The correct order of atomic radius of Li, Na, Be and O is (a) Na > Li > Be > O (b) Na > Be > Li > O (c) Be > Li > Na > O (d) O > Be > Li > Na

The atomic radius of elements follows specific periodic trends: it increases down a group and decreases from left to right across a period [1]. Sodium (Na) and Lithium (Li) belong to Group 1 (alkali metals). Since Na is in the 3rd period and Li is in the 2nd, Na has a larger radius because it has more electron shells. Lithium (Li), Beryllium (Be), and Oxygen (O) are all in the 2nd period. As we move from left to right (Li to Be to O), the atomic number and nuclear charge increase, which pulls the electron cloud closer to the nucleus, thereby decreasing the atomic radius. Consequently, Na (Group 1, Period 3) is the largest, followed by Li (Group 1, Period 2), then Be (Group 2, Period 2), and finally O (Group 16, Period 2) as the smallest. The correct order is Na > Li > Be > O.

Sources

1. [1] https://en.wikipedia.org/wiki/Atomic_radii_of_the_elements_(data_page)