Atomic Number and Atomic Mass: Get here the easy notes on atomic number and atomic mass for all the first 30 elements. Know how to calculate atomic mass using atomic number.

The standard atomic weight is the average mass of an element in atomic mass units. One atomic mass unit, or amu, is equal to 1/12 the mass of a single carbon-12 atom.

The exceptions are elements that have only one naturally occurring isotope, such as fluorine, where the numerical value of the isotopic mass of fluorine-19 would be equal to the atomic weight.

Atomic Number and Atomic Mass Atomic Number: The atomic number of an atom is determined by the number of protons it has. It uniquely identifies each element on the periodic table.

Atomic structure and the periodic table Atoms, elements and compounds - AQA Chemists use symbols and formulae to represent elements and compounds.

Some elements have more than one stable (nonradioactive) isotope —variants of the same substance, but with different numbers of neutrons in their atomic nuclei that alter the mass.