Question

Arrange the elements in each of the following groups in (b) F, order of increasing electron affinity: (a) \(\mathrm{Li}, \mathrm{Na}, \mathrm{K}\) Cl. Br. I.

Short answer

(a) \\( \text{K} < \text{Na} < \text{Li} \\); (b) \\( \text{I} < \text{Br} < \text{Cl} \\).

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 to form a negative ion. In general, across a period in the periodic table, electron affinity increases, and down a group, it usually decreases. This is due to the increased nuclear charge across periods and increased electron shielding down groups.

Analyzing Halogens

The group consisting of \( ext{Cl, Br, I}\) represents the halogens, which are in Group 17 of the periodic table. In this group, as you move down, electron affinity tends to decrease because the added electron is further from the nucleus and experiences more electron shielding.

Ordering Halogens by Electron Affinity

Based on the trend, the order of increasing electron affinity for the halogens is: \( ext{I} < ext{Br} < ext{Cl}\). Chlorine (Cl) has the highest electron affinity among these elements.

Analyzing Alkali Metals

The group consisting of \( ext{Li, Na, K}\) represents the alkali metals. In Group 1, electron affinity generally decreases down the group due to increased electron shielding and the added electron being placed in a higher energy level further from the nucleus.

Ordering Alkali Metals by Electron Affinity

For the alkali metals, the order of increasing electron affinity is \( ext{K} < ext{Na} < ext{Li}\). Lithium (Li) has the highest electron affinity among the three elements.

Key concepts

Periodic Table

The periodic table is a systematic way of organizing the chemical elements, ordered by their atomic number, electron configuration, and recurring properties. Elements are arranged in rows, called periods, and columns, known as groups or families. Each element in a period shares the same number of electron shells, while elements in a group have similar chemical properties. The organization allows us to understand various trends, including electron affinity, across the periodic table.

Electron affinity is an important concept when discussing the periodic table. It is the amount of energy released when an electron is added to a neutral atom to form a negative ion. Generally, electron affinity increases across a period due to increased nuclear charge that attracts the added electron more strongly.

• As you move from left to right across a period, elements are more likely to gain electrons to achieve stable electronic configurations.
• As you move down a group, elements tend to show decreased electron affinities because the increasing size reduces the influence of the nucleus on added electrons.

Understanding these trends helps predict the chemical behavior of elements, particularly their ability to form ions, making the periodic table not only a tool for organization but also a predictive model for chemical reactions.

Halogens

Halogens comprise Group 17 of the periodic table and include elements like fluorine (F), chlorine (Cl), bromine (Br), and iodine (I). These elements are known for their high electron affinities because they have seven valence electrons and need only one more electron to achieve a stable, octet configuration. Their ability to gain an extra electron makes them very reactive, often forming ionic bonds with metals.

Among the halogens, however, there are differences in electron affinity.

• Fluorine has a lower electron affinity than expected, due to its small size which results in electron-electron repulsion in the crowded outer shell.
• Chlorine has the highest electron affinity of the halogens, making it very effective in gaining electrons.
• As you move down the group to bromine and iodine, electron affinity decreases. This trend is due to increased atomic size and electron shielding, which weaken the core-nucleus interaction with the added electron.

Therefore, when ranking these elements by electron affinity, the order is generally: I < Br < Cl, with chlorine having the highest affinity among them. This understanding is essential when predicting their reactivity and role in various chemical reactions.

Alkali Metals

Alkali metals make up Group 1 of the periodic table. This group includes elements such as lithium (Li), sodium (Na), and potassium (K). These metals are characterized by having a single electron in their outermost shell, which they can easily lose to form positive ions, rather than gaining electrons. As such, their electron affinities tend to be lower compared to nonmetals like the halogens.

However, within the alkali metals, electron affinity decreases down the group.

• Lithium has the highest electron affinity among these metals. Its smaller atomic size means that incoming electrons are relatively closer to the nucleus and under its influence, though still less so than nonmetals.
• As you move to sodium and then to potassium, the increasing atomic size and electron shielding reduce the nuclear pull on an added electron.

The decreasing electron affinity in alkali metals as you go from Li < Na < K is attributed to these factors. Nonetheless, alkali metals are more known for their tendency to lose electrons and form cations, making them highly reactive, especially in reactions with halogens to form salts.