Question

Lithium is similar to magnesium in many properties. This is because (a) Both have nearly the same size. (b) The ratio of the charge to size is nearly the same (c) Both have similar electronic configurations (d) Both have nearly the same \(\mathrm{N} / \mathrm{P}\) ratio.

Short answer

(b) The ratio of the charge to size is nearly the same.

Step-by-step solution

Understanding the Elements

First, let's examine lithium (Li) and magnesium (Mg). Lithium is an alkali metal with atomic number 3, and magnesium is an alkaline earth metal with atomic number 12. While they belong to different groups in the periodic table, they exhibit diagonal similarities due to their position.

Analyzing Atomic and Ionic Sizes

Lithium has an atomic radius of about 152 pm, whereas magnesium has an atomic radius of approximately 160 pm. Their ionic sizes differ significantly, with Li+ having a smaller ionic radius than Mg2+. Thus, statement (a) is not correct.

Evaluating Charge to Size Ratio

The charge to size ratio (also known as charge density) affects the chemical reactivity and polarizing ability of ions. Both Li+ and Mg2+ have similarly high charge densities, leading to similar chemical behavior in some contexts. This makes statement (b) a correct observation.

Examining Electronic Configurations

Lithium has an electronic configuration of 1s² 2s¹, while magnesium has an electronic configuration of 1s² 2s² 2p⁶ 3s². Since these configurations are quite different, statement (c) is incorrect.

Checking Neutron-to-Proton Ratio

In general, the neutron-to-proton ratio varies among elements and isotopes. However, the N/P ratio is not a primary reason for chemical similarities between Li and Mg. Therefore, statement (d) is not the correct answer.

Key concepts

Element Properties

Elements in the periodic table have distinct properties due to their unique atomic structure. These properties dictate how each element behaves and reacts chemically.

• Chemical Reactivity: This describes how an element interacts with other substances. For instance, lithium is highly reactive, especially when exposed to water.
• Metallic Character: Refers to an element's ability to lose electrons and form positive ions. Both lithium and magnesium share this trait, contributing to their reactive similarities.
• Electronegativity: This is the ability of an atom to attract electrons. Although different, both lithium and magnesium generally exhibit low electronegativity values, which is a common feature in metals.

Understanding these properties is crucial, as they provide a foundation for explaining how elements like lithium and magnesium can show similar characteristics despite being in different groups of the periodic table.

Charge Density

Charge density is a key concept in understanding why lithium and magnesium exhibit diagonal relationships. It is defined as the charge of an ion divided by its volume.

• Lithium ion (\( Li^+ \)) and magnesium ion (\( Mg^{2+} \)) have high charge densities.
• High charge density means that the ions have a strong ability to polarize, which affects their chemical interactions.

This property leads to similarities in their chemical behavior, such as forming similar types of salts. The charge to size ratio plays a significant role here, enabling lithium and magnesium to depolarize other molecules effectively.

Atomic Radius

The atomic radius is a measure of the size of an atom's electron cloud. It plays a crucial role in defining an element's reactivity.

• Lithium has an atomic radius of about 152 pm.
• Magnesium has a slightly larger atomic radius of about 160 pm.

Despite these differences, lithium and magnesium can still exhibit similar properties because their atomic sizes fall within a comparable range. However, it is important to note that other factors such as electronic configuration and charge density also influence the behavior of these elements.

Ionic Size

Ionic size is different from atomic size as it measures the radius of an ion rather than a neutral atom.

• The lithium ion (\( Li^+ \)) is significantly smaller compared to the magnesium ion (\( Mg^{2+} \)).
• Such differences in ionic size influence how these ions interact with other atoms and molecules, determining the structure and stability of compounds they form.

Interestingly, despite the size differences, the charge densities of lithium and magnesium ions are quite similar. This can lead to similar polarizing effects, compensating for the difference in ionic sizes between the two elements.

Periodic Table Trends

The periodic table is carefully organized to reflect the trends and characteristics of elements as their atomic number increases. These trends include variations in atomic radius, ionization energy, and electronegativity.

• Atomic Radius: As you move across a period, atomic radius generally decreases, while it increases down a group.
• Ionization Energy: The energy required to remove an electron generally increases across a period and decreases down a group.
• Electronegativity: Typically increases across a period and decreases down a group.

Lithium and magnesium demonstrate diagonal relationships due to these trends. Their position on the periodic table allows them to share similarities, such as high charge density, despite being in separate groups.