Formation of Ions and Predicting Their Charges
Quick Notes
- Metal atoms usually lose electrons to form positive ions called cations.
- Non-metal atoms usually gain electrons to form negative ions called anions.
- The charge on an ion can be predicted from an element’s electron configuration and its group number in the periodic table.
- Transition metals can form ions with variable charges.
- This variability is explained by their successive ionization energies and electron configurations.
Full Notes
What Are Ions?
An ion is a charged particle formed when atoms gain or lose electrons.
- Cations are positively charged ions, formed when an atom loses electrons.
- Anions are negatively charged ions, formed when an atom gains electrons.
Note – metals usually lose electrons and non-metals gain electrons (there are some exceptions).
Example Sodium and Chlorine
When sodium and chlorine react together:
- Sodium atoms lose one electron and become positively charged ions.
- Chlorine atoms gain one electron and become negatively charged ions.
Na (1s2 2s2 2p6 3s1) → Na+ (1s2 2s2 2p6)
Cl (1s2 2s2 2p6 3s2 3p5) → Cl− (1s2 2s2 2p6 3s2 3p6)
Predicting Ion Charge from Electron Configuration
Elements lose or gain electrons to achieve a stable electron configuration, usually a noble gas configuration.
The charge is predicted from how many electrons are lost or gained.
For Example:
- Mg: has an electron configuration of [Ne] 3s² and so loses 2 electrons forming Mg²⁺
- O: has an electron configuration of [He] 2s² 2p⁴ and so gains 2 electrons forming O²⁻
Periodic Table and Ion Charge
Group number helps predict ion charge for main-group elements:
- Metals (Groups 1, 2, 3) usually lose electrons to form cations:
- Group 1 → +1 ions (e.g., Na+)
- Group 2 → +2 ions (e.g., Mg2+)
- Group 3 → +3 ions (e.g., Al3+)
- Non-metals (Groups 15, 16, 17) usually gain electrons to form anions:
- Group 15 → −3 ions (e.g., N3−)
- Group 16 → −2 ions (e.g., O2−)
- Group 17 → −1 ions (e.g., Cl−)
- Group 18 (noble gases) do not form ions under normal conditions.
Transition Elements and Variable Charges
Unlike main-group elements, transition metals often form more than one ion.
This happens because transition metals have a partially filled d-subshell and successive ionization energies for 4s and 3d electrons are relatively close.
As a result, the energy difference between successive oxidation states is small.
This allows transition metals to lose different numbers of electrons and form multiple oxidation states that can be interchanged relatively easily.
Example Iron (Fe)
Fe can form both Fe2+ and Fe3+ ions
Summary
- Metals form cations by losing electrons.
- Non-metals form anions by gaining electrons.
- Ion charges are based on electron configurations and group numbers.
- Transition metals can form variable charges due to similar energy levels of 4s and 3d electrons.
Linked Questions
How does the position of an element in the periodic table relate to the charge of its ion(s)?
The charge of an ion formed by an element depends on its position in the periodic table, which reflects its number of valence electrons. Elements on the left (Groups 1 and 2) have few outer electrons and tend to lose them, forming positive ions (e.g. Na⁺, Ca²⁺). Elements on the right (Groups 16 and 17) have nearly full outer shells and tend to gain electrons, forming negative ions (e.g. O²⁻, Cl⁻). Transition metals (d-block elements) can lose different numbers of electrons from 4s and 3d orbitals, leading to multiple possible ion charges and variable oxidation states.
How does the trend in successive ionisation energies of transition elements explain their variable oxidation states?
Successive ionisation energy data for transition metals show relatively small increases between the removal of the outer 4s electrons and the inner 3d electrons. This indicates that both energy levels are close in energy and can participate in bonding. As a result, transition metals can lose different numbers of electrons under different conditions, forming ions with multiple oxidation states. For example, iron can form Fe²⁺ by losing two 4s electrons or Fe³⁺ by losing one additional 3d electron. This flexibility in electron loss explains their wide range of chemical behaviour.