The periodic table
Mendeleev's table needed one important modification before it became the modern periodic table – the use of atomic number to order the elements.
All atoms of the same element contain the same number of particles called protons, and this is called the element's atomic number.
Mendeleev put the 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. 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 organising 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!
You need to be able to locate the positions of the: