Ions & Ionic Bonds
Section: 2. Atoms, Elements & Compounds | Syllabus: Cambridge AS Level Physics 9702
What are Ions? Atoms can gain or lose electrons to become charged particles called ions. Understanding ions is essential for understanding chemical bonding and reactions. Ion An atom or group of atoms that has gained or lost electrons, giving it an overall electric charge.
How Ions Form Atoms form ions to achieve a stable electronic configuration - usually a full outer shell like noble gases. Atoms with few outer electrons tend to lose electrons Atoms with nearly full outer shells tend to gain electrons By gaining or losing electrons, atoms achieve stable noble gas configurations This follows the octet rule - atoms want 8 electrons in their outer shell (or 2 for the first shell) Types of Ions Cations (Positive Ions) Cation A positively charged ion formed when an atom loses one or more electrons.
Formed by atoms losing electrons More protons than electrons, so overall positive charge Mainly formed by metals (Groups 1, 2, 3) Charge = number of electrons lost Examples of Cations: Sodium ion (Na⁺): Na (2.8.1) loses 1 electron → Na⁺ (2.8) Magnesium ion (Mg²⁺): Mg (2.8.2) loses 2 electrons → Mg²⁺ (2.8) Aluminium ion (Al³⁺): Al (2.8.3) loses 3 electrons → Al³⁺ (2.8) Anions (Negative Ions) Anion A negatively charged ion formed when an atom gains one or more electrons.
Formed by atoms gaining electrons More electrons than protons, so overall negative charge Mainly formed by non-metals (Groups 5, 6, 7) Charge = number of electrons gained Examples of Anions: Chloride ion (Cl⁻): Cl (2.8.7) gains 1 electron → Cl⁻ (2.8.8) Oxide ion (O²⁻): O (2.6) gains 2 electrons → O²⁻ (2.8) Nitride ion (N³⁻): N (2.5) gains 3 electrons → N³⁻ (2.8) Remember Cations are PAWSitive: Positive And Written Start (metals on left of periodic table).
Anions are negative: Non-metals on right gain electrons. Working Out Ionic Charges Using Group Number For main group elements, the charge can be predicted from the group number: Group Electrons in Outer Shell Gain/Lose Ion Charge Example Group 1 1 Lose 1 1+ Na⁺, K⁺, Li⁺ Group 2 2 Lose 2 2+ Mg²⁺, Ca²⁺ Group 3 3 Lose 3 3+ Al³⁺ Group 5 5 Gain 3 3- N³⁻, P³⁻ Group 6 6 Gain 2 2- O²⁻, S²⁻ Group 7 7 Gain 1 1- Cl⁻, Br⁻, F⁻ Group 0 8 (or 2 for He) None 0 (no ion) Ne, Ar (stable) What is Ionic Bonding?
Ionic Bond The electrostatic force of attraction between oppositely charged ions (cations and anions). Forms between metals and non-metals Metal atom loses electrons to form a positive ion (cation) Non-metal atom gains electrons to form a negative ion (anion) Opposite charges attract, creating a strong electrostatic force This is the ionic bond Key Concept Ionic bonding involves the transfer of electrons from metal to non-metal.
This is different from covalent bonding, which involves sharing electrons. How Ionic Bonds Form Example 1: Sodium Chloride (NaCl) Sodium atom (Na): Electronic configuration 2.8.1 Has 1 electron in outer shell Loses 1 electron to achieve stable configuration like neon (2.8) Becomes Na⁺ ion (2.8) Chlorine atom (Cl): Electronic configuration 2.8.7 Has 7 electrons in outer shell Gains 1 electron to achieve stable configuration like argon (2.8.8) Becomes Cl⁻ ion (2.8.8) Result: Na⁺ and Cl⁻ ions attract each other Strong electrostatic attraction forms ionic bond Forms sodium chloride (NaCl) - table salt Na (2.8.1) + Cl (2.8.7) → Na⁺ (2.8) + Cl⁻ (2.8.8) → NaCl Example 2: Magnesium Oxide (MgO) Magnesium atom (Mg): Electronic configuration 2.8.2 Loses 2 electrons → Mg²⁺ (2.8) Oxygen atom (O): Electronic configuration 2.6 Gains 2 electrons → O²⁻ (2.8) Result: Mg²⁺ and O²⁻ attract to form MgO Mg (2.8.2) + O (2.6) → Mg²⁺ (2.8) + O²⁻ (2.8) → MgO Dot and Cross Diagrams Dot and cross diagrams show how electrons are transferred during ionic bonding: Electrons from one atom shown as dots (•) Electrons from the other atom shown as crosses (×) Shows which electrons came from which atom For ions, draw square brackets with the charge outside Drawing Tip For ionic compounds: (1) Draw separate ions in square brackets, (2) Show the charge on each ion, (3) Only show outer shell electrons, (4) Show electron transfer with arrows if required.
Formulas of Ionic Compounds Ionic compounds are electrically neutral overall - positive and negative charges must balance. Balancing Charges Total positive charge = Total negative charge Use subscripts to show the ratio of ions Find the lowest common multiple of the charges Examples: Cation Anion Balancing Formula Na⁺ Cl⁻ 1:1 ratio NaCl Mg²⁺ Cl⁻ 1:2 ratio (need 2 Cl⁻ for each Mg²⁺) MgCl₂ Ca²⁺ O²⁻ 1:1 ratio CaO Al³⁺ O²⁻ 2:3 ratio (2×3=6, 3×2=6) Al₂O₃ Na⁺ S²⁻ 2:1 ratio (need 2 Na⁺ for each S²⁻) Na₂S Exam Tip To work out formulas: (1) Write symbols and charges, (2) Swap the numbers in the charges, (3) Simplify if possible, (4) Check total charges balance to zero.
Polyatomic Ions Some ions contain more than one atom bonded together: Polyatomic Ion An ion made up of two or more atoms covalently bonded together, with an overall charge. Common Polyatomic Ions to Know: Hydroxide: OH…
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