How do atomic radius vary in a period and in a group? How do you explain the variation?
Variation of atomic radii in a group. As we move down a group in the periodic table, the atomic radii go on increasing.
Reasons :
(i) Down a group, the nuclear charge goes on increasing. As a result, atomic radii must decrease.
(ii) As we move down a group, a new enthalpy shell is added at each succeeding element though the number of electrons in the valence shell the remain the same. As a result, atomic radii must increase.
The effect of the progressive addition of a new shell outweighs the effect of increased nuclear charge. Hence atomic radii of elements increase with the increase in atomic number as we move from top to bottom down a group.
Variation of atomic radii in a period. As we move from left to right in a period, atomic radii go on decreasing.
Reason: As we move from left to right in a particular period, the atomic number i.e. nuclear charge increases by one unit in each succeeding element but the corresponding addition of electron takes place in the same enthalpy shell. As a result, electrons are pulled little closer to the nucleus thereby making each individual shell smaller and smaller.
Why do the noble gas elements have exceptionally high atomic radii?
Or
In a period, generally the size of the atom decreases with rise in atomic number, but at the end of each period, atomic size of the noble gas increases abruptly. Why is it so?
