Simple Nature - Light and Matter

The relatively complex spectra of elements heavier than hydrogencan be understood as arising from the great number of possible combinationsof states for the electrons.A surprising thing happens, however, with lithium, the threeelectronatom. We would expect the ground state of this atom tobe one in which all three electrons settle down into n = 1 states.What really happens is that two electrons go into n = 1 states, butthe third stays up in an n = 2 state. This is a consequence of a newprinciple of physics:The Pauli Exclusion PrincipleOnly one electron can ever occupy a given state.There are two n = 1 states, one with s z = +1/2 and one withs z = −1/2, but there is no third n = 1 state for lithium’s thirdelectron to occupy, so it is forced to go into an n = 2 state.It can be proved mathematically that the Pauli exclusion principleapplies to any type of particle that has half-integer spin. Thustwo neutrons can never occupy the same state, and likewise for twoprotons. Photons, however, are immune to the exclusion principlebecause their spin is an integer.Deriving the periodic tableWe can now account for the structure of the periodic table, whichseemed so mysterious even to its inventor Mendeleev. The first rowconsists of atoms with electrons only in the n = 1 states:HHeLiBe. . .OFNe1 electron in an n = 1 state2 electrons in the two n = 1 statesThe next row is built by filling the n = 2 energy levels:2 electrons in n = 1 states, 1 electron in an n = 2 state2 electrons in n = 1 states, 2 electrons in n = 2 states2 electrons in n = 1 states, 6 electrons in n = 2 states2 electrons in n = 1 states, 7 electrons in n = 2 states2 electrons in n = 1 states, 8 electrons in n = 2 statesIn the third row we start in on the n = 3 levels:Na 2 electrons in n = 1 states, 8 electrons in n = 2 states, 1electron in an n = 3 state...We can now see a logical link between the filling of the energylevels and the structure of the periodic table. Column 0, for example,consists of atoms with the right number of electrons to fill allthe available states up to a certain value of n. Column I containsatoms like lithium that have just one electron more than that.This shows that the columns relate to the filling of energy levels,i / The beginning of the periodictable.j / Hydrogen is highly reactive.Section 13.4 The Atom 891