The mass number, or atomic mass, of potassium is 39.0983 amu at its stable state. Potassium is an element with an atomic number of 19 and the chemical symbol K. It is an alkali metal that exists as a solid at room temperature.
The atomic mass of an element is based on the number of protons and neutrons in the nucleus. The number of neutrons in an element can be calculated by subtracting the number of protons from the atomic mass. For example, potassium has 20 neutrons. The atomic mass has a decimal because it is the average of potassium's different isotopes.