Question

Which of the alkali metal is having least melting point? (a) Na (b) \(\mathrm{K}\) (c) \(\mathrm{Rb}\) (d) Cs

Short answer

Cesium (Cs) has the lowest melting point among the alkali metals listed.

Step-by-step solution

Understanding Alkali Metals

Alkali metals are found in Group 1 of the periodic table and include elements like Sodium (Na), Potassium (K), Rubidium (Rb), and Cesium (Cs). These metals are known to have one electron in their outermost shell, which makes them highly reactive and affects their physical properties such as melting points.

Melting Points Trend in Alkali Metals

As we move down the group in the periodic table from Lithium to Cesium, the atomic size increases. This leads to weaker metallic bonds in larger atoms and results in lower melting points. Hence, the melting points decrease as you go from top to bottom in the alkali metal group.

Identifying the Element with Lowest Melting Point

Based on the trend, Cesium (Cs), being the furthest down the group, is expected to have the lowest melting point among the alkali metals given in options: Na, K, Rb, and Cs.

Key concepts

Melting Points

Melting points refer to the temperature at which a solid substance turns into a liquid. For alkali metals, this physical property is significantly influenced by their atomic structure and size. These metals are characterized by a single electron in their outer shell, which forms metallic bonds. As the atomic size increases, the strength of these metallic bonds decreases. This decrease in bond strength results in lower melting points. For instance, among the alkali metals, Cesium (Cs) has a notably low melting point due to its larger atomic size compared to other members of its group.

Group 1 Elements

Group 1 elements are commonly known as the alkali metals, including Lithium (Li), Sodium (Na), Potassium (K), Rubidium (Rb), Cesium (Cs), and Francium (Fr). These elements are highly reactive, especially with water, and are characterized by a silvery luster and softness. Each of these metals has one electron in its outermost energy level, which contributes to its high reactivity.

• They are good conductors of electricity.
• Have low densities compared to other metals.
• React vigorously with water.

The reactivity increases as you move down the group due to the increasing atomic radius, which makes it easier for the outer electron to be lost in chemical reactions.

Metallic Bonds

Metallic bonds are the force of attraction between valence electrons and metal atoms. These bonds are characterized by the 'sea of electrons' model, where electrons move freely, giving metals their conductive properties.

• The strength of these bonds affects the melting and boiling points of metals.
• Stronger metallic bonds usually result in higher melting points.

In alkali metals, because each atom donates only one electron to the electron sea, the resultant metallic bond is weaker than that found in metals with more valence electrons. This contributes to the relatively low melting points observed in Group 1 elements.

Periodic Table Trends

Periodic table trends refer to the patterns observed in the properties of elements as you move across the periodic table. In Group 1, alkali metals show clear trends in several properties, like atomic radius, ionization energy, and melting point.

• As you move down the group, atomic radius increases.
• Ionization energy decreases, leading to heightened reactivity.
• Melting points decrease due to weaker inter-atomic bonds.

These trends are due to the additional electron shells as you go down the group, which increases the distance between the nucleus and the outermost electron, reducing the effective nuclear charge experienced by valence electrons.