Why does the first ionization energy generally increase across a period for the main group elements?

• A. Because the number of energy levels increases, increasing electron shielding.
• B. Because the elements become more metallic, which increases ionization energy.
• C. Because the atomic radius increases, making electrons easier to remove.
• D. Because the nuclear charge increases while the shielding effect remains relatively constant, making it harder to remove an electron.

Answer: (D)

Verified step by step guidance

1. Understand that ionization energy is the energy required to remove an electron from an atom in its gaseous state.
2. Recognize that as you move across a period from left to right, the number of protons in the nucleus (nuclear charge) increases, which means the positive charge attracting the electrons becomes stronger.
3. Note that the electrons are being added to the same principal energy level (same shell), so the shielding effect, which is the repulsion caused by inner electrons, remains relatively constant and does not increase significantly.
4. Because the increased nuclear charge is not offset by increased shielding, the effective nuclear charge experienced by the outermost electrons increases, pulling them closer to the nucleus.
5. This stronger attraction makes it more difficult to remove an electron, so the first ionization energy generally increases across a period.