Science MCQs

Two hydrogen atoms share one pair of electrons forming a single covalent bond (H-H).

Aluminum (electron configuration 2 8 3) loses its 3 valence electrons to form an Al³⁺ ion.

Add the subscripts: 6 carbon + 12 hydrogen + 6 oxygen = 24 atoms total.

Oxygen has a valency of 2-. There are three oxygen atoms total valency 6-. This is balanced by two iron atoms so each iron atom must have a valency of 3+ (Fe³⁺).

Zinc ion is Zn²⁺. Carbonate ion is CO₃²⁻. Their valencies are equal and cancel out so the formula is ZnCO₃.

The key difference is the mechanism: ionic bonds form through the transfer of electrons while covalent bonds form through the sharing of electrons.

Oxygen has 8 electrons. They are arranged in shells: first shell holds 2 the second shell holds the remaining 6 (2 6).

Ammonium ion is NH₄⁺ (valency 1+). Phosphate ion is PO₄³⁻ (valency 3-). Three ammonium ions are needed to balance one phosphate ion: (NH₄)₃PO₄.

Element X has valency 3+. Oxygen has valency 2-. The formula is X₂O₃ to balance the charges (2x3+ = 6+ 3x2- = 6-).

Calcium's electron configuration is 2 8 8 2. It has 2 valence electrons and loses both to form Ca²⁺ and achieve a stable octet.