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CHEMISTRY / CHEMICAL BONDS 02

One electron moves. Two ions appear.

Table salt contains sodium and chlorine. What changes when these elements form an ionic compound? Begin with the electrons, keep track of charge, then step back to see the repeating crystal.

01A useful pattern: the octet rule

For many main-group atoms, eight electrons in the outermost shell is a common stable arrangement—the octet rule. Treat it first as an observed pattern that helps predict formulas, not a wish that atoms consciously fulfil or a universal law. Hydrogen uses two, and there are many other exceptions.

Na + Cl → Na⁺ + Cl⁻28 electrons in total, including any in flight
NaCl2 · 8 · 12 · 8 · 711 p⁺nucleus17 p⁺nucleus11 protons · 11 electrons17 protons · 17 electronsElectron-count diagram · shells are not orbital paths · sizes not to scale
The transferred electronOther electrons
Sodium has 2, 8, 1 electrons in its shells; chlorine has 2, 8, 7. Focus on the outermost shell: sodium can lose its one outer electron, and chlorine can gain one to complete eight.

02Where do + and − come from?

A proton carries +e and an electron carries −e; a neutron has no net electric charge. The positive number e is the elementary charge. Add the charges: equal proton and electron counts cancel; an imbalance leaves a net charge.

e=1.602176634×10−19 C,Q=(Np−Ne)ee=1.602176634\times10^{-19}\,\mathrm{C},\qquad Q=(N_p-N_e)e
QQNet electric charge
The total charge after adding all positive and negative charges; measured in coulombs (C).
NpN_pNumber of protons
Count the protons. Each contributes +e.
NeN_eNumber of electrons
Count the electrons. Each contributes −e.
eeElementary charge
A positive amount of charge: 1.602176634 × 10⁻¹⁹ C.

Na → Na⁺ + e⁻

QNa+=(11−10)e=+eQ_{\mathrm{Na^+}}=(11-10)e=+e

Losing a negative electron leaves one extra positive charge.

Cl + e⁻ → Cl⁻

QCl−=(17−18)e=−eQ_{\mathrm{Cl^-}}=(17-18)e=-e

Gaining a negative electron leaves one extra negative charge.

Read the symbols

Nₚ and Nₑ count protons and electrons. C 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⁺.

A fragment of the NaCl latticeSelected Na⁺ · 6 nearest neighbours
Drag to orbit · arrow keys also rotate · Home resets the view
Na⁺Cl⁻Nearest-neighbour connections

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.

OpenStax · Ionic bondingNIST · Elementary chargeNext: sharing electrons →