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Ashby W. | Patient chemistry tutor with graduate level experiencePatient chemistry tutor with graduate le...
5.0 5.0 (3 lesson ratings) (3)

The Atomic number is the number of protons in an atom.  The mass number is found by taking the sum of all of the protons and neutrons in an atom and adding them together.

However, the mass number you calculate will always be slightly different than the mass on the periodic table because the periodic table also takes into account the percents of isotopes that exist naturally on Earth.


Again, "mass number" and "atomic mass" are two very different things, that happen to approximately correspond.

"Mass number" is simply the number of nuclear particles (protons and neutrons).

"Atomic mass" is the actual mass of the atom.  Keep in mind that protons and neutrons do not have masses of exactly one amu.  Even if they did, the mass of the nucleus is not the sum of the masses of the nuclear particles.

There is only one isotope of one element for which the mass number and atomic mass are exactly the same.  That is carbon-12, and happens because the amu is defined as 1/12 of the mass of a carbon-12 atom.

Jennifer A. | 10 years Experienced Chemistry, Biology, Science, & Math Tutor10 years Experienced Chemistry, Biology,...
4.5 4.5 (13 lesson ratings) (13)

Students often confuse the two. 

The Atomic Number is the number of protons ONLY.

The Mass Number is the number of protons + neutrons. 

NOTE: You can also obtain the atomic number from the number of electrons, since this number is the same as the number of protons (if the atom has not lost or gained any electrons through an external process).  

Dick B. | Math and Science TutorMath and Science Tutor
4.9 4.9 (72 lesson ratings) (72)

The mass number is not the same as the mass of the atom.

The mass number is obtained by counting the number of protons and neutrons.

Each proton and neutron is approximately 1 u, but not exactly.  Also, in atoms, the total is not the sum of the parts.  A small amount of mass is converted to energy to keep the nuclear particles together.

In summary, the atomic mass is the actual mass of the atom, while the mass number is simply the number of nuclear particles present.

Vera H. | ESL/ESOL, Russian, Math (Pre-Algebra, Geometry, Algebra I)ESL/ESOL, Russian, Math (Pre-Algebra, Ge...


The mass number  corresponds to the mass of the atom, that is obtained by adding the number of protons and neutrons present.

The mass numbers appears under the element symbol in the elements table. For instance, the atomic mass of oxygen is = 16. For nitrogen  = 14.


Atomic number  is the number of protons .

Atomic mass (mass number) is the total of protons plus neutrons.

So, the number of neutrons is Atomic mass minus Atomic number.