Structure 2.1.1 — Formation of Ions

  What Happens When Atoms Gain or Lose Electrons?

• Metals lose electrons → form positive ions (cations).
• Non-metals gain electrons → form negative ions (anions).

How to Predict the Charge on an Ion

You can predict the charge based on the number of valence electrons and the element’s position in the periodic table:

• Group 1 → lose 1 electron → charge = +1 (e.g., Na⁺)
• Group 2 → lose 2 electrons → charge = +2 (e.g., Mg²⁺)
• Group 13 → lose 3 electrons → charge = +3 (e.g., Al³⁺)
• Group 15 → gain 3 electrons → charge = -3 (e.g., N³⁻)
• Group 16 → gain 2 electrons → charge = -2 (e.g., O²⁻)
• Group 17 → gain 1 electron → charge = -1 (e.g., Cl⁻)
• Group 18 → full outer shell → no ions formed under normal conditions.

Example:

Sodium (Na)

Electron configuration: \( 1s^2\,2s^2\,2p^6\,3s^1 \)

It loses the 3s¹ electron → becomes \( \text{Na}^+ \) with configuration \( 1s^2\,2s^2\,2p^6 \)

Chlorine (Cl)

Electron configuration: \( 1s^2\,2s^2\,2p^6\,3s^2\,3p^5 \)

Gains one electron → becomes \( \text{Cl}^- \) with full outer shell: \( 3p^6 \)

Transition Metals: Variable Charges

Transition metals can form ions with multiple possible charges. This is because:

• They can lose different numbers of electrons from both the outer 4s and inner 3d subshells.
• The energy levels of the 4s and 3d orbitals are close together.

Examples:

• Iron (Fe): Can form \( \text{Fe}^{2+} \) (loses 2 electrons) or \( \text{Fe}^{3+} \) (loses 3 electrons)
• Copper (Cu): Can form \( \text{Cu}^+ \) or \( \text{Cu}^{2+} \)