Describe the formation of ionic bonds between elements from Group I and Group VII, including the use of dot-and-cross diagrams

Atoms, Elements and Compounds – Ions and Ionic Bonds

Quick Recap: Valence Electrons – The “Electron Wallet”

Think of electrons as coins in a wallet.

🔑 Group I (alkali metals) carry 1 coin in their outer shell.

⚡ Group VII (halogens) carry 7 coins in their outer shell.

When the wallet is full (8 coins) the atom is stable – this is called the octet rule.

How Ions are Made – The Transfer of Coins

  1. Group I atom loses its one coin → becomes a cation with charge +1: \$Na^+\$.

  2. Group VII atom gains one coin → becomes an anion with charge –1: \$Cl^-\$.

  3. Both ions now have 8 coins (octet) and are stable.

⚡ The lost coin becomes a free electron that is attracted to the halogen’s empty spot.

Dot‑and‑Cross Diagrams – Visualising the Transfer

Use dots (•) for electrons and crosses (✗) for the ion’s charge.

ElementElectrons (before)Ion (after)Diagram
Na• • • • • • • •Na+Na+ ✗
Cl• • • • • • •Cl-Cl- •
NaCl——Na+ • • • • • • • • Cl-

💡 In the diagram the dot that moves from Na to Cl is the free electron that completes the halogen’s octet.

Why the Bond Forms – Electrostatic Attraction

After the transfer, the Na+ cation and Cl- anion have opposite charges.

⚡ They attract each other like a magnet’s north and south poles, forming a strong ionic bond.

Mathematically: \$Na^+ + Cl^- \rightarrow NaCl\$

🔄 The bond is not a sharing of electrons (as in covalent bonds) but a pull of the negative charge towards the positive.

Exam Tip: Spotting Ionic Compounds

  • Look for elements from Group I or Group VII in the formula.

  • Check the charges: if they balance to zero, it’s likely ionic.

  • Remember the octet rule – atoms aim for 8 electrons.

  • Use dot‑and‑cross diagrams to visualise electron transfer if the question asks.

📌 Practice: Write the dot‑and‑cross diagram for MgCl2 and explain the charges.

Quick Quiz – Test Your Understanding

  1. Which element will form a cation in an ionic bond?

  2. How many electrons does a halogen need to complete its octet?

  3. Draw a dot‑and‑cross diagram for K+ + F-.

🧠 Answering these will help you memorise the key concepts and ace the exam!