2. Let’s Review
• Dalton’s Atomic Theory
• Thomson’s Model – Plum Pudding
• Rutherford’s Model- Nuclear
• Bohr’s Model – Planetary
• Quantum Mechanical Model –
cloud of probability
8. Bohr’s Model of the
Atom
• Einstein’s theory of light’s dual
nature accounted for several
unexplainable phenomena, but
it did not explain why atomic
emission spectra of elements
were discontinuous.
9. Bohr’s Model of the
Atom
• In 1913, Niels Bohr, a Danish
physicist working in Rutherford’s
laboratory, proposed a quantum
model for the hydrogen atom that
seemed to answer this question.
• This model correctly predicted the
frequency lines in hydrogen’s atomic
emission spectrum.
10. How does the Bohr model
explain the line spectrum of
an element?
• According to Bohr, electrons do not
fall on a nucleus because of
attractive forces that hold them by
their particular orbit. Hence each of
the electron in an orbit possesses
potential energy. Electrons are
located in different circular orbits at
a certain distance away from the
nucleus.
11. Bohr’s Model of the
Atom
• Bohr suggested that an electron
moves around the nucleus only
in certain allowed circular
orbits.
12. Bohr’s model was Imperfect
• The model of an electron in a
circular orbit around a nucleus
worked only for Hydrogen,
Lithium but not Boron, the
model was ineffective.
13. • Limitations of the Bohr Model
• The Bohr model explains the line spectrum
of the hydrogen atom, but not (accurately)
the spectra of other atoms.
• Also, the Bohr model assumes the electron
behaves as a particle.
• Electrons also have wave-like properties.
• However, Bohr model is important
because:
➢It shows electrons as existing in only
certain discrete energy levels, which are
described by quantum numbers.
➢Energy is involved in moving an electron
from one level to another.
13
14. 6.4 The Wave Nature of
Matter
• In the years after development of the
Bohr model, the dual nature of light
became known: Light can exhibit both
particle-like (photon) character as well
as wave-like character.
• Louis de Broglie (in 1924) extended this
idea to electrons, proposing a
relationship between the wavelength of
an electron (or any other particle), its
mass, and velocity:
14
16. Quantum Mechanical
Model
• LOUIS DE BROGLIE (1892–1987)
hypothesized that particles,
including electrons, could also
have wavelike behaviors.
• Electrons do not behave like
particles flying through space.
• We cannot, in general, describe their
exact paths.
17. Quantum Mechanical
Model
• Heisenberg showed it is impossible to
take any measurement of an object
without disturbing it.
• The HEISENBERG UNCERTAINTY
PRINCIPLE states that it is
fundamentally impossible to know
precisely both the velocity and
position of a particle at the same
time.
18. The Uncertainty Principle
• Heisenberg showed that the more
precisely the momentum of a particle
is known, the less precisely is its
position
• In many cases, our uncertainty of the
whereabouts of an electron is greater
than the size of the atom itself!
• In sum, you cannot accurately know
both an electron’s position and
momentum at the same time.
18
19. Quantum Mechanical
Model
•The only quantity that
can be known is the
probability for an electron
to occupy a certain
region around the
nucleus.
20. Quantum Mechanical
Model
• Schrödinger treated electrons as
waves in a model called the
• SCHRÖDINGER’S equation
applied equally well to elements
other than hydrogen (unlike
Bohr’s model).
21. A Quantum of energy
• A packet of energy required to
move an electron from its
present energy level to a higher
one.
• Planck’s Hypothesis - energy is
given off in little packets, or
quanta, instead of continuously.
22. Quantum Mechanical Model
•The quantum mechanical model
makes no attempt to predict the path
of an electron around the nucleus.
•Bohr orbits were replaced with
quantum-mechanical orbitals.
23. Quantum Mechanical
Model
• Orbitals are different from orbits in that
they represent probability maps that
show a statistical distribution of where
the electron is likely to be found.
24. Quantum Mechanical
Model
• According to this model,
electrons occupy definite
energy levels called
PRINCIPAL ENERGY LEVELS
represented by a number
called QUANTUM NUMBER,
(n). n= 1 2 3 4…
25. Quantum - Mechanical Model
• Each principal energy level
consists of one or more energy
SUBLEVELS:
• s = sharp
• p = principal
• d = diffuse
• f = fundamental
30. Quantum mechanical Model
• s sublevel has 1 s- orbital
• p sublevel has 3 p-orbitals
• d sublevel has 5 d – orbitals
• f sublevel has 7 f - orbitals
31. Quantum –Mechanical Model
• Each orbital can accommodate
a maximum of two (2) electrons
only.
• 1 s-orbital has 2 electrons
• 3 p-orbital has 6 electrons
• 5 d-orbital has 10 electrons
• 7 f-orbital has 14 electrons
32. How many electrons are
there in?
•s orbital
•p orbital
•d orbital
•f orbital
