Quantum Theory and the
Atom
Learning Objective
• Describe the relationship between electron
levels, sublevels and atomic orbitals.
Real-World Link
A house number is not enough to deliver a letter
to the correct address. More information, such
as a street name, city, and state, is necessary to
deliver the letter. Similarly, chemical elements
are identified according to details about the
arrangement of their electrons.
Bohr’s Model of the Atom
• Bohr suggested that
electrons are arranged
in concentric circular
orbits around the
nucleus.
Heisenberg’s Uncertainty Principle
• Heisenberg’s uncertainty
principle – states that it is
impossible to know both
the momentum and
position of an electron at
the same time.
• In other words, it is
impossible to know the
exact location of an
electron.
Heisenberg’s Uncertainty Principle
• The Heisenberg
uncertainty principle
means that it is
impossible to assign fixed
paths for electrons like
the circular orbits in
Bohr’s model.
• The only quantity that can
be known is the
probability for an electron
to occupy a certain region
around the nucleus.
Quantum Mechanical Model of the
Atom
• Like Bohr’s model, the
quantum mechanical
model limits an
electron’s energy to
certain values.
• Unlike Bohr’s model,
the quantum
mechanical model does
not attempt to describe
the electron’s path
around the nucleus.
Electron’s Probable Location
• Schrodinger’s wave
function (an equation
that treated the hydrogen
atom electron as a wave)
predicts a threedimensional region
around the nucleus,
called an atomic orbital,
which describes the
electron’s probable
location.
• An orbital can hold up to
two electrons.
Quantum Numbers and Atomic
Orbitals
• A set of four quantum
numbers is needed to
describe an electron’s
location/properties within
an atom.
– Principal quantum number
(n)
– Orbital angular momentum
quantum number (l)
– Magnetic quantum number
(ml)
– Electron spin quantum
number (ms)
Principal Quantum Number
• The principal quantum
number (n) describes the
average distance of the
orbital from the
nucleus—and the energy
of the electron in an
atom.
– It can only be a positive
integer (whole number):
n = 1, 2, 3, 4, 5…
• The larger the value of n,
the higher the energy and
the larger the orbital.
Principal Quantum Number
• n specifies the atom’s
major energy levels
• Each major energy level
is called a principal
energy level or energy
level.
Orbital Angular Momentum Quantum
Number
• The orbital angular momentum quantum number (l)
determines the shape of an orbital.
• Within each energy level, electrons are further
grouped into energy sublevels.
Energy Sublevels
• Sublevels are labeled s, p,
d, and f according to the
shapes of the atom’s
orbitals.
• S sublevel – spherical (has
1 orbital)
• P sublevel – dumbbellshaped (has 3 orbitals)
• Not all d or f sublevels
have the same shape
– d sublevel has 5 orbitals
– f sublevel has 7 orbitals
Magnetic Quantum Number
• The magnetic quantum
number (ml) describes
how the various orbitals
are oriented in space.
– Orbitals can be oriented
along x, y, and z axes.
– Examples: 2px, 2py, 2pz
Electron Spin Quantum Number
• The electron spin quantum number (ms) describes
the direction the electron is spinning, either
clockwise or counterclockwise.
• The two possible values of the spin quantum number
are + ½ and - ½.
4f (14 electrons)
4d (10 electrons)
4p (6 electrons)
4s (2 electrons)
3d (10 electrons)
3p (6 electrons)
3s (2 electrons)
Level 4
4 sublevels
Level 3
3 sublevels
Level 2
2 sublevels
2p (6 electrons)
2s (2 electrons)
Level 1
1 sublevel
1s (2 electrons)
Summary
• Bohr’s atomic model attributes hydrogen’s
emission spectrum to electrons dropping from
higher-energy to lower-energy orbits.
• The quantum mechanical model assumes that
electrons have wave properties.
• Electrons occupy three-dimensional regions of
space called atomic orbitals.