It shows the type of atoms/elements in a substance
Different types of atoms have different emission spectrum - a concept supported by Bohr quantum theory - hence one is able to identify elements in a substance using a method called spectroscopy.
Explanation:
When an electron jumps from a low energy orbital to a higher energy orbital in an atom, it absorbs a specific wavelength of electromagnetic radiation (This is called absorption spectrum). Vice versa, if the same electron jumps from a higher to a lower energy orbital it releases the equivalent quantum energy in electromagnetic wave (This is called emission spectrum). Different types of atoms of different elements have a unique spectrum identifier.
An element is a mixture of two isotopes. One isotope has an atomic mass of 34.969 amu and an abundance of 64.88%. The other isotope has an atomic mass 36.966 amu.