Nuclear Symbol
A nuclear symbol (also called nuclide symbol) is a compact notation used to represent a specific isotope of an element, showing the element symbol with its mass number (A) as a superscript and atomic number (Z) as a subscript on the left. The mass number A equals the sum of protons and neutrons in the nucleus, while Z equals the number of protons (which defines the element). Nuclear symbols allow chemists and physicists to unambiguously specify isotopes in nuclear equations, radioactive decay series, and isotope chemistry.
Key Formula
A/Z X where A = Z + N
LaTeX: ^{A}_{Z}X \quad \text{where } A = Z + N
| Symbol | Meaning | Unit |
|---|---|---|
| A | Mass number (protons + neutrons) | dimensionless |
| Z | Atomic number (number of protons) | dimensionless |
| X | Chemical symbol of the element | — |
| N | Number of neutrons | dimensionless |
Worked Example
Problem
An isotope has the nuclear symbol ²³⁸₉₂U. Determine its number of protons, neutrons, and electrons (neutral atom).
Solution
Step 1 — Identify A and Z from the symbol: A (mass number, superscript) = 238 Z (atomic number, subscript) = 92 Step 2 — Number of protons = Z = 92 Step 3 — Number of neutrons = A − Z = 238 − 92 = 146 Step 4 — For a neutral atom, electrons = protons = 92
Answer
Protons: 92 | Neutrons: 146 | Electrons: 92
Reading Nuclear Symbols for Common Isotopes
| Nuclear Symbol | Element | Z (Protons) | N (Neutrons) | A (Mass Number) |
|---|---|---|---|---|
| ¹H (protium) | Hydrogen | 1 | 0 | 1 |
| ²H (deuterium) | Hydrogen | 1 | 1 | 2 |
| ¹²C | Carbon | 6 | 6 | 12 |
| ¹⁴C (radiocarbon) | Carbon | 6 | 8 | 14 |
| ²³⁵U | Uranium | 92 | 143 | 235 |
| ²³⁸U | Uranium | 92 | 146 | 238 |
Interactive Tools
NIST Nuclear Data — Chart of Nuclides
Comprehensive database of nuclide properties including all known isotopes.
Open ToolPhET Isotopes and Atomic Mass
Interactive simulation for exploring isotopes and mass numbers.
Open ToolWolframAlpha Isotope Data
Look up protons, neutrons, and nuclear properties for any nuclide.
Open Tool
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Related Terms
Atomic Mass
Atomic mass is the weighted average mass of all naturally occurring isotopes of an element, expressed in atomic mass units (u or amu), where 1 u is defined as one-twelfth the mass of a carbon-12 atom. Because most elements exist as mixtures of isotopes with different natural abundances, the atomic mass reported on the periodic table is not a whole number. Atomic mass is essential for converting between grams and moles of a substance using the molar mass concept.
Atomic Radius
Atomic radius is a measure of the size of an atom, typically defined as half the distance between the nuclei of two identical adjacent atoms in a covalent bond (covalent radius) or in a metallic lattice (metallic radius). Atomic radius decreases across a period (left to right) because increasing nuclear charge pulls electrons closer to the nucleus, while it increases down a group because additional electron shells increase the average distance of the outermost electrons from the nucleus. These periodic trends directly influence bond lengths, ionic sizes, and many physical properties.
Effective Nuclear Charge
Effective nuclear charge (Z_eff) is the net positive charge experienced by a valence electron after accounting for the shielding (screening) effect of inner electrons, which partially cancel the attraction from the nucleus. It is calculated as Z_eff = Z − S, where Z is the actual atomic number and S is the shielding constant. Effective nuclear charge increases across a period because additional protons are added while shielding remains approximately constant, explaining trends in atomic radius, ionization energy, and electron affinity.
From Latin "nucleus" (kernel, diminutive of "nux" meaning nut) and Greek "symbolon" (token, sign). The nuclide notation was standardised in the 20th century following the discovery of the neutron by James Chadwick in 1932.