Q.Classify the following species into Lewis acids and Lewis bases and show how these act as such:

The key idea is the Lewis acid-base theory: a Lewis acid accepts an electron pair, and a Lewis base donates an electron pair.

Step 1 — Identify electron-rich species (Lewis bases):

Species with lone pairs or negative charge can donate electrons.

• (a) $\text{HO}^-$ has three lone pairs on oxygen and a negative charge — it is electron-rich.
• (b) $\text{F}^-$ has four lone pairs and a full octet with a negative charge — it is also electron-rich.

Step 2 — Identify electron-deficient species (Lewis acids):

Species with an incomplete octet or positive charge can accept electrons.

• (c) $\text{H}^+$ has no electrons at all — it readily accepts a lone pair.
• (d) $\text{BCl}_3$ has only six electrons around boron (incomplete octet) — it accepts a lone pair to complete its octet.

Step 3 — Show how each acts:

• $\text{HO}^-$ donates a lone pair to form a coordinate bond (e.g., with $\text{H}^+$ to give $\text{H}_2\text{O}$). …

The Lewis theory of acids and bases is beautifully simple: it’s all about electron pairs. A Lewis acid is any species that can accept a pair of electrons (it’s electron-deficient). A Lewis base is any species that can donate a pair of electrons (it’s electron-rich). This is broader than the Brønsted-Lowry idea (which focuses on protons), because it covers molecules like $\text{BCl}_3$ that have no hydrogen at all.

Let’s look at each species one by one.

1. $\text{HO}^-$ (hydroxide ion) The hydroxide ion has a negative charge and three lone pairs on oxygen. It is electron-rich and loves to share a lone pair. For example, when it reacts with $\text{H}^+$, it donates a lone pair to form a coordinate bond:

Here, $\text{HO}^-$ is the donor — so it is a Lewis base.

2. $\text{F}^-$ (fluoride ion) Fluoride ion has a full octet and four lone pairs. It is also electron-rich and readily donates a lone pair. For instance, with $\text{BF}_3$:

The fluoride ion donates its lone pair to boron. So $\text{F}^-$ is a Lewis base.

3. $\text{H}^+$ (proton) A proton has no electrons at all — it’s just a bare nucleus. It desperately needs an electron pair to become stable (as in $\text{H}_2\text{O}$ or $\text{NH}_3$). When it accepts a lone pair from a base, it forms a coordinate bond. So $\text{H}^+$ is a Lewis acid. …