One electron moves. Two ions appear.
Sodium reacts vigorously with water, and chlorine gas is poisonous. Yet sodium chloride, formed from them, is the main substance in table salt. Why is the result so different? Simply placing two substances beside each other does not explain that change. We need to look inside their atoms and follow one small particle: an electron.
An atom’s protons carry positive charge and its electrons carry negative charge. Equal numbers balance, leaving the atom neutral overall. Place one sodium atom beside one chlorine atom in the model and count both kinds of particle. The shells from the atom model help us locate the outer electrons. Now let one electron move from sodium to chlorine. Before memorising the resulting charges, check what each atom has gained or lost. Counting again lets us account for those charges ourselves.
A charged atom is an ion. These two ions now have opposite charges and attract, just as opposite charges do on the Coulomb’s law page. This is our starting point for an ionic bond. A grain of salt contains far more than one pair, though. Once this transfer is clear, we will widen the view to the lattice and examine each ion’s neighbours.
01A useful pattern: the octet rule
For many atoms, eight electrons in the outermost shell is a common stable arrangement—the octet rule. It helps us guess the ratio of elements in a compound. It is not something atoms want to do. Hydrogen uses two, and there are many other exceptions.
02Where do + and − come from?
A proton carries +e and an electron carries −e; a neutron has no net electric charge. The positive number is the elementary charge: the amount carried by one proton or one electron. Add the charges: equal proton and electron counts cancel; an imbalance leaves a net charge. The net charge is (number of protons − number of electrons) times one elementary charge, so losing one electron leaves one extra positive charge.
- Net electric charge
- The total charge after adding all positive and negative charges; measured in coulombs (C).
- Number of protons
- Count the protons. Each contributes +e.
- Number of electrons
- Count the electrons. Each contributes −e.
- Elementary charge
- A positive amount of charge: . This charge is tiny: 10⁻¹⁹ means 1 divided by 10 nineteen times, so the first 1 sits at the 19th decimal place, and written out it is about 0.00000000000000000016 coulomb.
Na → Na⁺ + e⁻
Losing a negative electron leaves one extra positive charge.
Cl + e⁻ → Cl⁻
Gaining a negative electron leaves one extra negative charge.
Read the symbols
and count protons and electrons. means coulomb, the unit of electric charge—not carbon in this formula. Na⁺ means +1 elementary charge, not +1 coulomb.
03Salt is a crystal, not isolated Na–Cl pairs
Ionic bonding is the electrostatic attraction between opposite ions throughout the solid. In the interior of a sodium chloride crystal, each Na⁺ has six nearest Cl⁻ neighbours, and each Cl⁻ has six nearest Na⁺ neighbours. Rotate the fragment to find all six gold connections around the selected interior Na⁺.
This is an electron-accounting story, not the microscopic reaction pathway of sodium metal with chlorine gas. The transfer path and sphere boundaries are schematic. NaCl is an extended ionic solid; NaCl records its 1:1 ion ratio, not a separate molecule.
