Laboratory 03

Shedding Light on the Structure of the Atom

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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.

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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.

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•Cathode ray tubes

•Millikan’s oil drop experiment

•Rutherford’s gold foil experiment

Atomic model

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A nucleus containing protons and neutrons, with electrons forming a relatively large “cloud” around the nucleus

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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.

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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

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Spectrum of the hydrogen atom

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How are these lines generated?According to the Niels Bohr model,

Electrons in an atom are locatedin fixed orbits that encircle thenucleus.

An electron in a given orbit has a specific energy,

m = mass of the electrone = 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 .

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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.

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Hydrogen Spectrum