Download Atomic definitions

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.