In the video, you learned that the masses of both protons and neutrons are very small. The mass of the electron is even smaller.
This table summarizes the masses of protons, neutrons, and electrons. Notice that protons and neutrons have almost the same mass. But it would take almost 2000 electrons to equal the mass of a proton.
| Particle | Actual Mass (g) | Mass (u) | Relative Mass |
|---|---|---|---|
| Electron | \(9.11 \times 10^{- 28}\) | 0.000549 | \(\frac{1}{1836}\) |
| Proton | \(1.674 \times 10^{- 24}\) | 1.007726 | 1 |
| Neutron | \(1.675 \times 10^{- 24}\) | 1.008665 | 1 |
The mass of an electron is so small that it does not contribute to the overall mass of an atom. Because of this, the mass number of an atom, or number of protons and neutrons, is very close to the atomic mass of that atom.
Because the mass of an atom depends mainly on the number of protons and neutrons it contains, and because protons and neutrons have masses that are very close to 1 u, you might expect the atomic mass of an element to always be very near a whole number. However, this is not the case. This is because the atomic mass of an element represents a weighted average of the masses of all the isotopes of that element. For example:
The average atomic mass of the element chlorine is displayed on the periodic table as 35.45 u. If the atomic mass of an atom is essentially equal to the number of protons and neutrons in the nucleus, why is this number not close to a whole number?
Chlorine has two stable, naturally occurring isotopes: chlorine-35 and chlorine-37. The numbers that follow the name of the element, chlorine, represent the mass number of that isotope. The data for chlorine's stable isotopes is provided in this table.
| Isotope | Percent Abundance | Atomic Mass (u) |
|---|---|---|
| Cl-35 | 75.77% | 34.969 |
| Cl-37 | 24.23% | 36.996 |
Notice the differences between chlorine-35 and chlorine-37. Their mass numbers are different because they have different numbers of neutrons. Their abundance in nature also differs, as chlorine-35 is a more common isotope. The average atomic mass of chlorine in nature must be calculated as a weighted average atomic mass that considers the percent abundance of each isotope.
Question
What are the values of the masses of a proton, neutron, and electron?
Protons and neutrons each have a mass of approximately 1 u. Electrons are essentially massless.
Question
Why is the atomic mass of an element not a whole number?
The atomic mass of an element is not a whole number because it is a weighted average of all the different isotopes of that element.
