Question

Define the following terms:
i. Valency
ii. Atomic Size
iii. How do the valency and the atomic size of the elements vary while going from left to right along a period in the modern periodic table?

Answer 1

Hint An atom is occupied by electrons, which suggests that electrons are responsible for the interaction of atoms and the formation of chemical bonds. In the electronic configuration the outermost shell of the element needs to attain its complete octet. The electrons which an element either gain, loose or share are known as valence electrons and it represents the valency of that particular element.

Complete answer:
i. Valency is the maximum combining capacity of an atom. The number of bonds that an atom can form as part of a compound is expressed by the valency of the element. An element’s valence is the number of hydrogen atoms which can combine with or replace (directly or indirectly) one of the element’s atoms An atom’s valence is equal to the number of electrons in the outer shell when that number is four or fewer. Then the valence in the outer shell is equal to maximum capacity in the shell minus the number of electrons present in the octet. The oxidation state of an atom in a molecule gives the number of valence electrons it has gained or lost. In contrast to the valency number, the oxidation state can be positive or negative for an atom.

ii. Atomic size: Atomic size is the distance between the centre of the nucleus of an atom and its outermost shell. The atomic size of a chemical element is a measure of the size of its atoms. It represents the mean distance from the nucleus to the boundary of the surrounding cloud of electrons in their outermost shell.

iii. The atomic size decreases as we move from left to right in a period and it increases when we go down a group. This is because in periods the valence electrons are in the same outermost shell. The atomic number increases within the same period while moving from left to right which in turn increases the effective nuclear charge. The increase in attractive forces reduces the atomic size of the elements.

Note
Similarly, While moving left to right across a period, the number of valence electrons of elements increases and varies between 1 to 8 as the valence electron is being added to the same outermost shell varying its valency.