Valency is a fundamental concept in chemistry that describes the ability of an atom to combine with other atoms. It is determined by the number of electrons in the outermost shell (valence shell) of an atom. Understanding valency is essential for predicting how elements will react and form compounds.

1. Definition of Valency:

Valency is defined as the combining capacity of an atom. It can be understood as the number of electrons an atom can gain, lose, or share to achieve a stable electronic configuration, usually resembling that of the nearest noble gas. Valency can be expressed in terms of:

• The number of electrons lost or gained.
• The number of bonds formed with other atoms.

2. Types of Valency:

Valency can be classified into several types:

• Positive Valency: This occurs when an atom loses electrons. For example, sodium ($\text{Na}$) has a valency of +1 because it loses one electron to form $\text{Na}^+$.
• Negative Valency: This occurs when an atom gains electrons. For example, chlorine ($\text{Cl}$) has a valency of -1 because it gains one electron to form $\text{Cl}^-$.
• Zero Valency: Some elements have a valency of zero, indicating that they do not readily combine with other elements. Noble gases like helium ($\text{He}$) and neon ($\text{Ne}$) have a complete outer shell of electrons.
• Variable Valency: Some elements can exhibit more than one valency. For example, iron ($\text{Fe}$) can have a valency of +2 or +3, depending on the compound it forms.

3. Determining Valency:

To determine the valency of an element, we can follow these guidelines:

• For main group elements, the valency is typically equal to the number of electrons in the outermost shell.
• Elements in Group 1 (alkali metals) have a valency of +1, while Group 2 (alkaline earth metals) have a valency of +2.
• Group 17 elements (halogens) usually have a valency of -1, while Group 16 elements (chalcogens) have a valency of -2.

4. Examples of Valency:

Here are some common elements and their valencies:

• Hydrogen ($\text{H}$): Valency of +1
• Carbon ($\text{C}$): Valency of +4 (can form four bonds)
• Nitrogen ($\text{N}$): Valency of +3 or -3
• Oxygen ($\text{O}$): Valency of -2
• Sodium ($\text{Na}$): Valency of +1
• Chlorine ($\text{Cl}$): Valency of -1

5. Importance of Valency:

Valency is crucial in understanding chemical bonding and the formation of compounds. It helps in predicting how different elements will combine to form chemical formulas. For instance, the valency of magnesium ($\text{Mg}$), which is +2, allows it to combine with two chlorine atoms (valency -1) to form magnesium chloride ($\text{MgCl}_2$).

By knowing the valencies of different elements, chemists can easily write balanced chemical equations, understand reaction mechanisms, and predict the properties of compounds. Valency also plays a key role in determining the stability and reactivity of chemical substances.