- The electronic configuration of the outermost shell is same for all the elements belonging to the same group.
- So, the number of valence electrons for all the elements in a group is the same.
- The valency of an element is determined by the valence electrons.
- Hence, elements belonging to the same group have the same valency.
Describe the merits of Mendeleev’s periodic table.
Answer:- To give the proper place in the periodic table, atomic masses of
some elements were revised in accordance with their properties. For example,
the previously determined atomic mass of beryllium, 14.09, was changed to the
correct value 9.4, and beryllium was placed before boron.
(2) Mendeleev had kept some vacant places in the periodic table for
elements that were yet to be discovered. Three of these unknown elements
were given the names eka-boron, eka-aluminium and eka-silicon from the
known neighbours and their atomic masses were indicated as 44, 68 and 72,
respectively. Their properties were also predicted. Later on these elements were
discovered subsequently and were named as scandium (Sc), gallium (Ga) and
germanium (Ge) respectively. The properties of these elements matched well with
those predicted by Mendeleev. This brought out the importance of Mendeleev’s
periodic table.
(3) There was no place reserved for noble gases in Mendeleev’s original
periodic table. When noble gases such as helium, neon and argon were
discovered, Mendeleev created the ‘zero group’ without disturbing the original
periodic table. The noble gases were placed very well in the new table.