The total weight of an atom is called the atomic weight. It is approximately equal to the number of protons and neutrons, with a little extra added by the electrons.

Where is the atomic weight located?

Over 99.9 percent of an atom's mass resides in the nucleus. The protons and neutrons in the center of the atom are about 2,000 times heavier than the electrons orbiting around it. Because the electrons are so light by comparison, they represent only a tiny fraction of a percent of the atom's total weight.

How do you find the atomic weight of an element?

To calculate the atomic mass of a single atom of an element, add up the mass of protons and neutrons. Example: Find the atomic mass of an isotope of carbon that has 7 neutrons. You can see from the periodic table that carbon has an atomic number of 6, which is its number of protons.