Question

The melting points of alkaline earth metals are many times higher than those of the alkali metals. Explain this difference on the basis of atomic properties. Name three other physical properties for which Group \(2 \mathrm{~A}(2)\) metals have higher values than the corresponding \(1 \mathrm{~A}(1)\) metals.

Short answer

Group 2A metals have higher melting points, densities, boiling points, and hardness compared to Group 1A metals due to stronger metallic bonds and higher effective nuclear charge.

Step-by-step solution

- Understand the Metals Groups

Group 1A (alkali metals) includes elements like lithium, sodium, and potassium. Group 2A (alkaline earth metals) includes elements like beryllium, magnesium, and calcium.

- Compare Melting Points

Alkaline earth metals have higher melting points compared to alkali metals. This is due to the stronger metallic bonding in Group 2A metals, as each atom contributes two electrons to the metallic bond, compared to just one electron from Group 1A metals.

- Examine Atomic Properties

The effective nuclear charge experienced by the valence electrons is higher in Group 2A metals. Consequently, they have stronger interatomic attractions, leading to higher melting points.

- Investigate Other Physical Properties

In addition to melting points, Group 2A metals have higher densities, higher boiling points, and greater hardness compared to Group 1A metals. These differences can also be attributed to the stronger metallic bonds and higher effective nuclear charge in Group 2A metals.

Key concepts

Metallic Bonding

Metallic bonding plays a crucial role in determining the physical properties of metals, such as melting points. In metals, atoms release some of their electrons to create a 'sea of electrons', which move freely around the positively charged ions. This creates a strong electrostatic attraction that holds the ions together.

• Alkali Metals (Group 1): These metals contribute only one electron per atom to the electron sea.
• Alkaline Earth Metals (Group 2): These metals contribute two electrons per atom.

This difference means that the bonds in alkaline earth metals are stronger, leading to higher melting points. More electrons in the metallic bond means stronger attractions between the ions and the 'sea of electrons'.
The stronger the metallic bonds, the more energy is required to break them, resulting in higher melting points for the alkaline earth metals compared to the alkali metals.

Effective Nuclear Charge

Effective Nuclear Charge (ENC) is the net positive charge experienced by an electron in a multi-electron atom. It affects how tightly electrons are held by the nucleus.

• Group 1 Metals: The ENC experienced by the single valence electron is lower.
• Group 2 Metals: The ENC experienced by the two valence electrons is higher.

Higher ENC means stronger attraction between the nucleus and the valence electrons. This makes it harder to separate the atoms, resulting in stronger metallic bonds and hence, higher melting points for alkaline earth metals. Moreover, a higher effective nuclear charge in Group 2 metals also contributes to other physical properties, such as higher densities and greater hardness, since the atoms are more tightly bound together.

Physical Properties of Metals

Different groups of metals show varied physical properties due to their atomic structure and bonding. Here are some valuable comparisons:

• **Density**: Alkaline earth metals tend to have higher densities compared to alkali metals because of their smaller atomic radii and higher atomic masses.
• **Boiling Points**: Similar to melting points, the boiling points of alkaline earth metals are higher due to stronger metallic bonds.
• **Hardness**: Alkaline earth metals are harder than alkali metals. This is because the stronger metallic bonding in Group 2 metals leads to a more rigid structure.

These differences in physical properties underline the significance of metallic bonding and effective nuclear charge in determining the characteristics of metals. To sum up, the stronger bonds and higher ENC cause alkaline earth metals to possess higher melting points, densities, boiling points, and hardness compared to alkali metals.