Chemistry 2e · Chemical Bonding and Molecular Geometry
Ionic Bonding
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Atoms bond when doing so lowers their energy. In ionic bonding, the energy-lowering step is the complete transfer of one or more electrons from a metal atom to a nonmetal atom, producing a positively charged cation Positively charged ion (lost electrons) Full entry → and a negatively charged anion Negatively charged ion (gained electrons) Full entry →. The electrostatic attraction between these opposite charges is the ionic bond Electrostatic attraction between oppositely charged ions Full entry →. Table salt is the classic example: sodium gives its single outer electron to chlorine, creating Na⁺ and Cl⁻, which pack into an extended three-dimensional lattice Repeating three-dimensional arrangement of ions in an ionic solid Full entry → rather than a discrete molecule.
Why this matters
Ionic compounds are everywhere — on the table, in the ground, and inside the body. Sodium chloride seasons food and forms the basis of saline IV solutions; calcium phosphate gives bones their hardness; sodium and potassium ions maintain the concentration gradients that nerve cells use to fire signals. In health care, electrolyte A substance whose dissolved or molten ions conduct electricity Full entry → panels measure dissolved ions precisely because ions, not neutral atoms, carry charge in body fluids. Ionic bonding explains why salts melt at high temperatures (NaCl melts at 801 °C), why they conduct when molten or dissolved, and why ceramics and batteries depend on ionic materials. It is also where electron configuration, periodic trends, and charge first come together — and it sets up the lattice-energy reasoning used again in thermochemistry.
The college version
Core Concepts
What makes a bond ionic
An ionic bond typically forms between a metal (low electronegativity) and a nonmetal (high electronegativity). The electronegativity difference ΔEN for NaCl is 3.16 - 0.93 = 2.23, large enough that chlorine essentially pulls sodium's electron entirely onto itself. A common guideline: ΔEN > 1.7 suggests mostly ionic behavior. It is a rule of thumb, not a law — bonding is a continuum, and compounds like AlCl₃ (ΔEN = 1.55) sit in between.
Electron transfer produces octets
Sodium, 1s2 2s2 2p6 3s1, loses its single 3s1 electron to become Na⁺ with the neon configuration, 1s2 2s2 2p6. Chlorine, [Ne]3s2 3p5, gains one electron to reach the argon configuration, 3s2 3p6, as Cl⁻. Both ions now have a full outer shell of eight electrons — the octet rule Tendency of atoms to gain, lose, or share electrons to reach eight valence electrons Full entry → in action. The pattern repeats across the periodic table: group 1 metals form 1+ ions, group 2 metals form 2+ ions, group 17 nonmetals form 1− ions, and group 16 nonmetals form 2− ions.
Electrostatic attraction and the crystal lattice
The attraction between oppositely charged ions follows Coulomb's law. For two point charges Q1 and Q2 separated by distance r, the energy is
E = k Q1 Q2r
with k = 8.99 × 109 J m C-2. Because the charges have opposite signs, E is negative — the ion Atom or group of atoms with a net charge from losing or gaining electrons Full entry → pair is more stable than the separated ions. The ions do not stop at one pair: each Na⁺ is surrounded by six Cl⁻ neighbors (and vice versa) in the repeating rock-salt lattice. There is no "NaCl molecule"; the formula unit Smallest neutral ratio of ions in an ionic compound, such as NaCl Full entry → simply states the 1:1 ratio of Na⁺ to Cl⁻ in the crystal.
Properties that follow from the lattice
Strong electrostatic attractions give ionic compounds their signature properties: high melting and boiling points, hardness, and brittleness. The solids do not conduct electricity because the ions are locked in place; molten salts and aqueous solutions do, because the ions can move. When a crystal is struck, layers of like-charged ions slide past each other and repel — salt shatters instead of bending.
How It Works / Step-by-Step Process
- Identify the metal (cation former) and the nonmetal (anion former) in the compound.
- Write each ion with its charge, using group position or the electron configuration.
- Combine the ions in the smallest whole-number ratio that makes the total charge zero.
- Name the compound: cation name + anion root with -ide (NaCl = sodium chloride).
- Predict properties from the lattice model: high melting point, hardness, conductivity only when molten or dissolved.
Common Confusions
| Do not confuse | With | Difference |
|---|---|---|
| An "NaCl molecule" | An NaCl lattice | Salts are extended crystals; the formula unit is a ratio, not a discrete molecule |
| ΔEN > 1.7 as an absolute law | A useful guideline | Bonding is a continuum; many bonds (Al–Cl, for one) are intermediate |
| Formal charge or partial charge | Actual ionic charge | Crystal ions carry full integer charges; bookkeeping charges (Topic 4) are separate |
| "Ionic compounds always conduct electricity" | "They conduct when molten or dissolved" | Solid lattices have locked ions and do not conduct |
| "Any metal + nonmetal pair is ionic" | A strong general trend | Most are largely ionic, but transition metals and polyatomic ions complicate matters; NH₄Cl is ionic with no metal |
| "Electron transfer is effortless" | Transfer costs energy, repaid by the lattice | Ionization costs energy; lattice formation repays it, making the process favorable |
Eli explains
The same idea, in plain words
Explain it like I’m 10
A metal atom has an extra toy it doesn't really need, and a nonmetal atom wants one more. The metal hands its toy over, so the metal is now missing one (positive) and the nonmetal has an extra (negative) — they stick together like magnets. Millions of these pairs stack into a crystal: table salt.
Worked example
Example 1: Predicting the formula of aluminum oxide
Aluminum (group 13) loses three electrons to form Al³⁺; oxygen (group 16) gains two to form O²⁻. The formula must balance charge: two Al³⁺ ions give 2(+3) = +6, and three O²⁻ ions give 3(-2) = -6, so the neutral combination is Al₂O₃, with 2(+3) + 3(-2) = 0. The crisscross method — swapping the charge magnitudes (3 and 2) for subscripts — reaches the same answer, but always verify that the charges sum to zero.
Example 2: Quantifying the attraction in NaCl with Coulomb's law
Estimate the electrostatic energy between one Na⁺ and one Cl⁻ in the crystal. Use Coulomb's law:
E = k Q1 Q2r
with k = 8.99 × 109 J m C-2, Q1 = +1.602 × 10-19 C, Q2 = -1.602 × 10-19 C, and r equal to the sum of the ionic radii: 102 pm + 181 pm = 283 pm.
First convert the distance to meters:
r = 283 pm × 1 m1012 pm = 2.83 × 10-10 m
Substitute:
E = (8.99 × 109)(+1.602 × 10-19)(-1.602 × 10-19)2.83 × 10-10 = -8.15 × 10-19 J per ion pair
Convert to a per-mole quantity using Avogadro's number, 6.022 × 1023 mol-1:
E = (-8.15 × 10-19 J) × (6.022 × 1023 mol-1) = -4.91 × 105 J mol-1 = -491 kJ mol-1
The measured lattice energy of NaCl is about -787 kJ mol-1. The pairwise estimate is smaller because a real ion interacts with all its neighbors — attractions to six Cl⁻ and repulsions from twelve Na⁺ — not just one partner. Negative sign: the lattice forms with a release of energy.
Key takeaways
- Ionic bonds form by electron transfer from a metal to a nonmetal; the resulting ions attract electrostatically.
- Cations are positive (lost electrons); anions negative (gained electrons). Charges follow group position: Na⁺, Mg²⁺, Al³⁺, F⁻, O²⁻, N³⁻.
- The octet rule guides transfer: ions usually adopt noble-gas electron configurations.
- Ionic compounds are extended lattices, not molecules; the formula unit is the smallest neutral ion ratio.
- Signature properties: high melting points, hardness, brittleness, no conductivity as a solid, conductivity when molten or dissolved.
- Coulomb's law E = kQ1Q2/r: bigger charges strengthen the attraction; larger separation weakens it.
Check yourself
5 review questions from the chapter. Try each one, then open the answer.
Why does sodium transfer exactly one electron to chlorine?
Show answer
Sodium has one valence electron; losing it gives Na⁺ with a full neon octet. Chlorine needs exactly one electron to reach the argon configuration.
Write the formula for calcium fluoride (Ca²⁺, F⁻).
Show answer
Calcium forms Ca²⁺ and fluorine forms F⁻, so two F⁻ are needed per Ca²⁺: CaF₂.
Why does solid NaCl not conduct electricity, while molten NaCl does?
Show answer
In the solid, ions are locked in the lattice and cannot move. When molten, the lattice breaks down and the ions become mobile charge carriers.
Two ionic solids have identical ion sizes, but one contains 1+ and 1− ions and the other 2+ and 2− ions. Which has the stronger lattice, and why?
Show answer
The 2+/2− compound. Coulomb's law depends on the product Q1Q2, so doubling both charges quadruples the attraction.
What does the formula unit "NaCl" actually describe?
Show answer
It states the 1:1 ratio of Na⁺ to Cl⁻ in the crystal lattice — not a molecule.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- ion
- Atom or group of atoms with a net charge from losing or gaining electrons
- cation
- Positively charged ion (lost electrons)
- anion
- Negatively charged ion (gained electrons)
- ionic bond
- Electrostatic attraction between oppositely charged ions
- lattice
- Repeating three-dimensional arrangement of ions in an ionic solid
- formula unit
- Smallest neutral ratio of ions in an ionic compound, such as NaCl
- octet rule
- Tendency of atoms to gain, lose, or share electrons to reach eight valence electrons
- electrolyte
- A substance whose dissolved or molten ions conduct electricity
Sources & references
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