Ion
An ion is an atom or molecule that has gained or lost one or more electrons, resulting in a net electric charge — positive ions (cations) form when electrons are lost, and negative ions (anions) form when electrons are gained. Ions play a fundamental role in chemical bonding (ionic compounds), electrochemistry, biological processes (nerve impulses rely on Na⁺ and K⁺ ions), and analytical chemistry. The charge of an ion is written as a superscript, such as Ca²⁺ or Cl⁻.
Key Formula
Ion charge = (Number of protons) - (Number of electrons)
LaTeX: \text{Ion charge} = Z - e^-_{\text{actual}}
| Symbol | Meaning | Unit |
|---|---|---|
| Z | Number of protons (atomic number) | dimensionless |
| e⁻_actual | Actual number of electrons in the ion | dimensionless |
Worked Example
Problem
Magnesium (Mg, Z = 12) loses 2 electrons to form an ion. What is its charge and symbol?
Solution
Step 1: Neutral Mg has 12 protons and 12 electrons. Step 2: Loses 2 electrons → electrons remaining = 12 - 2 = 10. Step 3: Charge = 12 protons - 10 electrons = +2.
Answer
The ion is Mg²⁺ (magnesium ion), with a +2 charge.
Common Ions and Their Charges
| Ion | Name | Type | Electrons Lost/Gained | Charge |
|---|---|---|---|---|
| Na⁺ | Sodium ion | Cation | -1 electron | +1 |
| Ca²⁺ | Calcium ion | Cation | -2 electrons | +2 |
| Al³⁺ | Aluminium ion | Cation | -3 electrons | +3 |
| Cl⁻ | Chloride ion | Anion | +1 electron | -1 |
| O²⁻ | Oxide ion | Anion | +2 electrons | -2 |
| N³⁻ | Nitride ion | Anion | +3 electrons | -3 |
Interactive Tools
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Related Terms
Electron
An electron is a negatively charged subatomic particle that occupies the space around an atom's nucleus in regions called orbitals or electron shells, with a charge of -1 elementary charge and a mass of 9.109 × 10⁻³¹ kg (about 1/1836 the mass of a proton). Electrons govern chemical bonding, electrical conductivity, and the optical properties of matter by determining how atoms interact with one another. In a neutral atom, the number of electrons equals the number of protons.
Electron Configuration
Electron configuration is the distribution of electrons of an atom or molecule among its atomic orbitals, described by the principal quantum number (n), sublevel (s, p, d, f), and the number of electrons in each sublevel. Electron configuration determines how an element behaves chemically — its valence electrons (outermost shell) govern reactivity, oxidation states, and bonding capacity. The Aufbau principle, Pauli exclusion principle, and Hund's rule govern how electrons fill orbitals.
Atomic Number
The atomic number (symbol Z) is the number of protons in the nucleus of an atom and uniquely identifies the chemical element — all atoms of the same element have the same atomic number. In a neutral atom, the atomic number also equals the number of electrons, which determines the element's chemical behavior, reactivity, and position on the periodic table. The atomic number ranges from 1 (hydrogen) to 118 (oganesson) for currently known elements.
From Greek "ion" meaning "going" (present participle of "ienai", to go), coined by Michael Faraday in 1834 to describe charged particles that "go" toward electrodes during electrolysis. Faraday also introduced the terms "cation" (from Greek "kation", going down) and "anion" (going up) for positive and negative ions respectively.