L13. Ions and Electric Charge

Chemical Bonding

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L13. Ions and Electric Charge

Why does salt water conduct electricity while pure water does not?

A curious observation: salt water makes a bulb light

Try this simple observation in class or imagine it: a small light bulb and a 9 V battery are connected with two wires that dip into a cup of water. If the cup holds pure distilled water the bulb stays dim or off. If you dissolve a teaspoon of table salt (NaCl) in the water the bulb glows. That visible change is the phenomenon we will explain: charged particles in the liquid can move and carry electrical current, while neutral molecules do not.

• Observation: Light bulb off in pure water, on in salt solution. Key measurable change: solution conducts electric current (bulb brightness). What we must explain: where the mobile charges come from in the salt solution.

What is an ion? A particle-level model

An ion is an atom or group of atoms that has a net electric charge because the number of electrons does not equal the number of protons. Protons (in the nucleus) are positively charged; electrons (in shells) are negatively charged. When an atom loses one or more electrons it becomes a positive ion (cation). When an atom gains electrons it becomes a negative ion (anion). From earlier lessons on the periodic table you already know which elements more readily lose or gain electrons; here we apply that idea to charges.

• Cation example (loss of electrons): Mg → Mg2+ + 2 e− (atoms, charge balanced: 0 → +2 + (−2)). Anion example (gain of electrons): Cl + e− → Cl− (neutral atom plus electron → negatively charged ion). Charge units: single electron has charge −1.6 × 10−19 C (coulombs).

How ions make solutions conduct and relate to static charge

When salt dissolves, its solid pieces separate into free ions that can move through the liquid. In a salt-water circuit the battery pushes positive and negative ions in opposite directions; the moving charges carry electrical energy that lights the bulb. This explains the first observation without invoking bonding details: the moving particles are ions, not neutral salt molecules. Ions also explain some static-electric phenomena, but with a difference. Static cling (a rubbed balloon sticking to hair) is often caused by excess electrons moving from one surface to another; that leaves objects temporarily charged. Those charged objects attract or repel nearby materials because of electric forces between charges, not because of dissolved ions. Both cases follow the same rule: opposite charges attract, like charges repel. A tiny calculation helps connect particles to measurable charge. If one electron has charge −1.6 × 10−19 C, three extra electrons give a net charge of 3 × (−1.6 × 10−19 C) = −4.8 × 10−19 C. Showing units (C) keeps particle ideas tied to instruments and measurements students can trace.

Putting the ideas together

Ions are atoms (or groups of atoms) with net electric charge because they have gained or lost electrons. Particle-level equations such as Mg → Mg2+ + 2 e− show how charge is conserved even when electrons move. Dissolving a salt produces free ions in solution and those mobile charges let electric circuits work through the liquid.

You saw two everyday ways charge matters: static electricity (movement of excess electrons on surfaces) and conduction in solutions (movement of ions). Both follow the same core rules about charges and conservation and lead directly into the next topic: how charged ions interact to form solid compounds.