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.
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
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