Valency

Key Notes:

Definition of Valency:

• Valency is the combining capacity of an element, indicating how many atoms of other elements it can combine with to form a compound.
• It reflects the number of electrons an atom can lose, gain, or share to achieve a stable electronic configuration (usually that of a noble gas).

Valence Electrons:

• The electrons in the outermost shell of an atom are known as valence electrons.
• These electrons determine the valency of an atom as they participate in chemical bonding.

Types of Valency:

• Positive Valency: If an atom loses electrons to achieve stability (commonly seen in metals), its valency is positive.
• Negative Valency: If an atom gains electrons to achieve stability (common in non-metals), its valency is negative.
• Zero Valency: Noble gases have a valency of zero since they already have a stable electronic configuration.

Valency and the Octet Rule:

• Atoms strive to have 8 electrons in their outermost shell (octet rule), which is why they either lose, gain, or share electrons during chemical reactions.
• For elements with fewer electrons in their outermost shell (like hydrogen, helium, and lithium), achieving a stable configuration often means having 2 electrons (duplet rule).

Valency of Common Elements:

• Hydrogen (H): 1
• Oxygen (O): 2
• Nitrogen (N): 3
• Carbon (C): 4
• Sodium (Na): 1
• Chlorine (Cl): 1

Determining Valency:

• For elements in Groups 1, 2, and 13 of the periodic table, valency is typically equal to the number of valence electrons.
• For elements in Groups 15, 16, and 17, valency is calculated by subtracting the number of valence electrons from 8.
• Example: For sulfur (Group 16), valency = 8 − 6 = 2

Variable Valency:

• Some elements can have more than one valency (e.g., iron can have a valency of 2 or 3).
• This happens because certain elements can lose different numbers of electrons under different conditions.

Applications of Valency:

• Understanding valency helps predict the chemical formulas of compounds.
• For example, water (H2O) has 2 atoms of hydrogen (valency 1) combining with 1 atom of oxygen (valency 2).