How to Find the Number of Neutrons in an Atom

Co-authored by Bess Ruff, MA

Last Updated: October 22, 2024 Fact Checked

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Although all atoms of the same element contain the same number of protons, their number of neutrons can vary. Knowing how many neutrons are in a particular atom can help you determine if it's a regular atom of that element or an isotope, which will have either extra or fewer neutrons.^{[1]
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Research source} Determining the number of neutrons in an atom is fairly simple and doesn’t even require any experimentation. To calculate the number of neutrons in a regular atom or an isotope, all you need to do is follow these instructions with a periodic table in hand.

Method 1

Method 1 of 2:

Finding the Number of Neutrons in a Regular Atom

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Locate the element on the periodic table. For this example, we’ll look at osmium (Os), which is in the sixth row down.^{[2]
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Find the element’s atomic number. This tends to be the most visible number pertaining to a given element and usually sits above the element symbol, either in the middle of the box or in the upper left corner. (On the chart we're using, in fact, no other numbers are listed.) The atomic number is the number of protons in a single atom of that element.^{[3]
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Research source} Os is number 76, meaning one atom of osmium has 76 protons.
- The proton number never changes in an element; it's basically what makes that element that element.^{[4]
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This number is usually found beneath the atomic symbol. Note that the chart in this example is based solely on atomic number and doesn’t list the atomic weight. This won’t always usually be the case. Osmium has an atomic weight of 190.23.^{[5]
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Round off the atomic weight to the nearest whole number to find the atomic mass.^{[6]
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Research source} In our example, 190.23 would be rounded to 190, resulting in an atomic mass of 190 for osmium.
- The atomic weight is an average of the isotopes of the element, so that's why it's not usually a whole number.^{[7]
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Since the vast majority of an atom’s mass is made up of its protons and neutrons, subtracting the number of protons (i.e. the atomic number) from the atomic mass will give you the calculated number of neutrons in the atom. The numbers after the decimal point represent the usually very small mass of the electrons in the atom. In our example, this is: 190 (atomic weight) – 76 (number of protons) = 114 (number of neutrons).^{[8]
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Remember the formula. To find the number of neutrons in the future, simply use this formula:^{[9]
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- N = M – n
  - N = number of Neutrons
  - M = atomic Mass
  - n = atomic number

Method 2

Method 2 of 2:

Finding the Number of Neutrons in an Isotope

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As an example, we’ll look at the carbon-14 isotope. Since the non-isotopic form of carbon-14 is simply carbon (C), find carbon on the periodic table (in the second row down).^{[10]
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This tends to be the most visible number pertaining to a given element and usually sits above the element symbol. (On our example chart, in fact, no other numbers are listed.) The atomic number is the number of protons in a single atom of that element.^{[11]
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Research source} C is number 6, meaning one atom of carbon has 6 protons.
Find the atomic mass. This is incredibly easy with isotopes, as they are named according to their atomic mass. Carbon-14, for example, has an atomic mass of 14. Once you find the atomic mass of the isotope, the process is the same as it is for finding the number of neutrons in a regular atom.^{[12]
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Since the vast majority of an atom’s mass is found its protons and neutrons, subtracting the number of protons (i.e. the atomic number) from the atomic mass will give you the calculated number of neutrons in the atom.^{[13]
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Research source} In our example, this is: 14 (atomic mass) – 6 (number of protons) = 8 (number of neutrons).
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Remember the formula. To find the number of neutrons in the future, simply use this formula:^{[14]
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Research source}
- N = M – n
  - N = number of Neutrons
  - M = atomic Mass
  - n = atomic number

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How do you find the number of electrons, neutrons and protons?

Bess Ruff, MA
Environmental Scientist
Bess Ruff is a Geography PhD student at Florida State University. She received her MA in Environmental Science and Management from the University of California, Santa Barbara in 2016. She has conducted survey work for marine spatial planning projects in the Caribbean and provided research support as a graduate fellow for the Sustainable Fisheries Group.

Bess Ruff, MA

Environmental Scientist

Expert Answer

Fortunately, there's a WikiHow article that can help you! It's called Find the Number of Protons, Neutrons, and Electrons. While the answer section here doesn't allow links, you can search for it in the search box at the top of the page using this title.

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How many neutrons are in oxygen?

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Atomic mass minus the atomic number. Atomic mass of Oxygen is 16, and the atomic number is 8. 16 - 8 = 8.

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Where can I locate the atomic mass in a modern periodic table?

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The modern periodic table has the atomic number on the top of each element's symbol and the atomic mass right below the symbol.

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Protons and neutrons make up almost all of the weight of the elements, whereas electrons and miscellaneous particles represent negligible mass (approaching zero mass). Since one proton has approximately the same weight as one neutron, and the atomic number represents the number of protons, we can simply subtract the number of protons from the total mass.^{[15]
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If you’re ever unsure which number is which in a periodic table, just remember that the table is usually designed around the atomic number (i.e. number of protons), which starts at 1 (hydrogen) and ascends one unit at a time from left to right, ending at 118 (oganesson). This is because the number of protons in an atom determines what that atom is, making it the easiest elemental trait to organize by. (Ex. an atom with 2 protons is always helium, just as an atom with 79 protons is always gold.)

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