Q.Identify the positions of Al (z=13) and S (z=16) in the periodic table with the help of their electronic configurations. Predict the formula of the compound formed between them.

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 …