Isotope
Isotopes are atoms of the same element (same atomic number Z) that have different numbers of neutrons, resulting in different mass numbers (A) but identical chemical behavior. Some isotopes are stable while others are radioactive (radioisotopes) and undergo decay, emitting radiation until they reach a stable configuration. Isotopes have widespread applications in medicine (radioactive tracers, cancer treatment), archaeology (radiocarbon dating), and nuclear energy.
Key Formula
Average atomic mass = Σ (fractional abundance × mass number) for all isotopes
LaTeX: A_{\text{avg}} = \sum_{i} (\text{abundance}_i \times A_i)
| Symbol | Meaning | Unit |
|---|---|---|
| A_avg | Average atomic mass of the element | atomic mass units (u) |
| abundance_i | Fractional natural abundance of isotope i | dimensionless (0 to 1) |
| A_i | Mass number of isotope i | u |
Worked Example
Problem
Chlorine exists as two isotopes: Cl-35 (abundance 75.77%) and Cl-37 (abundance 24.23%). Calculate the average atomic mass of chlorine.
Solution
Step 1: Convert percentages to decimals: 0.7577 and 0.2423. Step 2: Apply formula: A_avg = (0.7577 × 35) + (0.2423 × 37). Step 3: A_avg = 26.520 + 8.965 = 35.485 u.
Answer
Average atomic mass of chlorine = 35.45 u (matches the periodic table value of 35.45).
Stable and Radioactive Isotopes of Carbon
| Isotope | Protons | Neutrons | Mass Number | Stability | Application |
|---|---|---|---|---|---|
| Carbon-12 | 6 | 6 | 12 | Stable | Standard atomic mass reference |
| Carbon-13 | 6 | 7 | 13 | Stable | NMR spectroscopy |
| Carbon-14 | 6 | 8 | 14 | Radioactive (t½ = 5730 yr) | Radiocarbon dating |
Interactive Tools
PhET Isotopes and Atomic Mass
Build isotopes and calculate weighted average atomic mass interactively
Open ToolNIST Atomic Weights and Isotopic Compositions
Official database of isotopic abundances and masses for all elements
Open ToolPtable – Isotope Explorer
View all known isotopes, half-lives, and decay modes for each element
Open Tool
Wikimedia Commons, CC BY-SA
Related Terms
Neutron
A neutron is an electrically neutral subatomic particle found in the nucleus of all atoms except ordinary hydrogen, with a mass of approximately 1.675 × 10⁻²⁷ kg, slightly greater than the proton. Neutrons contribute to the mass of the nucleus and play a crucial role in nuclear stability by reducing electrostatic repulsion between protons through the strong nuclear force. Atoms with the same number of protons but different numbers of neutrons are called isotopes.
Mass Number
The mass number (symbol A) is the total number of nucleons — protons and neutrons — in the nucleus of an atom, and is always a whole number. It is used to distinguish between different isotopes of the same element, as isotopes have the same atomic number (Z) but different mass numbers (A) due to differing neutron counts. The mass number is approximately equal to the atomic mass in atomic mass units (u), though not exactly because of binding energy effects.
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 "isos" (equal) + "topos" (place), coined by Frederick Soddy in 1913 to describe atoms that occupy the same place on the periodic table (same element) but differ in mass. Soddy received the Nobel Prize in Chemistry in 1921 partly for this concept.