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An oxidizing agent undergoes reduction by accepting the formal Electron inventory. In the zinc-copper displacement experiment, ${\mathrm{Cu}}^{2+}$ plays this role.^[ref]

Consider the reacting zinc and copper species in the zinc-copper displacement experiment. Split the net chemical equation into two half-reactions:

The zinc oxidation state rises from $0$ to $+2$; the copper oxidation state falls from $+2$ to $0$. Add the half-reactions: two released Electrons match two accepted Electrons. Cancelling $2e^{-}$ from both sides gives^[ref]Thus the closed net chemical reaction has zero net production of free Electrons. Cancellation does not destroy Electrons: it removes a matched internal exchange from the written chemical equation. The half-reactions alone are incomplete descriptions of the whole reacting system. In a galvanic cell, the exchange can pass through a wire; in the zinc-copper displacement experiment, the reacting materials meet at the same metal surface. The equations do not require a reservoir of free Electrons in the bulk chemical solution.

Check both kinds of balance. The oxidation line has net electric charge $0$ on the left and $(+2-2)e=0$ on the right; the reduction line has $(+2-2)e=0$ on the left and $0$ on the right. The net chemical equation contains one zinc and one copper chemical atom on each side and has net electric charge $+2e$ on each side, where $e$ is the positive elementary charge. The written reacting species need not be electrically neutral as a group: the bulk chemical solution also contains compensating anions.

There is an even more literal check. Zinc has atomic number $30$ and copper $29$, so the initial pair contains $30+(29-2)=57$ Electrons. The final pair contains $(30-2)+29=57$ Electrons. Only their distribution has changed. Conserved chemical element counts give the usual chemical mass conservation inventory; changing the counts of neutral and charged species does not create or remove their atomic nuclei.

Matching Electrons is necessary but does not replace balancing every chemical element. The acidic permanganate-iron balance illustrates unequal one- and five-Electron exchanges, with water and protons also needed. Conversely, a balanced chemical equation alone establishes neither a reaction rate nor a detailed mechanism. Its usefulness is that it fixes the stoichiometry even when those additional questions remain open.

A reducing agent undergoes oxidation by supplying the formal Electron inventory. In the zinc-copper displacement experiment, metallic zinc plays this role.^[ref]