Ionic Bonding

Dot and Cross Diagrams

• Representation: Use dots for electrons from one atom and crosses for electrons from the other to track the transfer process clearly.
• Brackets and Charges: Always enclose the final ion in square brackets with the charge (e.g., $2+$, $1-$) written at the top right-hand corner to indicate the ion's state.

Formula Writing (The Criss-Cross Method)

• Step 1: Write the symbols of the metal and non-metal with their respective ionic charges.
• Step 2: Cross over the numerical value of the charges to become the subscripts for the opposite element.
• Step 3: Simplify the ratio to the lowest whole numbers to represent the empirical formula of the giant structure.

• Ionic vs. Covalent: Ionic bonds involve the transfer of electrons and form lattices, whereas covalent bonds involve the sharing of electrons and often form discrete molecules.

• Melting Point Trends: When comparing compounds, look at the charge density. Ions with higher charges (e.g., $Mg^{2+}$ vs $Na^+$) exert stronger electrostatic forces, resulting in significantly higher melting points.

• Conductivity Explanations: Always specify that ions are the charge carriers in ionic compounds. Mentioning 'electrons' moving in an ionic melt is a common error that loses marks.

• State Symbols: In equations, remember that ionic compounds are almost always solid ($s$) at room temperature due to their high melting points.

• Lattice Description: Use the phrase 'regular arrangement of alternating positive and negative ions' to describe the structure for maximum marks.

• Discrete Molecules: A common mistake is thinking that $NaCl$ exists as a single pair of atoms. In reality, it is a giant structure where each ion is surrounded by multiple neighbors.

• Electron Conductivity: Students often confuse metallic bonding with ionic bonding, claiming that 'delocalized electrons' allow ionic compounds to conduct. In ionic substances, only ions move.

• Brittleness: Students may forget why ionic crystals shatter. When hit, layers of the lattice shift, bringing ions of the same charge together; the resulting repulsion causes the crystal to split.