Atomic size decreases

Analyze the question: We are asked where atomic size (atomic radius) gets smaller in the periodic table.

Explain the concept:

• Across a period (left to right): The number of protons (nuclear charge) increases, while electrons are added to the same energy level (shell). The stronger nuclear pull draws the electrons closer, so the atom becomes smaller.
• Down a group: New electron shells are added, and shielding by inner electrons increases. The outer electrons are farther from the nucleus, so atomic size increases.

Go through the options:

• A. Across a period from left to right: Correct. Nuclear charge increases and the shell number stays the same, so the radius decreases. Example: Na (0.186 nm) is larger than Cl (0.099 nm) in Period 3.
• B. Across a period from right to left: Wrong. This is the reverse direction, so size increases.
• C. Diagonally down the group: Not a standard trend, and moving down generally increases size.
• D. Down a group: Wrong. Size increases because more shells are added.
• E. With decrease in atomic number: Wrong and too vague. Within a group, a lower atomic number does mean a smaller atom, but across a period a lower atomic number means a larger atom, so this isn't a general rule.

Answer: Atomic size decreases across a period from left to right, which is option A.

Tip: Remember "Period → Proton pull → Pack in tighter" (size shrinks across a period), and "Group down → Grows" (size grows down a group). Example: Li is smaller than Cs (down the group), but Li is larger than F (across the period).