Final answer:
if we consider the trend in atomic radius across a period, the correct answer is B) Decrease, and for the trend down a group, the correct answer is A) Increase.
Step-by-step explanation:
The trend in atomic radius for elements changes as you move across the periodic table. Within each period, the atomic radius decreases as the atomic number (Z) increases, for instance, from potassium (K) to krypton (Kr). Conversely, within each group such as the alkali metals, the atomic radius increases as the atomic number increases. This happens because as you move from left to right across a period, the effective nuclear charge experienced by the electrons increases, pulling them closer to the nucleus, resulting in a smaller atomic radius. Meanwhile, as you move down a group, the number of occupied principal energy levels increases and this effect outweighs the increase in nuclear charge, thus increasing the atomic radius.
So, if we consider the trend in atomic radius across a period, the correct answer is B) Decrease, and for the trend down a group, the correct answer is A) Increase.