What is Atomic Mass? - Definition & Examples

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  • 0:01 Definition of Atomic Mass
  • 1:15 Examples of Atomic Mass
  • 2:02 Lesson Summar
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Lesson Transcript
Instructor: John Williams
Atomic mass is a characteristic of an atom that is a measure of its size. It is plays a major role in the chemical properties of elements. This article discusses atomic mass and how it is calculated.

Definition of Atomic Mass

Atoms are the basic units of matter. They are the smallest components of a chemical element, which is a substance that cannot be broken down into simpler material. Atoms have specific properties that will determine their chemical and physical nature. One of these properties is their atomic mass. This video will discuss atomic mass and its role in chemistry.

Atomic mass is roughly equal to the sum of the individual particle masses of an atom. Atoms have three basic components: protons (positively charged particles), neutrons (non-charged particles), and electrons (negatively charged particles).

Protons and neutrons are the larger particles, and are found in the nucleus, which is the core of the atom. Atomic mass is typically calculated by adding the number of protons and neutrons together, ignoring the electrons because of their small size. Daltons are the standard units used for measuring atomic mass.

Atomic mass units, or amu, are also used to measure atomic masses, and they are equal to daltons. The atomic mass of an atom will change depending on its amount of protons and neutrons, and will be different for each individual element.

Examples of Atomic Mass

Hydrogen is the lightest naturally occurring element. It consists of one proton and one electron. The atomic mass for an atom of hydrogen is one dalton, which is calculated from the lone proton. Remember, electrons are too small to be considered in the atomic mass.

Helium is the second lightest element, and is larger than hydrogen. It consists of two protons and two neutrons. Because protons and neutrons are approximately the same size, the atomic mass of a helium atom is four daltons.

Let's look at a larger atom. Barium has an atomic mass of 137 daltons. This includes 56 protons and 81 neutrons. Once again, this does not consider electrons, as they are too small to be counted.

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