Which best describes the definition for the atomic mass of an element?
It is a weighted average of the masses of an element’s isotopes.
It is a ratio based on the mass of the carbon-12 atom.
It is the sum of the protons and neutrons in one atom of the element.
It is twice the number of protons in one atom of the element.
The Correct Answer and Explanation is :
The correct definition of the atomic mass of an element is:
“It is a weighted average of the masses of an element’s isotopes.”
Explanation:
Atomic mass, also referred to as atomic weight, represents the average mass of all the isotopes of an element, weighted by their natural abundance. Elements exist in nature as a mixture of isotopes, which are atoms of the same element with different numbers of neutrons, and thus different masses. For example, chlorine has two main isotopes, chlorine-35 and chlorine-37. These isotopes do not exist in equal proportions; about 75% of naturally occurring chlorine is chlorine-35, and about 25% is chlorine-37.
The atomic mass is calculated as a weighted average because the more abundant an isotope is, the more it contributes to the overall atomic mass of the element. So, instead of being a simple arithmetic mean, the atomic mass accounts for the relative abundance of each isotope.
For example, if chlorine-35 has a mass of about 35 amu (atomic mass units) and chlorine-37 has a mass of about 37 amu, the atomic mass of chlorine is calculated as:
[
(0.75 \times 35) + (0.25 \times 37) = 35.5 \, \text{amu}.
]
Therefore, the atomic mass of chlorine is approximately 35.5 amu, which reflects the weighted average of its isotopes.
This value is listed on the periodic table and represents the element as it is found in nature. It is distinct from the mass number, which refers to the number of protons and neutrons in a specific isotope. The atomic mass is a more practical measure for chemists who deal with elements in their natural isotopic forms.