Across a period from left to right, the general rise in first ionization energy is mainly due to

Correct Answer: Increase in nuclear charge

Explanation:

• As we move across a period from left to right in the periodic table, protons are added to the nucleus, so the nuclear charge increases.
• The additional electrons are placed in the same principal energy level, meaning the shielding (screening) effect does not increase significantly.
• The stronger attraction between the positively charged nucleus and the negatively charged electrons makes it more difficult to remove an electron.
• This leads to a general rise in the first ionization energy across the period.

Therefore, the main reason for the increase in ionization energy is the increase in nuclear charge.