Question

Which of the following has lowest melting point? (a) \(\mathrm{Cs}\) (b) \(\mathrm{Na}\) (c) Li (d) \(\mathrm{K}\)

Short answer

Cesium ( Mathrm{Cs}) has the lowest melting point.

Step-by-step solution

Analyze Periodic Trend

The melting point of alkali metals decreases as we move down the group in the periodic table. This is because the atomic size increases, leading to weaker metallic bonds.

Determine Group Membership

All given elements—Cs, Na, Li, and K—belong to the same group in the periodic table, which is Group 1 (alkali metals).

Compare Atomic Sizes

The atomic size increases from top to bottom in Group 1. Therefore, Li (lithium) has the smallest atomic size, and Cs (cesium) has the largest atomic size.

Apply Trend to Elements

Since the melting point decreases with increasing atomic size, lithium has the highest melting point, and cesium has the lowest.

Identify Element with Lowest Melting Point

Based on the trend and atomic sizes, cesium ( Mathrm{Cs}) has the lowest melting point among the given options.

Key concepts

Periodic Trends

Periodic trends refer to patterns in the properties of elements that repeat across periods and groups in the periodic table. These trends include variations in atomic size, ionization energy, electron affinity, and electronegativity. One important trend involving alkali metals—found in Group 1 of the periodic table—is their melting points.
As you move down a group, such as the alkali metals, certain properties change in predictable ways. For example:

• Atomic size increases.
• The strength of metallic bonds decreases.
• Melting and boiling points generally decrease.

This decrease in melting point as you move down the Group 1 elements is due to the increase in atomic size and the weakening of metallic bonds, making them less stable under heat.

Atomic Size

Atomic size, or atomic radius, is the distance from the nucleus of an atom to the outermost electrons. It informs us about the size of an atom and varies across the periodic table.
In a group such as the alkali metals (Group 1), atomic size increases as you move down the group. This trend occurs because with each successive element, an additional electron shell is added, increasing the distance between the nucleus and the outermost electrons.
This greater distance results in:

• Weaker attraction between the nucleus and the electrons.
• A larger atomic radius.
• Weaker metallic bonds due to the increased separation of atoms.

Understanding the concept of atomic size is crucial, especially when predicting and explaining periodic trends such as decreasing melting points in alkali metals.

Metallic Bonds

Metallic bonds are a type of chemical bonding that occurs between metal atoms. In these bonds, electrons are shared over many nuclei and can move freely, which is why metals are good conductors of electricity.
Strength of metallic bonds influences various properties of metals, such as:

• Melting and boiling points.
• Hardness and ductility.

In Group 1 elements (alkali metals), the strength of metallic bonds decreases as you move down the group. This weakening is due to the increased atomic size, which lessens the attraction between the nucleus and the electrons that partake in metallic bonding. As a result, elements such as cesium have weaker bonds and lower melting points compared to lithium, which has stronger bonds.

Group 1 Elements

Group 1 elements in the periodic table are known as alkali metals. This group includes lithium, sodium, potassium, rubidium, cesium, and francium. These metals are characterized by having one electron in their outermost electron shell, making them highly reactive.
Key properties of alkali metals include:

• Low melting and boiling points compared to other metals.
• High reactivity, especially with halogens and water.
• Softer compared to many other metals.
• Their tendency to lose the single electron in their outer shell, forming M^+ ions.

The arrangement of alkali metals within Group 1 demonstrates how their properties change consistently and predictably as you move down the group. For example, reactivity and atomic size increase, while melting points decrease. Understanding these properties helps explain why cesium has the lowest melting point among the alkali metals mentioned.