Among the elements of the main group, the first ionization energy increases in which direction on the periodic table?

• A. From right to left across a period
• B. From bottom to top within a group
• C. From left to right across a period
• D. From top to bottom within a group

Answer: (C)

Verified step by step guidance

1. Understand that ionization energy is the energy required to remove an electron from a gaseous atom or ion.
2. Recall that across a period (from left to right), the nuclear charge increases while the electron shielding remains relatively constant, causing a stronger attraction between the nucleus and the electrons.
3. Recognize that this stronger attraction makes it harder to remove an electron, so ionization energy increases from left to right across a period.
4. Consider the trend within a group (from top to bottom), where additional electron shells are added, increasing the distance between the nucleus and the outermost electron and increasing electron shielding.
5. This increased distance and shielding reduce the nuclear attraction on the outer electrons, causing ionization energy to decrease from top to bottom within a group.