Foundations — Acids and bases — Arrhenius, Brønsted-Lowry, Lewis definitions

Before you can read the parent note Acids and bases, you must be able to read its alphabet. The parent note builds up three theories, each a wider net than the last — here is what each one will claim, so you know where these symbols are heading:

Below is every symbol, arrow, and word these three theories quietly assume you already know. We build each one from nothing.

1. The atom, drawn once so we never guess again

Everything here is about tiny charged pieces of atoms. So we draw an atom first.

• A proton is drawn as a small $\oplus$. It carries one unit of positive charge.
• An electron is drawn as a tiny $\ominus$ or a dot. It carries one unit of negative charge.

Why the topic needs this: an acid-base reaction is literally one atom's proton or electron pair moving to another atom. If you can't picture a proton leaving, none of the three theories make sense.

2. The symbol ${\text{H}}^{+}$ — a hydrogen atom stripped naked

A hydrogen atom is the simplest atom: one proton, one electron, no neutrons.

Now remove the single electron. What is left? Just the one proton.

So in this chapter the words "${\text{H}}^{+}$", "hydrogen ion", and "proton" all point at the same picture: a lonely $\oplus$.

3. Charge superscripts: $+$, $-$, $2-$, $2+$

Any atom or group that has gained or lost electrons is called an ion. We label its charge as a superscript.

Why the topic needs this: the whole neutralization idea, "$+$ meets $-$ and they stick," is written entirely in these superscripts. A base like ${\text{OH}}^{-}$ is attractive to ${\text{H}}^{+}$ because one is $-$ and the other is $+$.

4. Subscripts: how many atoms

A subscript is the small number below and after a symbol; it counts atoms.

5. The lone pair — two electrons that stay home

This is the single most important picture for the Lewis theory, and the parent note assumes you can see it.

When atoms bond, they share electrons in pairs. But some pairs are not shared with anyone — they sit on one atom, unused.

6. The bond arrow → and the reversible arrow ⇌

Reactions are sentences; the arrow is the verb.

Why the topic needs this: "strong vs weak" in the parent note is entirely about which arrow you draw. $\text{HCl}\to$ (strong, one-way) versus ${\text{NH}}_3\rightleftharpoons$ (weak, balanced). The idea of equilibrium is developed fully in Le Chatelier's Principle and Acid-base equilibrium constants.

7. State labels: (aq), (l), (g), (s)

Each species carries a small tag in brackets telling you its physical form.

Why the topic needs this: Arrhenius only works in $\text{(aq)}$; that's literally its limitation. Brønsted-Lowry escapes this by working even in $\text{(g)}$. So the state label is what separates the first theory from the second.

8. Hydronium ${\text{H}}_3{\text{O}}^{+}$ — where the proton actually lands

A bare ${\text{H}}^{+}$ never floats around alone in water. It immediately sticks onto a water molecule.

The counting of these ions leads directly to pH and pOH.

9. Amphoteric — one molecule, both roles

Why the topic needs this: this word only makes sense after you accept the give/receive picture (§1) and the lone pair picture (§5). It reappears in Amphoteric oxides and Hydrolysis of salts.

10. Lewis acid, Lewis base, and the dative bond

The Lewis theory throws away the proton entirely and talks only about electron pairs — the lone pairs you met in §5. So we need its two roles named clearly.

When the base offers its pair and the acid accepts it, a new bond forms — but a special kind:

Look at the figure: nitrogen's lone pair (blue) swings across into boron's empty slot (pink). Nothing about a proton appears — yet this is a full acid-base reaction in the Lewis sense.

Why the topic needs this: this is exactly what happens in ${\text{BF}}_3+{\text{NH}}_3$ and in metal complexes like $[\text{Ag}({\text{NH}}_3{)}_2{]}^{+}$. These live in Coordination compounds. Once you can draw a lone pair (§5) moving into an empty slot, the whole Lewis section is just that picture repeated.

How these foundations feed the topic

Read this map top to bottom: the atom at the top splits into the ideas beneath it, and those flow down into the three theories at the bottom. Colours only group related branches (charge ideas, electron-pair ideas, reaction-writing ideas) — there is no hidden code to memorise.

Equipment checklist

Cover the right side and test yourself. If any answer surprises you, reread that section.