Q.Using the Periodic Table, predict the formulas of compounds which might be formed by the following pairs of elements;

Ionic Compound Prediction: From Intuition to Rule

Imagine you have a bag of positively charged magnets (cations) and negatively charged magnets (anions). If you just dump them together, they'll snap into a neutral clump — but only if the total positive charge exactly cancels the total negative charge. That's the core idea behind ionic compound formation: the compound must be electrically neutral overall.

The Intuition

Sodium ($\text{Na}$) wants to lose one electron to become $\text{Na}^+$. Chlorine ($\text{Cl}$) wants to gain one electron to become $\text{Cl}^-$.

If you put one $\text{Na}^+$ and one $\text{Cl}^-$ together, the charges cancel: $+1 + (-1) = 0$. That's why sodium chloride is $\text{NaCl}$ — one sodium ion for every chloride ion.

But what about magnesium ($\text{Mg}$) and chlorine? Magnesium loses two electrons to become $\text{Mg}^{2+}$. One $\text{Mg}^{2+}$ needs two $\text{Cl}^-$ ions to balance: $+2 + 2(-1) = 0$. So the formula is $\text{MgCl}_2$.

The rule is simple: the total positive charge must equal the total negative charge. You're just finding the smallest whole-number ratio of ions that makes this happen.

The Precise Statement

In plain language: the subscript on the cation ($a$) times its charge ($m$) must equal the subscript on the anion ($b$) times its charge ($n$).

How to Apply It — Step by Step

1. Write the ions with their charges.

   Example: calcium ($\text{Ca}^{2+}$) and phosphate ($\text{PO}_4^{3-}$).

2. Find the smallest numbers that balance the charges.

   The charges are $+2$ and $-3$. The least common multiple of 2 and 3 is 6.

   • To get $+6$ from $\text{Ca}^{2+}$, you need 3 calcium ions: $3 \times (+2) = +6$.
   • To get $-6$ from $\text{PO}_4^{3-}$, you need 2 phosphate ions: $2 \times (-3) = -6$.

3. Write the formula with those numbers as subscripts.

   $\text{Ca}_3(\text{PO}_4)_2$ — the parentheses around phosphate show it's a polyatomic ion taken as a unit.

Common Pitfalls

Another trap: forgetting to reduce the ratio. If you get $\text{Ca}_2\text{O}_2$, that's wrong — it should be $\text{CaO}$ (the smallest whole numbers are 1 and 1). Always simplify.

Why This Works

Ionic compounds form because opposite charges attract. But nature doesn't allow a net charge — a macroscopic crystal must be neutral. So the ions arrange themselves in a ratio that exactly cancels all charges. The formula you write is the simplest representation of that ratio.

Quick Reference Table

The last example shows polyatomic ions — treat the whole ion as a single unit, and use parentheses if you need more than one of them.

Final Takeaway

Ionic compound prediction is just charge balancing. Find the smallest whole numbers of each ion that make the total charge zero. That's it — no hidden complexity, just simple arithmetic with charges.

Predicting the correct formula of an ionic compound by balancing charges is a fundamental skill taught in the NCERT Class 11 Chemistry chapter on Chemical Bonding and Molecular Structure, and "how to write formula of ionic compounds" is a widely searched topic for CBSE board preparation. This charge-balancing method is also a quick-scoring technique frequently tested in "chemical bonding important questions" for competitive exams.