Ions
An ion is an atom or group of atoms with a net electric charge. The charge appears because proton and electron counts no longer match. The nucleus usually remains unchanged during ordinary ion formation.
Charge from electron bookkeeping
For a monatomic ion:
charge/e = number of protons − number of electrons
Al³⁺ has 13 protons and 10 electrons. O²⁻ has 8 protons and 10 electrons. Both are 10-electron species, but their nuclear charges differ.
Electron configurations of ions
Main-group ions often form by removing or adding electrons in the outermost occupied shell. Na: [Ne]3s¹ becomes Na⁺: [Ne]. Cl: [Ne]3s²3p⁵ becomes Cl⁻: [Ar].
Transition-metal ions need more care. Although 4s fills before 3d in neutral atoms, 4s electrons are normally removed first once the d subshell is occupied. Fe is commonly written [Ar]3d⁶4s²; Fe²⁺ is [Ar]3d⁶.
Polyatomic ions keep covalent bonds inside
NH₄⁺, SO₄²⁻ and NO₃⁻ are charged groups, not isolated charged atoms. Their atoms remain connected by covalent bonding while the group as a whole carries net charge.
Lewis structures and resonance help distribute that charge. A formal charge written on one atom in one Lewis form should not automatically be interpreted as a localized measured charge.
Ions in solids, melts and solutions
In an ionic solid, ions occupy an extended lattice. They cannot move freely, so the solid usually conducts poorly. When melted or dissolved, mobile ions can transport charge.
In water, ions are solvated: polar water molecules orient around them. Hydration changes effective interactions and is a major part of dissolution energetics.
Electron-count tool
Use signed charge: +2 for a 2+ cation, −1 for a 1− anion.
Ion formation is an energy balance
Removing an electron from a neutral atom requires ionization energy. Adding an electron has an electron-affinity energy change. Isolated Na does not spontaneously become Na⁺ merely because Na⁺ has a noble-gas electron configuration: energy must be supplied to remove the electron.
In NaCl formation, that cost is compensated by other favorable terms, especially attraction in the ionic lattice. In solution, hydration can provide strong stabilization. Stable ions arise from the total energy balance of the chemical environment.
Isoelectronic ions separate electron count from nuclear charge
O²⁻, F⁻, Ne, Na⁺, Mg²⁺ and Al³⁺ all contain 10 electrons. Their sizes are nevertheless different because their nuclei contain 8, 9, 10, 11, 12 and 13 protons respectively.
O²⁻ > F⁻ > Ne > Na⁺ > Mg²⁺ > Al³⁺ in radius.
Within one isoelectronic series, more protons pull the common electron population inward more strongly.
Charge balance determines ionic formulas
A bulk ionic compound is electrically neutral. The simplest whole-number ratio of cations and anions must make total positive and negative charge cancel.
| Ions | Neutral formula | Charge check |
|---|---|---|
| Na⁺ and Cl⁻ | NaCl | +1 −1 = 0 |
| Mg²⁺ and Cl⁻ | MgCl₂ | +2 + 2(−1) = 0 |
| Al³⁺ and O²⁻ | Al₂O₃ | 2(+3) + 3(−2) = 0 |
| Ca²⁺ and PO₄³⁻ | Ca₃(PO₄)₂ | 3(+2) + 2(−3) = 0 |
Subscripts describe the ratio of ions in the formula unit; they are not the charges themselves.
Variable charge in transition metals
Transition metals often form several stable ions because ns and (n−1)d states are close in energy. Iron commonly forms Fe²⁺ and Fe³⁺; copper forms Cu⁺ and Cu²⁺.
This is why names such as iron(II) chloride and iron(III) chloride specify oxidation state. The Roman numeral refers to the cation charge in those ionic compounds, not the number of atoms in the formula.
Ionization, dissociation and dissolution are different processes
When NaCl dissolves, pre-existing Na⁺ and Cl⁻ ions separate from the crystal and become hydrated; this is dissociation. When HCl dissolves in water, proton transfer produces H₃O⁺ and Cl⁻; this is often described as ionization because neutral HCl molecules form ions in the solvent.
These words should not be collapsed into “the compound turns into ions.” The microscopic starting point differs: an ionic lattice already contains ions, while a molecular substance may create ions through a chemical reaction with the solvent.
Hydration shells and ion mobility
Water molecules orient their oxygen end toward cations and their hydrogen ends toward anions. The first hydration shell is structured rather than random, and strongly charged small ions can bind water especially tightly.
Hydrated size can reverse expectations based on bare ionic radius. Li⁺ is a very small bare ion but strongly hydrates, so its effective hydrodynamic behavior in water differs from simply comparing crystal radii.
Electrical conductivity in solution depends on both the number of charge carriers and how rapidly each hydrated ion moves through the solvent.
Common ions and chemical names
| Ion | Name | Comment |
|---|---|---|
| NH₄⁺ | ammonium | common polyatomic cation |
| OH⁻ | hydroxide | base chemistry |
| NO₃⁻ | nitrate | resonance-delocalized anion |
| SO₄²⁻ | sulfate | 2− oxyanion |
| CO₃²⁻ | carbonate | central in acid–base equilibria |
| PO₄³⁻ | phosphate | 3− oxyanion |
Parentheses in formulas group a polyatomic ion when more than one is required: Ca(NO₃)₂ contains two nitrate ions for every Ca²⁺.
Ion pairing: in concentrated solutions, oppositely charged ions can remain associated for part of the time. Conductivity and thermodynamic behavior can therefore deviate from an ideal picture of completely independent hydrated ions.
Charge density matters: a small 2+ ion such as Mg²⁺ polarizes nearby solvent or anions much more strongly than a large 1+ ion. Ionic charge alone is not enough to predict interaction strength.
Exercises
Calcium ion
How many electrons are in Ca²⁺ (Z=20)?
Solution
18 electrons.
Parent size
Which is larger, Na or Na⁺?
Solution
Na. Forming Na⁺ removes the 3s electron and the entire n=3 valence shell.
Iron
Starting from [Ar]3d⁶4s², give Fe³⁺.
Solution
Remove two 4s electrons and then one 3d electron: [Ar]3d⁵.