Atomic Number, Atomic Mass, and Isotopic Average Mass

Atomic Structure Fundamentals

Atomic Number

The atomic number is a fundamental property of an element, representing the number of protons in the nucleus of an atom. It determines the identity of the element and its position on the periodic table.

• Symbol: Usually denoted as Z.

• Example: For carbon (C), the atomic number is 6, meaning every carbon atom has 6 protons.

Atomic Mass

The atomic mass (also called mass number) is the total number of protons and neutrons in an atom's nucleus. It is usually denoted as A.

• Formula:

$\text{Mass Number} = \text{Number of Protons} + \text{Number of Neutrons}$

• Example: For a typical carbon atom, if it has 6 protons and 6 neutrons, its mass number is 12.

Average Atomic Mass and Isotopes

Most elements exist as a mixture of isotopes, which are atoms of the same element with different numbers of neutrons. The average atomic mass is a weighted average of the masses of all naturally occurring isotopes of an element.

• Calculation: Multiply the mass of each isotope by its natural abundance (as a decimal), then sum the results.

$\text{Average Atomic Mass} = \sum (\text{Isotope Mass} \times \text{Fractional Abundance})$

• Example (Carbon):

$12(0.99) + 13(0.01) = 12.01$

• This calculation shows that the average atomic mass of carbon is approximately 12.01 amu, reflecting the presence of both carbon-12 and carbon-13 isotopes.

Key Terms

• Proton: Positively charged particle in the nucleus.

• Neutron: Neutral particle in the nucleus.

• Isotope: Atoms of the same element with different numbers of neutrons.

Additional info: The notes focus on carbon as an example, but the principles apply to all elements. The average atomic mass is what appears on the periodic table for each element.