Heisenberg’s Uncertainty Principle
In 1927, Werner
Heisenberg proposed
his uncertainty
principle which states
that the position and
the momentum of a
moving object cannot
simultaneously be
measured and known
exactly. This concept
becomes critical in
dealing with small
particles like
electrons.
The probability of
finding an electron in
various locations
around the nucleus
can be pictured in
terms of a blurry cloud
of negative charge.
The cloud is most
dense where the
probability of finding
the electron is highest.
The cloud is least
dense where the
probability of finding
the electron is lowest.
The density of an
electron cloud is
referred to as
electron density.
Orbitals and Energy
The probability of finding electrons in
certain regions of an atom is described
by orbitals.
 An atomic orbital is a region around the
nucleus of an atom where an electron
with a given energy is likely to be found.


Orbitals have
characteristics
shapes, sizes, and
energies. There are
several different kinds
of orbitals, each
having different
fundamental shapes.
These different kinds
of orbitals are
designed by the
letters s, p, d, and f.
All s orbitals are
spherical in shape,
whereas, p orbitals
are dumbbell shaped.
The shapes of d and f
orbitals are more
complex.

The main or
principal energy
levels in an atom are
designated by the
quantum number
n. There are seven
energy levels. Each
principal energy
level is divided into
one or more
sublevels (s, p, d,
and f). Each
sublevel is divided
into orbitals.
Electron Configuration

Electron configuration is the
distribution of electrons among the
orbitals of an atom. Electron
configurations of atoms are determined
by distributing the atom’s electrons
among levels, sublevels, and orbitals
based on a set of stated principles.
Summary of Principal Energy Levels,
Sublevels, and Orbitals
Diagonal Rule
Practice Problems
Write the electron configuration for the
following elements:
(1) H -
(3) Li -
(7) N -
(10) Ne -
(19) K -