Question

Find on-line a Grotrian diagram for the sodium atom. What wavelengths (in nm) are the most sensitive for the analysis of Na atoms in an atomic absorption experiment? What energy levels are involved in these electronic transitions? Na can also be measured by the process of atomic emission, where the heat from the flame is used to excite the atom to its excited state. What fraction of Na atoms are excited in an acetylene-air flame at a temperature of 2200C? How much would you expect the emission signal to increase if you employed a nitrous-oxide acetylene flame (operating at 2800C) instead?

Answer 1

Sodium atomic emission spectrum can be explained by assigning each line in the spectrum to an electronic transition on an energy level diagram. The diagram reveals the energy levels corresponding to the subshells in the sodium atom. It is called a Grotrian diagram on the name of its originator, Walter Grotrian.

For sodium atom the most intense line, the yellow emission at 589nm, is caused by an electronic transition from the 3p energy level down to the 3s energy level and is the result of a 3p to 3s transition. The orange line at 616nm is the result of a 5s to 3p transition. This line is at a longer wavelength than yellow line. Thus it appears as smaller energy gap in Grotrian diagram.

The fraction of Na atoms are excited in acetylene-air flame at temperature 2200oC can be expalined by Boltzman distribution theory and temperature effects.as follows

Temperature effects

Boltzmann Distribution – degeneration of energy states (describes the relative populations of diff. states at thermal equilibrium). Ground state atoms can absorb light to be promoted to the excited state. Excited state atoms can emit light to return to the ground state.

Therefore on rising the temperature by 600oC (873K) the emmision intensity increases by 349.2%.