How to figure out number of electrons, protons and neutrons
First step: Gather information
1. Go to the Periodic Table of Elements and click on your element.
2. Use the Table of Elements to find your element's atomic number and atomic weight. The atomic
number is the number located in the upper left corner and the atomic weight is the number located on
the bottom, as in this example for krypton:
Equal to number of
protons and electrons
Equal to total number of
protons and neutrons
Step 2 - The Number of Protons is...
The atomic number is the number of protons in an atom of an element. In our example, krypton's
atomic number is 36. This tells us that an atom of krypton has 36 protons in its nucleus. The interesting
thing here is that every atom of krypton contains 36 protons. If an atom doesn't have 36 protons, it
can't be an atom of krypton.
Step 3 - The Number of Electrons is...
By definition, atoms have no overall electrical charge. That means that there must be a balance between
the positively charged protons and the negatively charged electrons. Atoms must have equal numbers
of protons and electrons. In our example, an atom of krypton must contain 36 electrons since it
contains 36 protons.
Step 4 - The Number of Neutrons is...
1. Figure out the mass number. The number at the bottom of Kr atom.
2. Round the mass to the nearest whole number.
3. Remember that the nucleus is made up of protons and neutrons. So, if we want, we can write:
Mass Number = (Number of Protons) + (Number of Neutrons)
For krypton, this equation becomes:
84 = (Number of Protons) + (Number of Neutrons)
As mass number is equal to total number of protons and the number of neutrons, once we know
protons we can subtract protons from mass and we will be able to find neutrons. So In our case we
have 36 protons and now we can figure out number of neutrons.
So atomic mass- number of protons= Number of neutrons
84 – 36 = number of neutrons which is 48
If we are provided with number of neutrons and atomic mass, we can figure out protons as
well
Atomic mass- no of neutrons= no of protons