Chapter 3 · Question 8

Explain the formation of ionic compounds taking the examples of sodium chloride (NaCl) and magnesium chloride (MgCl₂). Show the electron transfer using electron-dot notation and state what type of ions are formed.

Answer Revealed

Direct Answer:

Ionic (electrovalent) compounds form when a metal atom transfers its valence electrons to a non-metal atom, creating oppositely charged ions held together by strong electrostatic forces. Na (electronic configuration 2,8,1) loses its one valence electron to form Na⁺ (2,8), attaining the stable octet of neon. Cl (2,8,7) gains that one electron to form Cl⁻ (2,8,8), attaining the stable octet of argon. Na⁺ and Cl⁻ ions attract to form NaCl — the ionic compound does not exist as discrete molecules but as an aggregate of ions. For MgCl₂: Mg (2,8,2) loses two valence electrons to become Mg²⁺ (2,8). Two chlorine atoms each gain one electron to become two Cl⁻ ions (2,8,8). The Mg²⁺ cation and two Cl⁻ anions form MgCl₂. The cations are Na⁺ and Mg²⁺; anions are Cl⁻.

Simple Explanation

Ionic bonding is about give and take. Sodium has one extra electron it is eager to lose, and chlorine desperately needs one more electron to complete its outer shell. So sodium gives its electron to chlorine: Na becomes Na⁺ (a positive ion), Cl becomes Cl⁻ (a negative ion). Opposite charges attract, and they stick together to make NaCl (table salt). Magnesium is even more generous — it has two electrons to give, so it hands one each to two chlorine atoms, creating MgCl₂. Think of it like sodium writing a cheque (an electron) to chlorine, and chlorine cashing it — both end up with a full outer shell of 8 electrons, like a noble gas.

Exam-Ready Structure

The formation of ionic compounds demonstrates how metals (electron donors) and non-metals (electron acceptors) react to achieve stable noble gas configurations, as illustrated in NCERT Figs. 3.5 and 3.6. 1. Driving force: All elements tend to attain a completely filled valence shell (octet), which gives the stable configuration of the nearest noble gas. Metals have 1–3 valence electrons and lose them; non-metals have 5–7 valence electrons and gain electrons. 2. Formation of NaCl (Fig. 3.5): (a) Na atom (atomic number 11, electronic configuration 2,8,1) has one electron in its M shell. If it loses this electron, its L shell (with 8 electrons) becomes the outermost shell — a stable octet like neon. The resulting species has 11 protons but only 10 electrons, giving it a net +1 charge: Na⁺ (sodium cation). Equation:

\text{Na} \rightarrow \text{Na}^+ + \text{e}^-

. (b) Cl atom (atomic number 17, configuration 2,8,7) has 7 valence electrons. By gaining one electron, it achieves the stable octet of argon (2,8,8) and becomes a negatively charged chloride anion:

\text{Cl} + \text{e}^- \rightarrow \text{Cl}^-

. (c) The Na⁺ and Cl⁻ ions are held together by strong electrostatic forces of attraction — this is the ionic (electrovalent) bond. NaCl does not exist as discrete molecules; it exists as aggregates of oppositely charged ions in a crystal lattice. 3. Formation of MgCl₂ (Fig. 3.6): Mg (configuration 2,8,2) loses its two valence electrons:

\text{Mg} \rightarrow \text{Mg}^{2+} + 2\text{e}^-

. Each of the two chlorine atoms accepts one electron:

2\text{Cl} + 2\text{e}^- \rightarrow 2\text{Cl}^-

. The Mg²⁺ cation and two Cl⁻ anions form the compound MgCl₂. 4. Cation: positively charged ion (e.g., Na⁺, Mg²⁺); Anion: negatively charged ion (e.g., Cl⁻).

Key Points

Ionic bonding: transfer of valence electrons from metal to non-metal → oppositely charged ions attract
Na → Na⁺ + e⁻ (loses 1 e⁻); Cl + e⁻ → Cl⁻ (gains 1 e⁻); Na⁺ and Cl⁻ form NaCl
Mg → Mg²⁺ + 2e⁻; 2Cl + 2e⁻ → 2Cl⁻; Mg²⁺ + 2Cl⁻ → MgCl₂
Na⁺ (2,8) attains Ne configuration; Cl⁻ (2,8,8) attains Ar configuration
Ionic compounds do NOT exist as individual molecules — they exist as aggregates of ions in a lattice

Explain the formation of ionic compounds taking the examples of sodium chloride (NaCl) and magnesium chloride (MgCl₂). Show the electron transfer using electron-dot notation and state what type of ions are formed.