The two certeria used by Mendeleev to classify the elements in his periodic table are:
- Atomic mass: elements are arranged in the order of increasing atomic masses.
- The formulae of the hydrides and the oxides formed by the various elements.
2.Mendeleev’s Periodic Law states that the properties of elements are a periodic function of their atomic masses. This means, that if elements are arranged in the increasing order of their atomic masses, then their properties will get repeated after regular interval.
3.Mendeleev positioned hydrogen in the first column above the alkali metals. He did so because hydrogen and alkali metals have similar properties. For example; hydrogen reacts with halogens, oxygen, and sulphur to form compounds whose formulae are similar to those of alkali metals.
However, hydrogen also resembles halogens in many ways. Like halogens, hydrogen is a gas, and exists as a diatomic molecule (H2). It forms covalent compounds like halogens and unlike alkali metals. Hence, it can also be placed above the halogen group.
(i) As we move across the periodic table from left to right in a period, atomic size decreases as effective nuclear charge increases.
(ii) As we move down a group in the periodic table, atomic size increases as new shells are being added.