# Atomic Mass

on . Posted in Quantum Mechanics

Atomic mass, also called atomic weight, is a measure of the average mass of the atoms in a chemical element.  The atomic mass of an element takes into account the masses of the protons, neutrons, and electrons in its atoms.  Protons and neutrons are found in the atomic nucleus, while electrons orbit around the nucleus.  Since the mass of an electron is much smaller compared to the mass of protons and neutrons, it is often neglected when calculating atomic mass.  Therefore, the atomic mass of an element is mainly determined by the sum of the protons and neutrons in the nucleus.

Because different isotopes of an element have varying numbers of neutrons while maintaining the same number of protons (thus having the same atomic number), the atomic mass of an element is usually given as a weighted average of the atomic masses of its naturally occurring isotopes.  This weighted average takes into account the relative abundance of each isotope in the element's natural state.

The atomic mass is listed on the periodic table for each element, usually below the element's symbol.  It is a crucial concept in chemistry as it is used to calculate molar mass, which is important for stoichiometry, the study of the quantitative relationships between substances in chemical reactions.

### Atomic Mass FORMULA

$$u = p + n$$     (Atomic Mass)

$$p = u - n$$

$$n = u - p$$

Symbol English Metric
$$u$$ = atomic mass $$amu$$ $$amu$$
$$p$$ = number of protons $$dimensionless$$
$$n$$ = number of neutrons $$dimensionless$$