Pauli Exclusion Principle                                Hund"s Rule
It states that, in an atom or molecule, no two           It states that, (1) Every orbital in a sublevel is
electrons can have the same four electronic              singly occupied before any orbital is doubly
quantum numbers. As an orbital can contain a             occupied. (2) All of the electrons in singly
maximum of only two electrons, the two                   occupied orbitals have the same spin (to
electrons must have opposing spins. This                 maximize total spin).
means if one is assigned an up-spin (+1/2), the
other must be down-spin (-1/2).                          Aufbau Principle : “to build” principle
Electrons in the same orbital have the same              It states that no two fermions (e.g., electrons)
first three quantum numbers, e.g., n = 1, l = 0,         in an atom can have the same set of quantum
ml = 0 for the 1s subshell. Only two electrons           numbers, hence they have to "pile up" or
can have these numbers, so that their spin               "build up" into higher energy levels. It means
moments must be either ms = −1/2 or ms =+                electrons are added to orbitals as protons are
1/2.                                                     added to an atom.
                                NUCLEAR CHEMISTRY
Radioactivity                                                The atomic number and mass depends
                                                              upon the type of radioactive rays emitted
    Atoms become unstable due to large                       during nuclear reaction. The decay of
     neutron to proton ratio. Such unstable                   radioactive parent nuclei to stable nuclei is
     nucleus emitted some radiations and                      known as radioactive decay or nuclear
     convert in to some other stable nucleus                  decay.
     and known as radioactive elements. These                The type of decay depends on the type of
     radiations are termed as radioactive rays.               radioactive particles emitted in decay. For
    The property of emission of radioactive                  example, Alpha decay, Beta decay and
     rays from radioactive elements is termed                 Gamma decay.
     as radioactivity. It was discovered by
     Henry Becquerel in 1896.
    Generally, elements with atomic number
     more than 82 show radioactivity and
     disintegrated to small nuclei with the
     emission of alpha, beta, proton, neutron
     particles or gamma rays. This nuclei with
     decomposed is called as parent nuclei and
     the product nuclei is termed as daughter
                                            Nuclear Radiation
    Name           Charge        Symbol                Shield                    Distance traveled
    alpha          positive       (2He4)2+ or α        paper or clothing               2-4 cm
    beta           negative      -1e or β              aluminum                         2-3 m
    gamma          neutral       γ                     lead or concrete                500 m