What is the unit of measure used to express the mass of an atom?

The atomic mass unit is the standard unit used to express the mass of an atom. It is defined as one twelfth of the mass of a carbon-12 atom, which is a practical reference point for measuring atomic masses of other elements. By using the atomic mass unit, chemists can conveniently compare the masses of different elements in relation to carbon-12. This unit is particularly useful because atomic masses are typically very small and using a larger unit like a kilogram would yield very unwieldy numbers.

While other units such as kilogram, mole, and gram are commonly used in chemistry, they serve different contexts. The kilogram is too large for atomic and molecular scales, the mole is a unit for amount of substance, and the gram, although closer in scale, is not the standard for atomic masses. The atomic mass unit allows for precision and simplicity in expressing and comparing atomic and molecular weights effectively.