Download Lecture-3: Atomic Structure

Atomic Spectra
A spectroscope is an instrument that disperses the
light emitted by an excited gas into the different
frequencies the light contains.
refraction
Wavelengths
separated
Atomic Spectra
An emission spectrum consists of
the various frequencies of light given
off by an excited substance. Below
is the part of the emission spectra of
sodium.
Atomic Spectra
An absorption spectrum
consists of the various
frequencies absorbed by
a substance when white
light is passed through it.
The frequencies in the
spectrum of an element
fall into sets called
spectral series. Seen
here is the spectral series
of hydrogen.
Origin of Absorption Spectra
Spectrum of hydrogen
• When energy is
supplied to the H gas
sample, individual
atoms absorb different
amount of energy and
electron shift to
different energy levels,
I,e., 3rd, 4th, 5th…….
• The electrons are in
higher energy levels
are relatively unstable
and drop back to lower
energy levels.
• This process of dropping back, energy is emitted in the form of
line spectrum containing various lines of particular frequency
and wavelength.
Spectrum of Hydrogen
Electron Waves and Orbits
An electron can
circle a nucleus only
in orbits that contain
a whole number of
de Broglie
Wavelengths. The
quantum number n
of an orbit is the
number of electron
waves that fit into
the orbit.
Electron Waves and Orbits
Electrons seemed
to be locked into
these wave patterns
around the nucleus.
Quantum Mechanics
Erwin Schrödinger
(1887-1961)
The theory of quantum mechanics was developed
by Erwin Schrödinger, Werner. According to quantum
mechanics, the position and momentum of a particle
cannot both be accurately known at the same time.
Only its most probable position or momentum can be
determined.
The most probable distance
between the proton and
electron for a hydrogen
atom turns out to be about
0.89Å, the same as Niels
Bohr.
Quantum Numbers and Atomic
Orbitals
n = principal quantum number
(shell)
n 1
describes orbital size
specifies primary energy level
greater n, higher E (like Bohr’s E)
l = angular momentum quantum number (subshell)(azimuthal
quantum number)

describes orbital shape
l = 0 = s orbital
l = 1 = p orbital inc E
l = 2 = d orbital within
0  n -1
l = 3 = f orbital
n
Quantum Numbers and Atomic
Orbitals
ml = magnetic quantum number
describes orbital orientation
  m 
the direction is space
also tells the number of orbitals of
same
energy (degenerate orbitals)
So, the combination of n, l, and ml, completely describes a
 and orientation.
specific orbital, its size , shape
To make a complete picture, a 4th quantum number added,
ms.
ms = spin quantum number - describes the electron spin in the
orbital
ms 1/2 or 
Quantum Numbers
principal quantum number n
1,2,3,…..
orbital quantum number l
0,1,2,….n-1
magnetic quantum number m
-l to +l
for n=2 -2,-1,0,1,2
spin magnetic quantum number +½ or –½ spin
Its like your address. To find where you are you
need to know 4 things: state, city, street, house #.
To know where or what state the electron is in you
need to know the four quantum numbers.
Quantum #’s are like an
Address.
What do you need to know to find out where you live?
State
Principle
Quantum # (n)
City
Angular
Quantum # (l)
Street
Magnetic
Quantum # (ml)
House
Spin Quantum #
(ms)
Shapes of Atomic Orbitals
P orbital
d orbital
s orbital
Quantum Numbers
3py
3d
2py
1s 2s
2px
2pz3pz
3s
3px
s and p orbital
Movie
d orbital
Movie
Aufbau Principle
-For an atom in its ground state ( the lowest energy
configuration) fill the lowest energy orbital first
then go up in energy until all the electrons are used.
Predicting Orbital Filling
Diagonal Rule
1s
2s 2p
3s 3p 3d
4s 4p 4d 4f
1s2s2p3s3p4s3d4p5s4d…..
5s 5p 5d 5f
6s 6p 6d 6f
7s 7p 7d 7f
Energy Levels
For H atom
4s_4p_ _ _ 4d_ _ _ _ _4f _ _ _ _ _ _ _ 4s _
All other atoms
Inc E
3p _ _ _
3s _ 3p _ _ _ 3d _ _ _ _ _
Inc E
3d _ _ _ _ _
3s _
2p _ _ _ (degenerate
orbitals)
2s _ 2p _ _ _(degenerate orbitals)
2s _
1s _
1s _
Pauli Exclusion Principle
• No two electrons in the same atom can have
the same four quantum numbers
• Each electron has a unique address
• Designated by a set of quantum numbers (n, l
,ml , ms)
• So: (1,0,0, +1/2) indicates an electron in a 1s
orbital
• While (3,1, -1, +1/2) would be a 3p electron
Which of the following does not represent a
3d electrons?
A. (3,2,1,+1/2)
B. (3,2,-1,+1/2)
C. (3,1,1,+1/2)
D. (3,2,2,-1/2)
E. (3,2,0,+1/2)
What are the atomic numbers of elements
whose outer most electrons are represented by
2p3 and 3p6?
N= 1s22s22p3
Ar=1s22s22p63s23p6
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