1925 – Austria
‘No two electrons in an atom can have the same quantum number’
A quantum number describes certain properties of a particle such as its charge and spin.
An orbital or energy level cannot hold more than two electrons, one spinning clockwise, the other anti-clockwise.
Electrons are grouped in shells, which contain orbitals. The shells are numbered ( n = 1,2,3 etc. ) outwards from the nucleus. These numbers are the ‘principle quantum numbers’.
An increase in n indicates an increase in energy associated with the shell, and an increase in the distance of the shell from the nucleus. The number of electrons allowed in a shell is 2n2. Each shell contains sub-shells or energy sub-levels. A shell can only have n sub-shells. A shell is given a number and a letter ( s,p,d,f,g,etc. ). For example, the electron shell structure of lithium is 1s22s1 (two electrons in ‘s’ sub-shell of the first shell, and one electron in ‘s’ sub-shell of the second shell; the superscript indicates the number of electrons in the shell).
The Pauli principle provided a theoretical basis for the modern periodic table.
1930 – Austria
‘The radioactive beta decay of an atomic nucleus in which a neutron turns into a proton and emits an electron does not seem to follow the law of conservation of energy.’
To account for the missing energy, Pauli postulated that a particle of zero charge and zero mass is released in such reactions.
A few years later ENRICO FERMI named the new particle a neutrino.
There are three known types of neutrino – muon, tau and electron.