Moving from left to right across a period, the general rise in the first ionization energy can be attributed to the?

Analysis

The question asks for the reason behind the general increase in first ionization energy as we move from left to right across a period in the Periodic Table.

Concept Explanation

Ionization Energy is the minimum amount of energy required to remove the most loosely held electron from an isolated gaseous atom.

As you move from left to right across a period:

1. Nuclear Charge: The number of protons in the nucleus increases. This creates a stronger positive pull on the electrons.
2. Atomic Radius: Because the nucleus is pulling the electrons more strongly, the electron cloud is pulled closer to the nucleus, causing the atomic radius to decrease.
3. Screening (Shielding) Effect: The number of inner electron shells remains the same across a period. Therefore, the screening effect remains relatively constant and does not significantly change.

Because the nuclear charge increases while the distance between the nucleus and the valence electrons decreases (and the screening effect stays constant), the electrostatic force of attraction between the nucleus and the valence electrons becomes much stronger. Consequently, it requires more energy to remove an electron.

Step-by-Step Solution

1. Identify the trend: Across a period, the atomic number increases, meaning the number of protons (nuclear charge) increases.
2. Relate to force: According to Coulomb’s Law, a higher nuclear charge exerts a stronger pull on the valence electrons.
3. Relate to energy: A stronger pull means the electrons are held more tightly. Therefore, more energy is needed to overcome this attraction to remove an electron.
4. Evaluate options: • A is incorrect because nuclear charge increases, not decreases. • B is correct because the increasing positive charge of the nucleus is the primary driver for the increased hold on electrons. • C and D are incorrect because the screening effect remains roughly constant across a period, so it is not the cause of the rise in ionization energy.

Exam Tip

Always remember: "Across a period, the nucleus gets stronger." If you ever get confused, think of the nucleus as a magnet. As you add more protons (moving right), the magnet gets stronger, making it harder to pull an electron away.