The mass number of an atom is the total number of protons and neutrons in its nucleus. It identifies the number of these two types of particles that make up the atom’s nucleus and helps distinguish different forms of the same chemical element.
Calculating mass number is straightforward: add the number of protons to the number of neutrons. For example, a carbon atom with six protons and six neutrons has a mass number of 12. A carbon atom with six protons and eight neutrons has a mass number of 14.
Although mass number is closely related to an atom’s mass, the two are not the same. Understanding this distinction helps explain how atoms are identified, why isotopes exist, and how scientists describe the structure of matter.
What is mass number?
Mass number is the total number of protons and neutrons in an atom’s nucleus. These particles are collectively called nucleons because they make up the nucleus.
Protons carry a positive electric charge, while neutrons have no electric charge. Both have approximately one atomic mass unit of mass, making them much heavier than electrons. Electrons occupy regions outside the nucleus and contribute very little to an atom’s total mass.
The mass number is always a whole number because it counts individual protons and neutrons rather than measuring mass directly. An atom with 11 protons and 12 neutrons has a mass number of 23, regardless of how many electrons it has.
Mass number is especially useful because the number of protons determines an element’s identity, while the number of neutrons can vary among atoms of that element. This variation produces different isotopes.
How to calculate mass number
To calculate an atom’s mass number, determine how many protons and neutrons its nucleus contains, then add the two values.
The formula is:
Mass number = Number of protons + Number of neutrons
For example, suppose an atom contains 8 protons and 8 neutrons.
Mass number = 8 + 8 = 16
The atom’s mass number is 16.
If an atom contains 17 protons and 18 neutrons, its mass number is 35. The calculation works the same way for every element, regardless of its size or chemical properties.
The number of protons is called the atomic number. If the atomic number and mass number are known, the number of neutrons can be calculated by rearranging the formula:
Number of neutrons = Mass number − Atomic number
For an atom with a mass number of 23 and an atomic number of 11, the calculation is:
Number of neutrons = 23 − 11 = 12
The atom therefore contains 11 protons and 12 neutrons.
Examples of mass number in common elements
The following examples show how proton and neutron counts determine mass number in several familiar elements.
| Element | Protons | Neutrons | Mass number |
|---|---|---|---|
| Hydrogen-1 | 1 | 0 | 1 |
| Helium-4 | 2 | 2 | 4 |
| Carbon-12 | 6 | 6 | 12 |
| Nitrogen-14 | 7 | 7 | 14 |
| Oxygen-16 | 8 | 8 | 16 |
| Sodium-23 | 11 | 12 | 23 |
| Chlorine-35 | 17 | 18 | 35 |
| Calcium-40 | 20 | 20 | 40 |
Hydrogen-1 is a useful example because its nucleus contains just one proton and no neutrons. Its mass number is therefore 1. Helium-4 has two protons and two neutrons, giving it a mass number of 4.
Carbon-12 contains six protons and six neutrons. Carbon-14, another form of carbon, contains six protons and eight neutrons. Both are carbon because they have the same number of protons, but their mass numbers differ.
These examples demonstrate that mass number depends on the composition of the nucleus, not simply on the element’s name.
How mass number differs from atomic number
Mass number and atomic number describe different properties of an atom. The atomic number tells you how many protons the nucleus contains, while the mass number tells you the combined number of protons and neutrons.
The atomic number determines which chemical element an atom is. Every carbon atom has six protons, every oxygen atom has eight, and every sodium atom has 11. Changing the number of protons changes the element itself.
Mass number can change without changing the element. A carbon atom with six neutrons has a mass number of 12, while a carbon atom with eight neutrons has a mass number of 14. Both remain carbon because both have six protons.
This distinction is important when reading the periodic table. Each element has a fixed atomic number, but it can have several isotopes with different mass numbers.
What are isotopes, and how does mass number identify them?
Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. Because the proton count remains unchanged, isotopes belong to the same element. Their different neutron counts give them different mass numbers.
Carbon provides a clear example. Carbon-12 has six protons and six neutrons, while carbon-14 has six protons and eight neutrons. Their mass numbers are 12 and 14, respectively.
These isotopes share many chemical characteristics because chemical behavior depends primarily on an atom’s electrons and the arrangement of those electrons. However, differences in their nuclei can affect their stability and nuclear behavior.
Some isotopes are stable, meaning their nuclei do not spontaneously undergo radioactive decay. Others are radioactive and transform into different nuclear configurations over time, releasing particles or energy. Carbon-12 is stable, whereas carbon-14 is radioactive.
Scientists use mass numbers to distinguish isotopes and describe nuclear reactions, radioactive decay, and other processes involving atomic nuclei.
How to read mass number in atomic notation
Chemists and physicists often represent an isotope using the element’s chemical symbol and its mass number. A common format places the mass number at the upper left of the symbol and the atomic number at the lower left.
For example, carbon-12 can be written as:
¹²₆C
In this notation, the number at the upper left, 12, is the mass number. The number at the lower left, 6, is the atomic number. The chemical symbol C identifies carbon.
The number of neutrons can be found by subtracting the atomic number from the mass number:
12 − 6 = 6 neutrons
For sodium-23, the notation is:
²³₁₁Na
Its mass number is 23, and its atomic number is 11. Subtracting 11 from 23 gives 12 neutrons.
An isotope can also be named by writing the element’s name followed by a hyphen and its mass number, as in sodium-23 or carbon-14. This form is common in textbooks and scientific writing.
Is mass number the same as atomic mass?
Mass number is not the same as atomic mass, even though the two concepts are closely related.
Mass number is a count of protons and neutrons. It is a whole number with no unit because it represents the number of particles in a nucleus.
Atomic mass, by contrast, is a measurement of an atom’s mass, commonly expressed in atomic mass units (u). An atomic mass unit is defined as one-twelfth of the mass of a carbon-12 atom in its ground state.
Protons and neutrons each have masses close to, but not exactly equal to, one atomic mass unit. In addition, the energy associated with binding particles together in a nucleus affects the nucleus’s total mass. As a result, an atom’s measured mass is not generally identical to its mass number expressed in atomic mass units.
For example, a carbon-12 atom has a mass number of exactly 12 and an atomic mass of exactly 12 u by definition. A carbon-13 atom has a mass number of 13, but its atomic mass is approximately 13.003 u.
The distinction becomes particularly important when reading a periodic table. The atomic mass shown for an element is often a weighted average of the masses of its naturally occurring isotopes, based on their relative abundances. It is therefore frequently a decimal rather than a whole number.
Chlorine, for example, occurs naturally mainly as chlorine-35 and chlorine-37. Its average atomic mass is approximately 35.45 u because the two isotopes occur in different proportions. This decimal does not mean that an individual chlorine atom has 35.45 protons and neutrons. Each individual atom has a whole-number mass number.
Does the number of electrons affect mass number?
No. Mass number counts only protons and neutrons, so electrons are not included in the calculation.
A neutral atom has equal numbers of protons and electrons, giving it no overall electric charge. When an atom gains or loses electrons, it becomes an ion. Its electric charge changes, but its mass number remains the same as long as its nucleus is unchanged.
For example, a neutral sodium-23 atom contains 11 protons, 12 neutrons, and 11 electrons. If it loses one electron, it becomes a positively charged sodium ion. It still has 11 protons and 12 neutrons, so its mass number remains 23.
This principle also explains why the mass number cannot be calculated simply by adding all the particles in an atom. Electrons are subatomic particles, but the mass number specifically refers to the nucleons in the nucleus.
Why mass number matters in science
Mass number helps scientists describe the composition of atomic nuclei and distinguish isotopes of the same element. This information is essential in nuclear physics, chemistry, geology, medicine, and other fields.
In nuclear reactions, scientists track mass numbers to account for the nucleons involved in a reaction. The total mass number is conserved in ordinary nuclear reactions, including radioactive decay, even when individual nuclei change into different elements or isotopes.
Isotope identification also supports radiometric dating. For example, scientists use the radioactive decay of carbon-14 to estimate the age of certain once-living materials. The difference between carbon-14 and stable carbon isotopes is determined by their neutron counts and mass numbers.
In medicine, radioactive isotopes can be used in diagnostic imaging and treatment. Their nuclear properties, including stability and decay behavior, help determine how they can be used.
Mass number also provides a foundation for understanding nuclear stability. The balance between protons and neutrons influences whether a nucleus remains stable or undergoes radioactive decay. However, mass number alone cannot predict stability; the specific numbers and arrangement of protons and neutrons matter as well.
Common mistakes when calculating mass number
One frequent mistake is confusing mass number with atomic number. The atomic number counts protons only, while the mass number counts both protons and neutrons. Adding electrons to either count does not produce the mass number.
Another mistake is using the decimal atomic mass from the periodic table as if it were the mass number. The decimal often represents the weighted average of naturally occurring isotopes, not the particle count in one particular atom. To calculate the mass number of a specific isotope, add its protons and neutrons.
It is also important not to assume that all atoms of an element have the same mass number. All atoms of the same element have the same number of protons, but different isotopes can have different numbers of neutrons.
Finally, remember that a neutral atom and an ion of the same isotope have the same mass number if their nuclei are unchanged. Gaining or losing electrons changes the atom’s charge, not its proton and neutron counts.
The essential rule is simple: mass number equals the number of protons plus the number of neutrons in an atom’s nucleus. Once the atomic number and mass number are known, the number of neutrons can be found by subtraction. Together, these values provide a straightforward way to describe an atom’s nuclear composition and distinguish one isotope from another.
