The atomic number and mass number describe two different properties of an atom. The atomic number tells you how many protons an atom has, while the mass number tells you the total number of protons and neutrons in its nucleus. Both numbers help scientists identify atoms, distinguish isotopes of the same element, and understand the structure of matter.

Although the two numbers are related, they serve different purposes. The atomic number determines which chemical element an atom is, while the mass number identifies a particular version of that element based on its nuclear composition.

What is the atomic number?

The atomic number is the number of protons in an atom’s nucleus. Protons are positively charged subatomic particles, and their number defines the identity of a chemical element.

Every element has a unique atomic number. Hydrogen has an atomic number of 1 because its nucleus contains one proton. Helium has an atomic number of 2, lithium has an atomic number of 3, and carbon has an atomic number of 6.

This sequence continues throughout the periodic table. Oxygen has eight protons, sodium has 11, and iron has 26. No two different elements have the same atomic number.

The atomic number is commonly represented by the letter Z. On a standard periodic table, the atomic number usually appears above or near the element’s chemical symbol.

For example, every carbon atom has six protons, regardless of how many neutrons it contains. If an atom has seven protons, it is nitrogen, not carbon. Changing the number of protons changes the element itself.

In a neutral atom, the number of electrons equals the number of protons. Electrons carry a negative charge and occupy regions around the nucleus. Because the positive and negative charges balance, the atom has no overall electrical charge. However, an atom can gain or lose electrons and become an ion without changing its atomic number.

What is the mass number?

The mass number is the total number of protons and neutrons in an atom’s nucleus. It is commonly represented by the letter A.

Protons and neutrons are collectively called nucleons because they are the particles that make up the nucleus. Both have masses close to one atomic mass unit, so counting them provides a convenient way to describe an atom’s nuclear composition.

The mass number is calculated by adding the number of protons to the number of neutrons:

Mass number = Number of protons + Number of neutrons

For example, a carbon atom containing six protons and six neutrons has a mass number of 12. A carbon atom containing six protons and eight neutrons has a mass number of 14.

Both atoms are carbon because each has six protons. Their mass numbers differ because their neutron counts are different.

Unlike the atomic number, the mass number is not fixed for an element. An element can have several forms with different numbers of neutrons, each with its own mass number.

The mass number is always a whole number because it counts individual protons and neutrons. It is not the same as an atom’s exact measured mass, which can differ slightly from the mass number because of the masses of the particles and the energy involved in binding the nucleus together.

The key differences between atomic number and mass number

The two quantities describe different aspects of an atom’s nucleus. The atomic number identifies the element, while the mass number describes the total number of nuclear particles.

The most important distinction is that the atomic number stays the same for every atom of a given element, but the mass number can vary.

For instance, all oxygen atoms have eight protons, so their atomic number is always 8. An oxygen atom with eight neutrons has a mass number of 16, while one with ten neutrons has a mass number of 18.

How to calculate the number of neutrons

Knowing an atom’s atomic number and mass number makes it possible to calculate how many neutrons its nucleus contains.

Rearranging the mass-number equation gives:

Number of neutrons = Mass number − Atomic number

Consider sodium-23. Sodium has an atomic number of 11, and the number 23 in its name represents its mass number.

The number of neutrons is therefore:

23 − 11 = 12 neutrons

A sodium-23 atom contains 11 protons and 12 neutrons. If the atom is electrically neutral, it also contains 11 electrons.

Now consider chlorine-37. Chlorine has an atomic number of 17, so its neutron count is:

37 − 17 = 20 neutrons

A chlorine-37 atom therefore contains 17 protons and 20 neutrons.

This calculation works for atoms of any element when their mass number and atomic number are known. It is particularly useful in chemistry and nuclear physics, where the composition of an atomic nucleus helps explain its properties and behavior.

Why isotopes have different mass numbers

Isotopes are atoms of the same element that contain different numbers of neutrons. Because they have the same number of protons, they share the same atomic number, but their mass numbers differ.

Carbon provides a familiar example. Carbon-12 and carbon-14 are both isotopes of carbon.

Carbon-12 contains six protons and six neutrons, giving it a mass number of 12. Carbon-14 contains six protons and eight neutrons, giving it a mass number of 14.

Comparing two carbon isotopes

¹²C

Carbon-12

6 protons

6 neutrons

Mass number: 12

¹⁴C

Carbon-14

6 protons

8 neutrons

Mass number: 14

Both isotopes have atomic number 6. Their different neutron counts produce different mass numbers and nuclear stability.

Isotopes generally have very similar chemical behavior because their electron arrangements are determined primarily by their proton counts in neutral atoms. However, differences in neutron number can affect nuclear stability, atomic mass, and some physical properties.

Some isotopes are stable, meaning their nuclei do not spontaneously undergo radioactive decay. Others are unstable and transform into different nuclei by releasing particles or energy. Carbon-12 is stable, whereas carbon-14 is radioactive.

These differences make isotopes valuable in science and medicine. Carbon-14, for example, is used in radiocarbon dating to estimate the age of once-living materials. Other isotopes have applications in medical imaging, cancer treatment, industrial measurements, and nuclear energy.

How atomic number and mass number appear in chemical notation

Scientists use a compact notation to show an isotope’s atomic number and mass number together. The chemical symbol identifies the element, while the two numbers provide information about its nucleus.

The standard notation places the mass number at the upper left of the chemical symbol and the atomic number at the lower left:

ᴬ_ZX

Here, X represents the chemical symbol, A is the mass number, and Z is the atomic number.

For example, carbon-12 can be written as ¹²₆C. The upper number, 12, is the mass number; the lower number, 6, is the atomic number.

Similarly, sodium-23 can be written as ²³₁₁Na. This notation tells you immediately that sodium has 11 protons and that its nucleus contains 23 protons and neutrons combined.

Subtracting the lower number from the upper number reveals the neutron count. For sodium-23, 23 − 11 = 12 neutrons.

In everyday writing, scientists often use a simpler form, such as carbon-12 or sodium-23. The element’s name or symbol identifies its atomic number, so the lower number is unnecessary when the element is already known.

Why mass number is not the same as atomic mass

Atomic number and mass number are also different from atomic mass, a related term that often causes confusion.

Mass number is a count of protons and neutrons in one nucleus. Atomic mass describes the mass of an atom, usually expressed in atomic mass units (u). The mass of a particular isotope is measured experimentally and is generally close to, but not exactly equal to, its mass number in atomic mass units.

The difference arises partly because protons and neutrons do not each have a mass of exactly 1 u. In addition, the energy released when a nucleus forms contributes to its total mass through the relationship between mass and energy. This effect is known as the mass defect.

The number shown for an element on many periodic tables is its standard atomic weight, which reflects the relative abundances of its naturally occurring isotopes. For elements with multiple naturally occurring isotopes, this value is generally a weighted average of their isotopic masses rather than a whole number.

For example, the periodic-table value for chlorine is approximately 35.45 u, not 35 or 36. Natural chlorine contains a mixture of chlorine-35 and chlorine-37, and the average reflects the relative amounts of those isotopes.

This distinction is important when reading a periodic table. The atomic number is a whole-number count of protons. The mass number is a whole-number count of protons and neutrons in a particular isotope. The listed atomic weight is a mass-related value that accounts for isotopic composition.

Why these numbers matter in chemistry and physics

The atomic number and mass number help explain different aspects of matter.

In chemistry, the atomic number is essential because the number of protons determines an element’s identity and, in a neutral atom, its electron count. Electron arrangements govern how atoms form chemical bonds, interact with other atoms, and participate in chemical reactions.

Mass number is especially useful when studying isotopes and nuclear reactions. It allows scientists to track the number of nucleons in a nucleus and helps distinguish different isotopes of the same element.

In nuclear reactions, the identities of nuclei can change, but the total number of nucleons is conserved in ordinary nuclear processes. Tracking mass numbers is therefore useful when balancing nuclear equations, alongside conservation of electric charge and other relevant quantities.

The neutron count also influences whether a nucleus is stable. A nucleus needs an appropriate balance of protons and neutrons for its particular size. If that balance is unfavorable, the nucleus may be radioactive. The exact conditions depend on nuclear structure and forces, not simply on whether the mass number is odd or even.

Together, atomic number and mass number provide a simple way to describe an atom’s nuclear composition. One tells you which element you are dealing with; the other tells you how many protons and neutrons its nucleus contains.

The simplest way to remember the difference

Remember that atomic number equals protons, while mass number equals protons plus neutrons.

The atomic number never changes for an atom of the same element. The mass number can change when the neutron count changes, producing a different isotope.

Once you know both numbers, you can identify the element, calculate its neutron count, and distinguish isotopes without needing any additional information about the atom’s nucleus.