5.6 Octet and Duet Rules

Octet Rule:

• The octet rule states that atoms tend to form molecules in such a way that they achieve the same number of electrons in their outermost energy level as the nearest inert gas, which is typically 8 electrons. This gives the atom a stable electronic configuration.

• For example, in the CH4 (methane) molecule, the central carbon atom has 8 electrons in its outermost shell. Four of these electrons are from carbon itself, and the remaining four come from hydrogen atoms.

• Example: Carbon in CH4:

H

|

C

|

H

|

H

|

H

Here, carbon shares electrons with hydrogen to complete its outer shell, achieving an octet.

Duet Rule:

• The duet rule is a modification of the octet rule and applies to atoms that need fewer electrons to achieve stability. It states that some elements, particularly hydrogen and helium, will tend to have 2 electrons in their outermost energy level (like inert gases such as helium).

• This rule is primarily applicable to elements that don’t require 8 electrons to be stable but rather 2 electrons in their outer shell, like hydrogen and helium.

• Example:

• Hydrogen (H): Hydrogen follows the duet rule, aiming to have 2 electrons in its outer shell. In the H2 molecule, two hydrogen atoms share one pair of electrons.

• Helium (He): Being an inert gas, helium already has 2 electrons in its outer shell and doesn’t need to form bonds to achieve a stable configuration.

Application of Octet and Duet Rules:

• Beryllium (Be) in BeCl2: The Be atom has 2 pairs or 4 electrons in its outermost shell, fulfilling the duet rule.

• Boron (B) in BF3: The B atom has 3 pairs or 6 electrons in its outermost shell, which is less than the octet but still stable in this case.

• Carbon (C) in CH4: The C atom in CH4 has 4 pairs or 8 electrons in its outer shell, following the octet rule.

• Chlorine (Cl) in Cl2: Chlorine atoms in Cl2 have 4 pairs of electrons in their outer shells, following the octet rule.

• Fluorine (F): Like chlorine, fluorine has 4 pairs of electrons in its outermost shell to complete its octet.

Key Points:

• Octet Rule: Elements tend to have 8 electrons in their outermost shell to achieve stability, except for hydrogen and helium.

• Duet Rule: Hydrogen and helium tend to have 2 electrons in their outermost shell to achieve stability, which is a modification of the octet rule.

• Elements 1–20 in the periodic table generally follow the octet and duet rules when forming molecules.