Understanding Variable Valency in Elements

The question asks us to identify which among the given elements exhibits variable valency. Valency refers to the combining capacity of an element, essentially how many bonds an atom of that element can form or the charge it typically carries as an ion. Variable valency means an element can show more than one valency in different compounds.

What is Variable Valency?

Elements with variable valency can exist in different oxidation states. This is common among transition metals and some non-metals, where electrons from different energy levels can participate in bonding.

Analyzing the Options

Let's examine each element provided in the options:

• Iron (Fe): Iron is a transition metal. Transition metals are well-known for exhibiting variable valency. Iron commonly forms two types of ions:
  • Fe²⁺ (Ferrous ion), where its valency is 2.
  • Fe³⁺ (Ferric ion), where its valency is 3.
  For example, in Iron(II) oxide (FeO), the valency of iron is 2, while in Iron(III) oxide (Fe₂O₃), the valency of iron is 3. Since Iron can have both 2 and 3 as its valency, it shows variable valency.
• Sodium (Na): Sodium is an alkali metal from Group 1 of the periodic table. Elements in Group 1 typically lose one electron to achieve a stable electron configuration, forming ions with a +1 charge (Na⁺). Sodium almost always exhibits a valency of 1 in its compounds. It does not show variable valency under normal conditions.
• Sulphur (S): Sulphur is a non-metal from Group 16. Sulphur can exhibit multiple valencies, such as 2, 4, and 6, in different compounds (e.g., H₂S, SO₂, SO₃). So, Sulphur also shows variable valency. However, we must select the best fit from the given options based on common examples of variable valency.
• Magnesium (Mg): Magnesium is an alkaline earth metal from Group 2 of the periodic table. Elements in Group 2 typically lose two electrons to achieve a stable electron configuration, forming ions with a +2 charge (Mg²⁺). Magnesium almost always exhibits a valency of 2 in its compounds. It does not show variable valency under normal conditions.

Conclusion on Variable Valency

Based on the analysis of each element, Iron exhibits variable valency (2 and 3). While Sulphur also shows variable valency, Iron is also a correct example and is often cited as a classic case of variable valency among the given options.

Therefore, among the provided choices, Iron is an element that has variable valency.

Revision Table: Elements and Valency

This table helps summarise the valency characteristics of some common elements.

Additional Information: Oxidation States and Variable Valency

Variable valency is closely related to the concept of oxidation states. The oxidation state of an element in a compound indicates the total number of electrons that have been removed from an element (positive oxidation state) or added to an element (negative oxidation state) to reach its present state. Many elements, particularly transition metals, can exist in multiple stable oxidation states, which directly corresponds to their variable valency.

For example, Iron can exist in +2 and +3 oxidation states. The presence of incompletely filled d-orbitals in transition metals allows for electrons from both s and d subshells to participate in bonding, leading to multiple possible oxidation states and thus variable valency.