Chemistry · Lesson
Ions, molecules and electrically neutral substances
Count electrons to determine charge, then use charge neutrality to check ionic formulas. Keep ions and molecules distinct.
Back to the Chemistry mapBy the end, you can
- Find an ion’s electron count from its proton count and charge.
- Distinguish monatomic and polyatomic ions.
- Check the net charge of a proposed ionic formula.
Charge follows the balance of charges
An ion is an atom or group of atoms with a net electric charge. A positively charged ion is a cation; a negatively charged ion is an anion. For a monatomic ion, the charge in elementary-charge units is proton count minus electron count. Ten protons and eight electrons give a net charge of +2; ten protons and eleven electrons give −1.
This is charge accounting, not a complete story about formation. Losing two electrons makes a monatomic ion two charge units more positive. Adding an electron makes it one unit more negative. Ordinary ion formation does not turn the atom into another element, because its proton count remains unchanged.
One atom or a charged group
Na⁺ is a monatomic ion: one sodium nucleus with an electron count giving charge +1. OH⁻ is a polyatomic ion: an oxygen and hydrogen group with net charge −1. The charge belongs to the whole group. It does not mean the group contains one extra hydrogen or that every constituent atom has that same net charge.
Useful introductory examples include calcium Ca²⁺, chloride Cl⁻, hydroxide OH⁻ and phosphate PO₄³⁻. Learning familiar ion names and charges can help read formulas. It is not a universal rule that every atom of an element always forms only one ion charge; transition metals, for example, often have several oxidation states in compounds.
Balance positive and negative contributions
A bulk ionic compound is electrically neutral overall. One Ca²⁺ and two Cl⁻ ions give +2 + 2(−1) = 0, so calcium chloride is written CaCl₂. The lower-right 2 gives the chloride-to-calcium ratio, while the superscript on an isolated ion gives its charge. Subscripts and charges do different jobs.
For calcium phosphate, three Ca²⁺ ions balance two PO₄³⁻ ions: 3(+2) + 2(−3) = 0. The formula is Ca₃(PO₄)₂. Parentheses show that two complete phosphate groups contribute to the ratio. Neutrality is a useful constraint; it does not by itself prove a proposed material is stable, available or correctly identified.
| Formula | Positive contribution | Negative contribution | Net charge |
|---|---|---|---|
| NaCl | 1 × (+1) | 1 × (−1) | 0 |
| CaCl₂ | 1 × (+2) | 2 × (−1) | 0 |
| Ca₃(PO₄)₂ | 3 × (+2) | 2 × (−3) | 0 |
Count charges of the stated ions. Zero net charge checks the ratio; it does not prove the existence or stability of an arbitrary suggested substance.
An ionic formula need not describe one molecule
A sodium chloride crystal contains an extended arrangement of sodium and chloride ions. NaCl expresses their 1:1 ratio; it is not a claim that the solid consists of separate NaCl molecules. A formula unit is a counting description based on that composition, rather than necessarily an isolated object you could pick out.
Representations depend on the setting. An ionic solid, dissolved ions and a gas-phase cluster are different physical situations. This lesson uses introductory solid compositions and ion-charge ledgers. Later bonding and solution chapters explain interactions and how the species present depend on the material and conditions.
Worked examples
Count a calcium ion
A calcium-40 ion has charge +2. Calcium has Z = 20. Give proton, neutron and electron counts.
- The calcium identity gives 20 protons.
- A = 40 gives 40 − 20 = 20 neutrons.
- For charge +2, the electron count is two fewer than the proton count: 18.
20 protons, 20 neutrons and 18 electrons. Removing electrons changes charge, not the calcium-40 nuclear identity.
Try it yourself
Sodium has Z = 11. How many electrons does Na⁺ have?
Show the answer and reasoning
10.
Eleven positive proton charges and ten negative electron charges leave net +1.
Chlorine has Z = 17. How many electrons does Cl⁻ have?
Show the answer and reasoning
18.
One more electron than protons gives net −1.
Which ratio balances Ca²⁺ and Cl⁻?
Show the answer and reasoning
One calcium ion for two chloride ions.
(+2) + 2(−1) = 0; the neutral composition is CaCl₂.
Does the NaCl formula establish separate NaCl molecules in the ordinary crystal?
Show the answer and reasoning
No.
The crystal is an extended ionic arrangement; the formula expresses its ion ratio.
What to carry forward
- For monatomic ions, charge = proton count − electron count.
- A polyatomic-ion charge applies to the whole group.
- Subscripts count composition; superscripts state charge.
- Charge neutrality checks ionic ratios within a stated species model.
References
Teaching resources and reference material supporting this lesson’s explanations and examples.