Concept:The chemical formula of an ionic compound is derived by "crisscrossing" the valencies (ionic charges) of the constituent elements to ensure the final compound is electrically neutral.
Formula:$$ \text{Ca}^{+2} + 2\text{Cl}^{-} \rightarrow \text{CaCl}_2 $$
Solution:- Calcium (Z=20) is in Group IIA. It loses two valence electrons to achieve an octet, forming the \( \text{Ca}^{+2} \) ion.
- Chlorine (Z=17) is a halogen in Group VIIA. It needs to gain exactly one electron to complete its octet, forming the \( \text{Cl}^{-} \) ion.
- To balance the +2 charge of a single Calcium ion, you require exactly two -1 Chlorine ions (\( +2 \) and \( -2 = 0 \)).
- Therefore, the formula is definitively \( \text{CaCl}_2 \).
Why other options are incorrect:- CaCl: Leaves a net positive charge of +1, which is electrically unstable.
- \( \text{Ca}_2\text{C} \): Brings Carbon into a question specifically asking about Calcium and Chlorine.
- \( \text{Ca}_2\text{Cl} \): Crisscrosses the valencies backward, implying Chlorine has a +2 charge and Calcium has a -1 charge, which is chemically impossible.
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