How do atomic radii vary (a) down the group and (b) along the period from left to right?
How do atomic radii vary (a) down the group and (b) along the period from left to right?
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a. The atomic radius increases down the group `(darr)`. This is due to (i) decrease of `Z_(eff)` (effective nuclear charge) and (ii) increase of number of shells or principal quantum number `(n)`.
i. Effective nuclear charge `(Z_(eff))`: The `Z_(eff)` decreases down the group, due to increase in sreening constant `(sigma)` with the increase in number of inner electrons. Lesser is `Z_(eff)` lesser is the attraction between the nucleus and outer most electrons and consequently the size of the atom increases.
ii. Number of shells: As the total number of shells or principal quantrum number `(n)` increases, the outer electrons get farther and farther away from the nucleus adn hence atomic and ionic radii increase.
b. The atomic radius decreases along the period `(rarr)`. Along the period, the outer electrons remain in the same shell but the nuclear charge increses by one unit at each succeeding element. Thus, `Z_(eff)` also increases along the period. Due to this increased `Z_(eff)` the attraction between nucleus and the outermost electrons increase and atomic size decreases.
i. Effective nuclear charge `(Z_(eff))`: The `Z_(eff)` decreases down the group, due to increase in sreening constant `(sigma)` with the increase in number of inner electrons. Lesser is `Z_(eff)` lesser is the attraction between the nucleus and outer most electrons and consequently the size of the atom increases.
ii. Number of shells: As the total number of shells or principal quantrum number `(n)` increases, the outer electrons get farther and farther away from the nucleus adn hence atomic and ionic radii increase.
b. The atomic radius decreases along the period `(rarr)`. Along the period, the outer electrons remain in the same shell but the nuclear charge increses by one unit at each succeeding element. Thus, `Z_(eff)` also increases along the period. Due to this increased `Z_(eff)` the attraction between nucleus and the outermost electrons increase and atomic size decreases.
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