed one important modification before it became the modern periodic table - the use of atomic number as the organizing principle for the periods.
Mendeleev ordered his elements in order of their relative atomic mass, and this gave him some problems. For example, iodine has a lower relative atomic mass than tellurium, so it should come before tellurium in Mendeleev's table - but in order to get iodine in the same group as other elements with similar properties such as fluorine, chlorine and bromine, he had to put it after tellurium, so breaking his own rules.
Using atomic number instead of atomic mass as the organizing principle was first proposed by the British chemist Henry Moseley in 1913, and it solved anomalies like this one. Iodine has a higher atomic number than tellurium - so, even though he didn't know why, Mendeleev was right to place it after tellurium after all!
Metals and non-metals
More than three quarters of the elements in the modern table are metals. They are mainly found in the two left-hand columns (Groups 1 and 2) and the central block (the transition metals). Less than a quarter of the elements are non-metals, and are found on the right-hand side of the periodic table.