Science (086) — AI-generated practice question

Electronic configurations:

• Na (11): 2, 8, 1 — has 1 electron in outermost shell
• Mg (12): 2, 8, 2 — has 2 electrons in outermost shell
• Cl (17): 2, 8, 7 — needs 1 electron to complete octet

NaCl: Sodium loses 1 electron to form Na⁺. One Cl atom gains that 1 electron to form Cl⁻. So one Na combines with one Cl → formula NaCl.

MgCl₂: Magnesium loses 2 electrons to form Mg²⁺. Each Cl atom can accept only 1 electron. So two Cl atoms are needed to accept both electrons, forming 2Cl⁻. Thus one Mg combines with two Cl → formula MgCl₂.

$$\text{Mg} \rightarrow \text{Mg}^{2+} + 2e^- \quad;\quad 2\text{Cl} + 2e^- \rightarrow 2\text{Cl}^-$$

Source: Chapter 3, Section 3.3

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Examiners look for:

1. Correct electronic configurations of all three elements written explicitly.
2. Clear statement of how many electrons Na and Mg lose (valency).
3. Linking Mg's 2 electrons lost to needing 2 Cl atoms — this is the key reasoning for MgCl₂ vs NaCl.
4. Equations showing ion formation are a bonus that fetch full marks.

Remember: the formula of an ionic compound is determined by balancing the charges of the ions formed, which directly follows from the number of electrons lost/gained based on electronic configuration.