Question

Choose the atom with the more exothermic electron affinity in each pair: (a) \(S\) or \(A s ;\) (b) \(S\) i or \(N\).

Short answer

For each pair, the atom with the more exothermic electron affinity is: (a) Sulfur (S) and (b) Nitrogen (N).

Step-by-step solution

Understanding Electron Affinity

Electron affinity refers to the energy change that occurs when an electron is added to a neutral atom in the gaseous state to form a negative ion. Generally, electron affinity becomes more exothermic (more negative value) across a period from left to right on the periodic table and less exothermic as you move down a group.

Comparing Electron Affinities in Pair (a)

For the pair Sulfur (S) and Arsenic (As), both elements are in the same period (period 3) of the periodic table. Sulfur (S) is to the left of Arsenic (As), and therefore Sulfur is expected to have a more exothermic (more negative) electron affinity than Arsenic.

Comparing Electron Affinities in Pair (b)

For the pair Silicon (Si) and Nitrogen (N), both are in period 3 as well. However, Nitrogen is to the right of Silicon, and thus Nitrogen is expected to have a more exothermic electron affinity than Silicon.

Key concepts

Periodic Table Trends

The periodic table is not simply a tool for classifying elements, but it also reveals several trends in the properties of elements, including their electron affinity. This property indicates the energy released when an electron is added to an atom, and understanding this concept is critical in chemistry.

As you move across a period from left to right, the electron affinity generally increases. This means that atoms are more inclined to gain an electron, releasing more energy in the process. In contrast, when you move down a group, from top to bottom, this affinity tends to decrease. This is because atoms further down have more electron shells, making it more difficult for the nucleus to attract additional electrons.

Furthermore, there are some anomalies to be aware of. For instance, elements in Group 18 of the periodic table—the noble gases—do not follow this trend since they already possess a complete valence shell and are not inclined to attract more electrons.

Exothermic Electron Affinity

What Makes Electron Affinity Exothermic?

When electron affinity is described as exothermic, it means that the process of adding an electron to an atom releases heat. This release of energy occurs because the electron added to the atom results in a more stable, lower energy state for the resultant ion.

The more exothermic the electron affinity, the more energy is released, indicating a stronger attraction between the atom and the added electron. This strong attraction can be due to several factors, including the atom's nuclear charge and the distance of the outermost electron shell from the nucleus. For instance, halogens, found in Group 17 of the periodic table, have highly exothermic electron affinities because they require only one additional electron to achieve a stable octet formation.

Chemical Periodicity

Chemical periodicity refers to the repetitive patterns observed in the properties of elements when they are arranged in the periodic table. This concept suggests that elements with similar properties tend to occur at regular intervals when the table is arranged according to atomic number.

The underlying reason for this periodicity is the arrangement of electrons into various shells and subshells. As the atomic number increases, electrons fill these structures in a predictable manner, leading to recurring chemical properties. Properties such as atomic radius, ionization energy, and electron affinity exhibit this periodic behavior – changing in a somewhat systematic way across periods and down groups.

Understanding chemical periodicity not only helps in predicting the chemical behavior of elements but is also essential for explaining anomalies. It allows scientists to infer properties of unknown elements and to explore relationships between the elements already discovered.