Fractional Atomic Mass

Fractional atomic mass refers to the average mass of an element’s isotopes relative to the mass of a carbon-12 atom, which is assigned a mass of exactly 12 atomic mass units (amu). It is expressed as a decimal fraction.

The fractional atomic mass takes into account the relative abundances of the isotopes of an element found in nature. Since many elements exist as a mixture of isotopes, each with its own mass and abundance, the fractional atomic mass provides a weighted average of these isotopic masses.

For example, carbon has two stable isotopes: carbon-12 and carbon-13, with natural abundances of approximately 98.9% and 1.1%, respectively. To calculate the fractional atomic mass of carbon, one would consider the mass of each isotope weighted by its abundance:

Fractional Atomic Mass of Carbon = (0.989 × 12) + (0.011 × 13)

This calculation yields the average atomic mass of carbon, taking into account the contributions of both isotopes based on their natural abundance.

Fractional atomic masses are essential for accurately representing the atomic masses of elements in the periodic table, which provides valuable information for chemical calculations and understanding the properties of elements.

Atomic mass is the total mass of all subatomic particles of an atom, including protons, neutrons, and electrons. However, the mass of electrons is incredibly small, so it’s typically neglected when determining an atom’s overall mass.

The unit commonly used to express atomic mass is the Atomic Mass Unit (AMU) and dalton, a non-SI unit. One dalton is equivalent to one-twelfth of the mass of a carbon-12 atom at rest in its ground state. This definition provides a standard reference point for measuring atomic masses.

The atomic mass of an individual atom is closely related to its mass number, which represents the total number of protons and neutrons in the nucleus. Since protons and neutrons contribute most to an atom’s mass, the atomic mass is nearly identical to the mass number. This relationship helps simplify calculations and understanding of atomic masses.