Question

The highest first ionization energy of the following is that of (a) \(\mathrm{Cs}_{i}\) (b) \(\mathrm{Cl}_{i}\) (c) I; (d) Li.

Short answer

The element with the highest first ionization energy is Cl

Step-by-step solution

Understanding the concept of ionization energy

Ionization energy is the energy required to remove an electron from an atom. It increases from left to right across a period and decreases from top to bottom down a group in the Periodic Table.

Positioning the given elements on the Periodic Table

On a Periodic Table, you can observe that Cs is placed at the bottom left, I and Cl are in the same group (Group 17 - Halogens), but I is placed below Cl, and Li is placed at the top left.

Identifying the trend

Considering the periodic trends in ionization energy, we can infer that Cl should have the highest ionization energy among the given elements because it's further to the right of Li (meaning it's more electronegative and holds onto its electrons more tightly) and above I (meaning it's less shielded and its effective nuclear charge is greater, which also means it holds onto its electrons more tightly). In contrast, Cs should have the lowest ionization energy because it's at the bottom left of the Periodic Table.

Key concepts

Periodic Trends

In chemistry, understanding periodic trends is key to predicting how elements behave. These trends refer to the patterns observed in the properties of elements as you move across and down the Periodic Table. One major trend is the change in ionization energy, which is especially relevant when you're comparing elements like Cl and Cs.
Ionization energy increases as you move from left to right across a period. Why? Because atoms hold onto their electrons more tightly as they have more protons creating a stronger positive charge in the nucleus, attracting the electrons. Conversely, ionization energy decreases as you move from top to bottom down a group. As you go down a group, atoms get larger with more electron shells, so their outer electrons are further from the nucleus, thus less tightly bound.

• Left to right: Ionization energy increases.
• Top to bottom: Ionization energy decreases.

Understanding these trends explains why Cl, which sits to the right of Li and above I in the Halogen group, has a higher ionization energy than either of them.

Electronegativity

Electronegativity is another important periodic trend closely related to ionization energy. It measures an atom's ability to attract and hold onto electrons in a chemical bond. Generally, electronegativity increases across a period and decreases down a group. This trend is similar to that of ionization energy, as atoms with high electronegativity also tend to have high ionization energies.
In the context of the given elements, Cl and I are both halogens and are in Group 17 of the Periodic Table. However, Cl is more electronegative than I, which is part of why Cl also has a higher ionization energy. Higher electronegativity means that Cl holds onto its electrons more strongly. This makes it less willing to give up an electron, thereby having a higher ionization energy.

• Electronegativity increases across a period.
• Electronegativity decreases down a group.

These electronegativity trends further reinforce why Chlorine is expected to have a higher first ionization energy compared to Iodine and Cesium.

Halogens

The Halogens, found in Group 17 of the Periodic Table, include fluorine, chlorine, bromine, iodine, and astatine. These elements are unique in their chemical properties largely due to their high electrularity and reactivity. Considered to be highly electronegative, halogens want to gain an electron to achieve a stable electronic structure, which makes them react readily, especially with alkali metals like lithium.
This high reactivity doesn't just lead to chemical reactions; it also links directly to the patterns in ionization energy we've discussed. As a group, they show decreasing ionization energy from top to bottom. Chlorine, being higher up in the group than iodine, has higher ionization energy because electrons are closer to the nucleus and more strongly attracted to it.
When reviewing periodic trends and ionization energies, it's essential to remember:

• The halogens display unique trends due to their reactivity and electronegativity.
• Chlorine's position above iodine makes it energetically less favorable to lose an electron.

This understanding of halogens further contributes to predicting which among Group 17 elements would exhibit the highest or lowest ionization energies.