Foundations — Batteries — primary (dry cell), secondary (lead-acid, Li-ion)

This is a foundations page. Before you meet the dry cell, the lead-acid car battery, or the Li-ion cell in your phone, you must be able to read every letter and squiggle they are written with. We build each idea from nothing, in an order where every new word only uses words we already defined, and we anchor each one to a picture. Nothing here is assumed. By the end you will be able to read a battery equation the way you read a sentence.

0. The starting picture: electrons want to fall

Look at the figure. On the left, an atom is holding an electron loosely — the electron "wants" to move to the right, where a different atom holds on tighter. Left to itself, the electron just jumps across in a tiny spark: chemistry, no electricity. A battery is what happens when we dig a canyon between the two sides and build a bridge (the wire) as the only crossing. The electron still falls — but now it falls through your device.

Keep this waterfall-across-two-cliffs image in your head. Every symbol below is a label on some part of this picture.

1. Electron ($e^{-}$) — the thing that actually moves

• Plain words: the smallest lump of "moving electricity" in ordinary chemistry.
• The picture: the blue dot sliding across the bridge in the figure above.
• Why the topic needs it: electricity from a battery is a stream of electrons. When you see $2e^{-}$ in an equation, read it as "two electrons are handed over here." Every half-reaction you meet later ends or begins with a pile of $e^{-}$.

2. Ions and aqueous species — atoms that are missing or carrying spare electrons

Before we can talk about electrons moving between things, we must be able to describe what an atom looks like after it has lost or gained some.

• The picture: a zinc atom that hands away two electrons is now short two negatives, so it is ${\text{Zn}}^{2+}$ — draw it as the same atom with a little "$2+$" tag. When it floats off into the electrolyte we tag it $(aq)$.
• Why the topic needs it: every battery works by turning neutral solids into charged ions that dissolve, and by turning ions back into solids. The symbols ${\text{Zn}}^{2+}$, ${\text{H}}^{+}$, ${\text{SO}}_4^{2-}$ that fill the parent note are all ions — you cannot read a single half-reaction without this word.

3. Oxidation & Reduction — losing vs gaining those electrons

Now we can name the two events precisely: what happens to make an ion, and what undoes it.

• The picture: in the figure, the yellow atom on the left lets go of electrons (oxidation — its charge climbs, it becomes a positive ion). The red atom on the right catches them (reduction — its charge drops).
• Why "reduction" for gaining? It sounds backwards. The name is historical: gaining electrons reduces the positive charge number. Look at the arrow pointing down on the red side — the charge is reduced.

4. Oxidation number — a scoreboard for electrons

To tell whether an atom lost or gained electrons, we need a number we can track before and after. That number is the oxidation number.

• Plain words: an electron scoreboard. If the number goes up, the atom was oxidised (lost electrons). If it goes down, it was reduced (gained).
• The picture: imagine a small counter floating over each atom. In the dry cell, zinc's counter reads $0$ (pure metal), then clicks up to $+2$ once it becomes ${\text{Zn}}^{2+}$.
• The everyday rules (enough for this topic): a pure element on its own is always $0$ (so $\text{Zn}(s)$ and $\text{Pb}(s)$ are $0$); a simple ion's number equals its charge (${\text{Zn}}^{2+}$ is $+2$); oxygen is usually $-2$ and hydrogen usually $+1$; and all the numbers in a neutral formula must add up to $0$.

$\underset{0}{\underbrace{\text{Zn}}}\to \underset{+2}{\underbrace{{\text{Zn}}^{2+}}}+2e^{-}\text{(counter went up by 2, so 2 electrons left)}$

5. The symbols on a chemical formula

You will read formulas like $\text{Zn}(s)$, ${\text{Zn}}^{2+}(aq)$, $2{\text{MnO}}_2$. Let's decode every mark.

• Why the topic needs it: the dry cell works because $\text{Zn}(s)$ (solid can) turns into ${\text{Zn}}^{2+}(aq)$ (dissolved ion). The state labels are telling you the metal is dissolving. Miss them and the chemistry is invisible.

6. Half-reactions — writing down the "split"

Now that we can score electrons, we can write each cliff of the waterfall separately.

Here is the simple oxidation half from the dry cell:

$\text{Oxidation half: }\text{Zn}(s)\to {\text{Zn}}^{2+}(aq)+2e^{-}$

And here is a reduction half, using the manganese dioxide of the dry cell as our example:

$\text{Reduction half (illustrative): }{\text{MnO}}_2(s)+{\text{H}}^{+}(aq)+e^{-}\to \text{MnO(OH)}(s)$

• The picture: each half-reaction is one cliff in the figure from §0. The electrons on the oxidation side and the electrons on the reduction side are the same electrons — they travelled through your wire in between.

7. Electrode, Anode & Cathode — naming the two cliffs

Before we name the two sides, we need the word for a "side" itself.

• The picture: in the figure the anode is the source (left cliff), the cathode is the destination (right cliff). Electrons flow through the outside wire from anode to cathode.
• The sign trap: in a battery delivering power (discharging), the anode is the negative (−) terminal and the cathode is the positive (+) terminal. This is why the parent note labels the zinc can as "Anode (−)".

8. Volt ($\text{V}$) and EMF — how hard the electrons are pushed

We know electrons move; now: how strong is the push?

• The picture: the height of the waterfall. A taller cliff (more volts) pushes the electrons harder. A 1.5 V dry cell is a short cliff; two in series (3.0 V) is twice as tall.

• Why subtract? The cathode's potential is where electrons want to go (high, positive); the anode's is where they start. The difference is the drop available to do work — exactly the cliff height in the picture. These $E^{\circ }$ numbers come from 2.7.05-Standard-electrode-potentials.

9. Current ($I$) and Charge ($Q$) — counting the flow

Voltage is how hard; current is how many per second.

• The picture: if voltage is the cliff height, current is how many electrons per second pour over the edge. A bright torch bulb draws more amps; a car starter motor draws a flood (200 A).
• Why the topic needs it: every worked example ("how much zinc is used up?", "how much ${\text{PbO}}_2$ is consumed?") starts by turning current-and-time into a total charge $Q$, then into a count of electrons.

10. The electron's charge and the mole — from flow to grams

The last bridge: connecting the flow of electrons to the mass of chemical used up.

• The picture: imagine scooping electrons by the bucket. One bucket = one mole = $6.022\times 10^{23}$ electrons, and that bucket weighs in at $96,485$ coulombs of charge.
• Why the topic needs it: to answer "how much zinc dissolved," we count electrons ($Q÷e$), turn that into moles ($÷N_A$, or use $Q÷F$ directly), then use the half-reaction's stoichiometry and the molar mass to reach grams. That chain — $t\to Q\to$ electrons $\to$ moles $\to$ grams — is the skeleton of every calculation in the parent note.

$\underset{\text{coulombs}}{\underbrace{Q}}\stackrel{÷F}{\to }\underset{\text{mol of }{e}^{-}}{\underbrace{n_{e^{-}}}}\stackrel{÷z}{\to }\underset{\text{mol}}{\underbrace{n_{\text{substance}}}}\stackrel{\times M}{\to }\underset{\text{grams}}{\underbrace{m}}$

where $M$ = molar mass (grams per mole).

11. Spontaneity — why the reaction runs at all

One last "why." What makes electrons want to fall in the first place?

• The picture: the waterfall flows downhill on its own (spontaneous, discharge). To pump water back up the cliff (charge), you need an external pump (the charger). This is the entire difference between a primary cell (can only fall) and a secondary cell (you can pump it back up).

Prerequisite map

Each node below is one foundation from this page; arrows mean "you need the lower box to understand the higher one." The short codes are just labels — their meanings: OR = Oxidation & Reduction, ON = Oxidation number, HR = Half-reactions, AC = Anode & Cathode, V = Voltage/EMF, SP = Spontaneity, COUNT = electron-counting with Faraday, BAT = the batteries topic itself.

Equipment checklist

Cover the right side and test yourself — you are ready for the parent note only if each reveal is obvious.