Q.How the atomic size vary in a group and across a period? Explain with suitable example.

Step 1. State the across-period trend. Moving left to right across a period, effective nuclear charge $Z_{eff}$ rises steadily (screening from core electrons stays roughly constant, section 7.5.1), pulling the valence electrons in more tightly — so atomic size DECREASES across a period.

Step 2. Give an example. Table 7.2, period 2: Li (152 pm) > Be (111 pm) > B (88 pm) > C (77 pm) > N (74 pm) > O (66 pm) > F (64 pm) — a steady shrink from group 1 to group 17.

Step 3. State the down-group trend. Moving down a group, a whole new outer shell is added at each period, and the resulting rise in core shielding outweighs the rise in nuclear charge, so $Z_{eff}$ on the valence electron actually FALLS — so atomic size INCREASES down a group. …

Stating both atomic-radius trends (section 7.5.2.1) with c …

• Giving the trend directions without any supporting numeric example from the chapter's own data. …