Electron Configurations and the Octet Rule in Main Group Elements

Electron Configurations and the Octet Rule

Main Group Elements and Valence Electrons

Main group elements tend to achieve a stable electron configuration by having 8 valence electrons or a filled outer shell. This tendency drives their chemical reactivity.

• Metals tend to lose electrons to achieve the electron configuration of the noble gas that is before them in the periodic table.

• Non-metals tend to gain electrons to achieve the electron configuration of the noble gas that is after them in the periodic table.

• Filled s and p subshells lead to increased stability and decreased chemical reactivity.

Example: How many electrons must magnesium atom (Z = 12) lose in order to obtain a filled outer shell?

• Answer: 2 electrons

Ions and the Octet Rule

Atoms form ions to achieve a stable electron configuration, often resembling that of a noble gas. The process involves either losing or gaining electrons:

Ion	Electron Configuration
Li	1s22s1
Li+	1s2
F	1s22s22p5
F-	1s22s22p6

Additional info: The table above compares the electron configurations of neutral atoms and their corresponding ions, illustrating how ions achieve filled outer shells.

Electron Configurations (Cations)

When forming cations (positively charged ions), electrons are removed from the highest energy level (highest n value):

• For example, sodium (Na) loses one electron to form Na+:

$\text{Na: } 1s^2\ 2s^2\ 2p^6\ 3s^1$

$\text{Na}^+: 1s^2\ 2s^2\ 2p^6$

• Example: Write the condensed electron configuration for the Na+ ion.

• Answer: Remove one electron from the 3s orbital.

Electron Configurations (Anions)

When forming anions (negatively charged ions), electrons are added to the orbitals with available space:

• For example, nitrogen (N) gains three electrons to form N3-:

$\text{N: } 1s^2\ 2s^2\ 2p^3$

$\text{N}^{3-}: 1s^2\ 2s^2\ 2p^6$

• Example: Write the full electron configuration for the N3- ion.

• Answer: Add three electrons to the 2p orbital.

Practice Problems

• Determine the electron configuration for the Cl- ion.

• Answer: $1s^2\ 2s^2\ 2p^6\ 3s^2\ 3p^6$

Summary Table: Electron Configurations of Selected Ions

Ion	Electron Configuration	How electrons change
Na	1s22s22p63s1	Neutral atom
Na+	1s22s22p6	Lose 1 electron
N	1s22s22p3	Neutral atom
N3-	1s22s22p6	Gain 3 electrons
Cl-	1s22s22p63s23p6	Gain 1 electron

Additional info: This table summarizes how electron configurations change when atoms form common cations and anions.