Laboratory 03
Shedding Light on the Structure of the
Atom
1
Objectives
• Build and calibrate a simple spectroscope.
• Use the spectroscope to observe the line spectrum of
sodium; calculate the energy and frequency associated with
the major transition(s).
• Use the spectroscope to observe the line spectra of
unknown substances and use line spectrum to identify
substance.
2
Atoms are too small to be observed directly.
Scientists in the late nineteenth and early twentieth centuries had to use
indirect methods to decipher the architecture of individual atoms.
•Cathode ray tubes
•Millikan’s oil drop experiment
•Rutherford’s gold foil experiment
3
Atomic model
A nucleus containing protons and neutrons, with electrons forming a relatively
large “cloud” around the nucleus
4
Scientists have used the absorption and emission of light by matter, to determine
the electronic structure of atoms.
The electrons in the lower orbit move
to the next level after absorbing a
quantized
amount
of
energy.
Once the excited state electrons loose
energy they emit light (various color
depending on energy level) by releasing
energy and coming back to its ground
state.
5
Spectrum of the hydrogen atom
When hydrogen gas (or other element) receive high energy spark it emits light with
specific L-wavelength ,
H2 (g) absorb energy (H-H bond breaks) - H atoms
6
Spectrum of the hydrogen atom
7
How are these lines generated?
According to the Niels Bohr model,
Electrons in an atom are located
in fixed orbits that encircle the
nucleus.
An electron in a given orbit has a specific energy,
m = mass of the electron
e = charge of the electron ( -1.6022 x10-19 C)
h = Planck’s constant (6.63 x 10-34 Js),
b = 2.178 X10-18 J (combination of all constants in the equation).
The variable n can have values 1, 2, . . . and is known as the principal quantum
number. Each orbit has a different value of n .
8
According to the Bohr’s model the electron can only have an
energy associated with one of the orbits; that is, the electron
can never have an intermediate energy.
Energy of an electron is quantized.
The deference between the energies of the two orbits between which the
electron moves reflects the amount of energy absorbed by the electron.
9
Hydrogen Spectrum
10