Question

For each of the following pairs of elements \((\mathrm{Mg}\) and \(\mathrm{K}) \quad(\mathrm{F}\) and \(\mathrm{Cl})\) pick the atom with a. more favorable (exothermic) electron affinity. b. higher ionization energy. c. larger size.

Short answer

For the pair Mg and K: Mg has a more favorable electron affinity, higher ionization energy, and smaller size. For the pair F and Cl: F has a more favorable electron affinity, higher ionization energy, and smaller size.

Step-by-step solution

Pair 1: Mg (Magnesium) and K (Potassium)

a. Electron affinity: Electron affinity generally increases across a period from left to right and decreases down a group in the periodic table. As potassium (K) is below magnesium (Mg) in Group 1 and Group 2, it has less favorable (exothermic) electron affinity, and Mg has more favorable (exothermic) electron affinity. b. Ionization energy: Ionization energy generally increases across a period from left to right and decreases down a group. Since potassium (K) is below magnesium (Mg) in the periodic table, K has a lower ionization energy compared to Mg. c. Atomic size: Atomic size generally decreases across a period from left to right and increases down a group. As potassium (K) is below magnesium (Mg) in the periodic table, its atomic size is larger than that of Mg.

Pair 2: F (Fluorine) and Cl (Chlorine)

a. Electron affinity: Electron affinity generally increases across a period from left to right and decreases down a group. Since chlorine (Cl) is below fluorine (F) in Group 17, it has less favorable (exothermic) electron affinity, and F has more favorable (exothermic) electron affinity. b. Ionization energy: Ionization energy generally increases across a period from left to right and decreases down a group. Since chlorine (Cl) is below fluorine (F) in the periodic table, Cl has lower ionization energy compared with F. c. Atomic size: Atomic size generally decreases across a period from left to right and increases down a group. As chlorine (Cl) is below fluorine (F) in the periodic table, its atomic size is larger than that of F.

Key concepts

Electron Affinity

Electron affinity is the energy change that occurs when an electron is added to a neutral atom in the gas phase, forming a negative ion. It is an important periodic table trend and helps us understand an atom's tendency to accept an electron. Elements with a more exothermic electron affinity release more energy when they gain an electron. This means that such elements have a stronger desire to gain electrons.
For example, in Pair 1, magnesium (Mg) and potassium (K), magnesium exhibits a more favorable electron affinity. Even though both are metals and typically do not have high electron affinities compared to nonmetals, Mg is positioned in the same period to the right of K, indicating it has a slightly higher electron affinity.
In Pair 2, between fluorine (F) and chlorine (Cl), fluorine has a more favorable electron affinity. Despite both being halogens,

• fluorine is smaller and has electrons closer to the nucleus,
• making it more energetically favorable for it to gain electrons and release energy.

This exothermic process aligns with their respective positions on the periodic table, where electron affinity usually increases across a period and decreases down a group.

Ionization Energy

Ionization energy refers to the energy required to remove an electron from a gaseous atom or ion. It is an indication of how strongly an atom holds onto its electrons.
Generally, ionization energy increases across a period and decreases down a group. This trend occurs due to increasing nuclear charge as you move across a period, which pulls electrons closer, and the addition of extra shells as you go down a group, which makes it easier for outer electrons to be removed.
For Pair 1, between

• magnesium (Mg) and potassium (K),
• magnesium has a higher ionization energy since it holds onto its electrons more tightly due to its position further right and up on the periodic table compared to potassium.

In Pair 2, between fluorine (F) and chlorine (Cl), fluorine possesses a higher ionization energy as it is positioned above chlorine in the periodic table.

• This means the electrons are closer and more tightly bound to the nucleus.

Both these examples demonstrate how position on the periodic table can predict ionization energy.

Atomic Size

Atomic size, or atomic radius, indicates the size of an atom. It is determined by the area in which electrons orbit the nucleus.
This property is crucial for understanding how atoms might interact with other atoms since larger atoms tend to form bonds differently compared to smaller ones. Atomic size generally decreases across a period and increases down a group.
In Pair 1, featuring magnesium (Mg) and potassium (K), potassium is larger because it is located down the group compared to magnesium. This increase in size is due to the addition of another electron shell in potassium.
For Pair 2, when comparing fluorine (F) and chlorine (Cl), chlorine is larger than fluorine. Although both are halogens, chlorine's additional electron shell compared to fluorine results in a larger atomic radius.

• These differences are essential for predicting chemical behavior.

A greater atomic size also influences the ease with which an atom can lose or gain electrons, which ties into other trends like ionization energy and electron affinity.