How Many Protons, Neutrons, and Electrons in an Atom?

Steps to Find Number of Protons, Neutrons, and Electrons

The periodic table organizes elements by the number of protons in their atoms.
The periodic table organizes elements by the number of protons in their atoms. Andrew Brookes/Corbis/Getty Images

The three parts of an atom are positive-charged protons, negative-charged electrons, and neutral neutrons. Follow these simple steps to find the number of protons, neutrons, and electrons for an atom of any element.

Get Basic Information About Elements

You'll need to gather basic information about the elements to find the number of protons, neutrons, and electrons. Fortunately, all you need is a periodic table.

For any atom, what you need to remember is:

Number of Protons = Atomic Number of the Element

Number of Neutrons = Mass Number - Atomic Number

Find the Number of Protons

Each element is defined by the number of protons found in each of its atoms. No matter how many electrons or neutrons an atom has, the element is defined by its number of protons. In fact, it's actually possible to have an atom consisting of only a proton (ionized hydrogen). The periodic table is arranged in order of increasing atomic number, so the number of protons is the element number. For hydrogen, the number of protons is 1. For zinc, the number of protons is 30. The element of an atom with 2 protons is always helium.

If you are given the atomic weight of an atom, you need to subtract the number of neutrons to get the number of protons. Sometimes you can tell the elemental identity of a sample if all you have is the atomic weight. For example, if you have a sample with an atomic weight of 2, you can be pretty certain the element is hydrogen. Why? It's easy to get a hydrogen atom with one proton and one neutron (deuterium), yet you won't find a helium atom with an atomic weight of 2 because this would mean the helium atom had two protons and zero neutrons!

If the atomic weight is 4.001, you can be confident the atom is helium, with 2 protons and 2 neutrons. An atomic weight closer to 5 is more troublesome. Is it lithium, with 3 protons and 2 neutrons? Is it beryllium with 4 protons and 1 neutron? If you're not told the element name or its atomic number, it's hard to know the correct answer.

Find the Number of Electrons

For a neutral atom, the number of electrons is the same as the number of protons.

Often, the number of protons and electrons is not the same, so the atom carries a net positive or negative charge. You can determine the number of electrons in an ion if you know its charge. A cation carries a positive charge and has more protons than electrons. An anion carries a negative charge and has more electrons than protons. Neutrons do not have a net electric charge, so the number of neutrons does not matter in the calculation. The number of protons of an atom cannot change via any chemical reaction, so you add or subtract electrons to get the correct charge. If an ion has a 2+ charge, like Zn2+, this means there are two more protons than electrons.

30 - 2 = 28 electrons

If the ion has a 1- charge (simply written with a minus superscript), then there are more electrons than the number of protons. For F-, the number of protons (from the periodic table) is 9 and the number of electrons is:

9 + 1 = 10 electrons

Find the Number of Neutrons

To find the number of neutrons in an atom, you need to find the mass number for each element. The periodic table lists the atomic weight for each element, which can be used to find mass number, For hydrogen, for example, the atomic weight is 1.008. Each atom has an integer number of neutrons, but the periodic table gives a decimal value because it is a weighted average of the number of neutrons in the isotopes of each element. So, what you need to do is round the atomic weight to the nearest whole number to get a mass number for your calculations. For hydrogen, 1.008 is closer to 1 than 2, so let's call it 1.

Number of Neutrons = Mass Number - Number of Protons = 1 - 1 = 0

For zinc, the atomic weight is 65.39, so the mass number is closest to 65.

Number of Neutrons = 65 - 30 = 35