GoalExplain how an ionic bond forms, the charges of ions and the formulas of ionic compounds.
New words
ion · ioncation and anion · kation va anionionic bond · ionli bog‘lanishionic compound · ionli birikma
Explanation
When atoms whose electronegativities differ sharply (usually a typical metal and a typical nonmetal) combine, no shared pair forms: the electron passes completely from the metal atom to the nonmetal atom. A charged particle is called an ion. The atom that gave electrons becomes a positive cation, the atom that accepted them becomes a negative anion; the charge of the ion equals the number of electrons given or accepted: Na – e⁻ → Na⁺, Cl + e⁻ → Cl⁻, Ca – 2e⁻ → Ca²⁺. Oppositely charged ions attract by electrostatic force, and this is called the ionic bond. The ions formed resemble a noble gas atom with a full outer shell: Na⁺ neon, Cl⁻ and K⁺ argon. For the compound to be neutral the positive and negative charges must add up to zero: in CaCl₂ one Ca²⁺ goes with two Cl⁻.
Worked examples
K₂S: each K gives one electron (K⁺), S takes two (S²⁻); for balance 2 K⁺ and 1 S²⁻: 2·(+1) + (–2) = 0.
CaO: Ca gives 2 electrons (Ca²⁺), O takes 2 (O²⁻), ratio 1 : 1. Mg²⁺ and Cl⁻ give MgCl₂, because +2 is balanced by two –1.
Class activity
“Ion cards”: cut paper cards for Na⁺, K⁺, Ca²⁺, Al³⁺ and Cl⁻, O²⁻, S²⁻ (use scissors with an adult’s help). Combine cards so the charges add up to zero and write the formula.
Practice
1
What are a cation and an anion? Give one example of each.
A cation is a positive ion (Na⁺), an anion is a negative ion (Cl⁻).
2
An aluminium atom gives 3 electrons. Write the ion and say how it combines in Al₂O₃.
Al³⁺; 2 Al³⁺ and 3 O²⁻: 2·3 = 3·2 = 6.
3
Formula of the compound made of Mg²⁺ and Br⁻ ions?
MgBr₂
4
Why is HCl counted as a polar covalent, not an ionic compound?
The EN difference between H and Cl is small, the electron does not transfer fully, the shared pair only shifts to Cl.