Question

Which of the following will have lowest electron affinity? (a) Nitrogen (b) Oxygen (c) Argon (d) Boron

Short answer

Argon (c) will have the lowest electron affinity.

Step-by-step solution

Understanding Electron Affinity

Electron affinity refers to the amount of energy released when an electron is added to a neutral atom in the gaseous state to form a negative ion. Generally, as you move from left to right across a period on the periodic table, electron affinity increases. However, within a group, electron affinity decreases as you move down, because larger atoms with more electron shells have a less effective nuclear charge seen by the valence electrons.

Comparing Nitrogen and Oxygen

Nitrogen (N) and Oxygen (O) are within the same period with Nitrogen being to the left of Oxygen. Oxygen has a higher electron affinity than Nitrogen typically because it is more to the right. However, due to the stable half-filled p-orbital configuration of Nitrogen, its electron affinity is comparably less than what might be predicted strictly based on its position.

Analyzing Boron and Argon

Boron (B) is to the left of Nitrogen and Oxygen within the same period, which typically suggests a lower electron affinity. Argon (Ar), being a noble gas, has a complete valence shell and is not keen to attract additional electrons, leading to a very low or positive electron affinity because extra energy is required to add an electron to a full shell.

Determining the Element With the Lowest Electron Affinity

Considering the trends and exceptions, Boron normally has a lower electron affinity than Nitrogen and Oxygen because it is earlier in the period. However, between Boron and Argon, Argon has the lowest electron affinity as its full valence shell makes it energetically unfavorable to add an extra electron.

Key concepts

Periodic Table Trends

Understanding the periodic table trends is essential for predicting and explaining various chemical behaviors and properties. One of these properties is electron affinity, the measure of an atom's likelihood to accept an electron. Moving from left to right across a period, electron affinity generally increases because the atomic number rises, leading to a stronger pull from the nucleus on additional electrons.

However, this trend isn't uniform due to subatomic peculiarities. For example, the presence of a half-filled or completely filled subshell can influence electron affinity. A half-filled p-orbital is notably stable, which is why elements such as Nitrogen may not follow the expected trend strictly. Similarly, filled shells found in noble gases also disrupt these trends, resulting in atypical electron affinities.

The trends are due to the effective nuclear charge and the distance from the nucleus to the electron shells. As you go down a group, the size of atoms increases with more electron shells, which reduces electron affinity due to the weaker pull on the valence electrons by the increasingly distant nucleus.

Noble Gases

Noble gases occupy the far right column of the periodic table and are known for their low reactivity. This is due to their valence shells being full, meaning they have a stable electronic configuration and a full octet in their outermost electron layer. This full outer shell reflects their unique place in periodic trends, particularly for electron affinity.

Noble gases, such as Argon, have a very low, and often positive, electron affinity. This means that rather than releasing energy when gaining an electron, energy is required to add an electron. This runs counter to the typical trend of increasing electron affinity across a period. Their stable configuration makes them generally uninterested in accepting additional electrons, leading to the energy cost of overcoming this stability.

Argon's electron configuration is a prime example of the noble gases' electronic complacency. With its valance shell full, it's energetically unfavorable for Argon to gain an electron, making its electron affinity the lowest among the given options.

Valence Electrons

Valence electrons are the electrons in the outermost shell of an atom and play a crucial role in an element's chemical properties, including electron affinity. They are the electrons available for bonding and are the most influenced by an atom's environment due to their distance from the nucleus.

The number of valence electrons in an atom contributes to the stability of its electron configuration. Atoms tend to seek a full valence shell, akin to the noble gas configuration, leading to highly reactive chemical behavior in those lacking a full outer shell. Conversely, a full valence shell often indicates chemical inertness, as in noble gases.

Valence electrons also influence the energy changes associated with electron affinity. For instance, the addition of an electron to a nearly full valence shell might release energy, thus indicating a higher electron affinity. But adding an electron to a full valence shell (like in noble gases) requires energy, showing a low or even positive electron affinity. This understanding helps to explain the exception of Argon in the exercise, which despite its position on a later period, has a lower electron affinity due to having a full valence electron shell.