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.
Key Formula
A = Z + N (Mass number = Atomic number + Number of neutrons)
LaTeX: A = Z + N
| Symbol | Meaning | Unit |
|---|---|---|
| A | Mass number | dimensionless |
| Z | Atomic number (protons) | dimensionless |
| N | Number of neutrons | dimensionless |
Worked Example
Problem
Chlorine-35 has 17 protons. Calculate its mass number and number of neutrons.
Solution
Step 1: Z = 17 (given — chlorine always has 17 protons). Step 2: Mass number A = 35 (given in the isotope name). Step 3: N = A - Z = 35 - 17 = 18 neutrons.
Answer
Chlorine-35 has a mass number of 35 and contains 18 neutrons.
Mass Numbers of Common Isotopes
| Element | Symbol | Z (Protons) | N (Neutrons) | A (Mass Number) |
|---|---|---|---|---|
| Hydrogen-1 | ¹H | 1 | 0 | 1 |
| Deuterium | ²H | 1 | 1 | 2 |
| Carbon-12 | ¹²C | 6 | 6 | 12 |
| Carbon-14 | ¹⁴C | 6 | 8 | 14 |
| Uranium-235 | ²³⁵U | 92 | 143 | 235 |
| Uranium-238 | ²³⁸U | 92 | 146 | 238 |
Interactive Tools
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Related Terms
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.
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.
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.
The term "mass number" came into use in the early 20th century following the work of Francis Aston, who developed the mass spectrograph in 1919 and demonstrated that elements consist of isotopes with different whole-number masses. The term distinguishes it from atomic mass, which is a weighted average.