The atoms of the same element, which logically have the same number of protons in the nucleus, but which have different number of neutrons, are called isotopes.	Lithium has three protons in the nucleus and three electrons on the periphery. But There are three isotopes of lithium that have three, four and five neutrons in the core.	The isotopes of an element have the same number of protons in the nucleus and electrons on the periphery. They only differ in the number of neutrons and, therefore, in its mass.

Scheme of a mass spectrometer

The isotopes of an element can be separated and detected in a mass spectrometer. A small amount of the atoms in gaseous state is introduced into a high vacuum tube. quantity of the atoms to be analyzed in the gaseous state. The atoms They pass through an area where an electron is knocked out from the atoms by a jet of electrons. In this way the atoms, lacking an electron, become positively-charged ions. They continue traveling through the tube and reach an area where there is a magnetic field of a certain intensity. The ions experience here a trajectory more or less curved, depending on its mass/charge ratio. Thus, for a certain magnetic field, only the ions 7Li can reach the slit and are separated from the 6Li ions and viceversa.

If the magnetic field is continuously varied and the electrical signal reaching the detector is recorded, we will see that for certain values of the field a signal with different intensity is detected, which corresponds to the different isotopes of an atom, of different mass because they have a different number of neutrons in the nucleus. So we obtain a mass spectrum of the Lithium atoms (or any other element), in which the relative intensity of the signals indicates the different abundance of its isotopes in Nature.

The Li isotope 8 (8Li) has a very low natural abundance and is only detected in ultrasensitive devices.