Why does the atomic radius decrease as you move from left to right across a period in the periodic table?

• A. Because electrons are lost from the outer shell, reducing electron-electron repulsion.
• B. Because additional electron shells are added, increasing the distance between the nucleus and outer electrons.
• C. Because the atomic mass decreases, causing the atom to shrink.
• D. Because the number of protons increases, resulting in a greater effective nuclear charge that pulls electrons closer to the nucleus.

Answer: (D)

Verified step by step guidance

1. Understand that as you move from left to right across a period in the periodic table, the number of protons in the nucleus increases. This increase in positive charge is called the nuclear charge.
2. Recognize that electrons are being added to the same principal energy level (same shell), so the distance between the nucleus and the outermost electrons does not increase significantly.
3. Know that the increased positive charge from the additional protons pulls the electrons closer to the nucleus, increasing the effective nuclear charge experienced by the electrons.
4. Realize that although electrons repel each other, the increase in nuclear charge outweighs the electron-electron repulsion, causing the atomic radius to decrease.
5. Summarize that the atomic radius decreases across a period primarily because the effective nuclear charge increases, pulling electrons closer and reducing the size of the atom.