The approximately circular level in which electrons travel
around the nucleus.
Electrons revolve around the positively charged
nucleus (made of neutrons and protons) in orbits
called shells. The shell closest to nucleus is called the
‘K shell’ (also called ‘1 shell), followed by ‘L shell’ (or
‘2 shell’), then ‘M shell’ (or ‘3 shell’) and so on. Each
shell can hold up to 2n2electrons, where n is the shell
number. The K shell can hold up to 2 electrons, the L
shell can hold up to 8 electrons, the M shell can
occupy up to 18 electrons.
Each shell is composed of one or more subshells. The
first K shell has one subshell, called ‘1s’; the L shell
has two subshells, called ‘2s’ and ‘2p’; the third shell
has ‘3s’, ‘3p’, and ‘3d’; and so on. A subshell is the set
of states defined by azimuthal quantum number, l,
within a shell. The values l = 0, 1, 2, 3 correspond s, p,
d and f subshells, respectively. The maximum
number of electrons which can occupy a subshell is
given by 2(2l + 1). This gives two electrons in an
s subshell, six electrons in a p subshell, ten electrons
in a d subshell and fourteen electrons in an f subshell.
Sharpe (s), Principle (p), Diffuse (d) and Fundamental
(f). px, py and pz are orbitals
ORBIT
ORBITAL
1. It is well-defined circular path 1.It is a region of space around the
followed by electron around nucleus where the probability of
nucleus.
finding an electron is maximum.
2. It represents two dimensional 2. It represents three dimensional
motion of electron around motion of electron around
nucleus.
nucleus.
3. The maximum no. of electrons 3. The maximum no. of electrons
in an orbit is 2n2.
in an orbital is 2.
4. Orbit is circular in shape.
\
4. Orbitals have different shapes.
http://www.cem.msu.edu/~reusch/VirtualText/intro3.htm#strc8a
http://www.cem.msu.edu/~reusch/VirtualText/intro3.htm#strc8a
http://www.cem.msu.edu/~reusch/VirtualText
/Spectrpy/UV-Vis/uvspec.htm#uv2
The p  p* transition involves orbitals that
have significant overlap, and the probability is
near 1.0 as they are “symmetry allowed”.
McGarvey and Gaillard, Basic Photochemistry at
http://classes.kumc.edu/grants/dpc/instruct/index2.htm
Organic compounds with -C≡C- or -C≡N groups, or transition
metals complexed by C≡N- or C≡O ligands, usually have “lowlying” p* orbitals
http://www.cem.msu.edu/~reusch/VirtualText/intro3.htm#strc8a
http://www.cem.msu.edu/~reusch/VirtualText
/Spectrpy/UV-Vis/uvspec.htm#uv2
The n-orbitals do not overlap at all well with the
p* orbital, so the probability of this excitation is
small. The e of the np* transition is about 103
times smaller than e for the pp* transition as
it is “symmetry forbidden”.
McGarvey and Gaillard, Basic Photochemistry at
http://classes.kumc.edu/grants/dpc/instruct/index2.htm
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Ultraviolet: 190~400nm
Violet: 400 - 420 nm
Indigo: 420 - 440 nm
Blue: 440 - 490 nm
Green: 490 - 570 nm
Yellow: 570 - 585 nm
Orange: 585 - 620 nm
Red: 620 - 780 nm
Steps in Developing a Spectrometric Analytical Method
2. Obtain a monochromatic
wavelength for the maximum
absorption wavelength.
3. Calculate the concentration of
your sample using Beer Lambert
Equation: A = ECL
Absorbance
1. Run the sample for spectrum
2.0
0.0
200
250
300
350
400
Wavelength (nm)
450
Spectrometer Reading
A
C
Slope of Standard Curve =
x
1.0
x
0.5
x
1
4
2
3
Concentration (mg/ml)
5
There is some A vs. C where graph is linear.
NEVER extrapolate beyond point known
where becomes non-linear.
Spectrometric Analysis Using Standard Curve
1.2
0.8
0.4
3
1
2
Concentration (g/l) glucose
4
Avoid very high or low absorbencies when drawing a
standard curve. The best results are obtained with 0.1 < A
< 1. Plot the Absorbance vs. Concentration to get a straight
line
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The position and intensity of an absorption
band may be slightly different in different
solvent.
This effect is more pronounced in carbonyl
compounds.
p* orbital is more polar than a p orital, as result
the difference between two states decrease and
less energy required for this transistion.

On the other hand n orbital is polarized than p*
orbital and energy difference increase. It is
shifted to shorter wavelength in a polar
solvent.

Both single and double beam
spectrophotometers are in common use for
absorptive measurements in the uv/vis region.
Power indicator light
Absorbance & Transmittance display
Sample
holder
Wavelength
selector
Power switch
Zero control
Absorbance & Transmittance control
Spectronic 20 spectrophotometer
Procedure
Scale of spectronic 20
spectrophotometer
LED
1)
Power on
2)
Select wavelength
3)
0% T adjustment
(Calibration)
4) Blank (Reference cell) is
inserted into cell holder
5) 100% T adjustment
6) Sample cell is placed in
the cell compartment
7)
Readout absorbance
8)
Power off
Spectronic 20 spectrophotometer
The dual-beam design greatly simplifies this process by simultaneously measuring P and
Po of the sample and reference cells, respectively. Most spectrometers use a mirrored
rotating chopper wheel to alternately direct the light beam through the sample and
reference cells. The detection electronics or software program can then manipulate the P
and Po values as the wavelength scans to produce the spectrum of absorbance or
transmittance as a function of wavelength.
HP8452A diode array UV-visible spectrophotometer