i) Valency: Valency is the power of an atom to combine with another atom by donating or receiving electrons in their outermost shell or Valency is the number of electrons gained or lost by an element to achieve stable noble gas configuration which is a fully filled octet.
For example: The electronic configuration of element Magnesium is: 2,8,2. The number of valence electrons in the outermost shell here as we can see is 2. Therefore the valency of element Magnesium is 2.
(Electronic configuration: It is the distribution of electrons of an atom or molecule in respective molecular or atomic orbitals.)
ii) Atomic size: This is the distance between the centre of the nucleus of an atom to its outermost shell. Hence when the number of shells increases, the atomic size increases too as the distance from the centre of the nucleus to the outermost shell increases.
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