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LESSON 12 What is the difference in atomic number, atomic mass, and mass number? Atoms of different kinds of elements have different numbers of protons and electrons. When scientists talk about different kinds of elements, they often refer to the element by its atomic number. The atomic number of an atom is the number of protons (and usually the number of electrons) in the atom. Scientists also describe atoms by their atomic mass. Scientists do not measure the mass of atoms in grams or ounces. They measure the mass of atoms in atomic mass units (amu). You can figure out the atomic mass by using the following information: Each proton has a mass of 1 amu Each neutron has a mass of 1 amu The mass number of an atom is the total number of protons and neutrons in the nucleus of the atom. What about electrons? Don’t they count? Electrons are very light. It takes almost 2,000 electrons to equal the mass of one proton. Because of this, their mass is not counted in the atomic mass. Sometimes two atoms of the same kind of matter do not have the same mass number. Why? They have a different number of neutrons. All atoms of the same kind of matter always have the same number of protons. They all have the same atomic number. Atoms of the same kind of matter that have different numbers of neutrons are called isotopes (i-suh-tohps). The atomic mass of an element is the average of the mass numbers of all of the isotopes of that element. You will find the atomic mass listed on the periodic table and it is usually a number with a decimal. The mass number is not listed on the periodic table, but you can calculate the mass number of the most common isotope of an element by rounding the atomic mass and subtracting the atomic number.