SPECTROSCOPY

 

Purpose

 

The purpose of this activity is to observe the character­istic colors of metal ion­s using flame spec­troscopy and the patterns of atomic line spectra from a gas discharge tube.

 

Introduction

 

Atoms and molecules are able to absorb energy by a number of different methods and later emit the energy in the form of light.  An incandescent light bulb converts electrical energy into heat, and the hot wire re-radiates the energy as light.  An ideal black body is able to absorb and emit all colors of light equally well, but no object is a perfect black body.  Therefore, heated materials usually have a particular shading or color associated with them.  Tungsten is chosen as a filament in bulbs because it shows very little of this effect.

 

Another way to produce radiant energy is to heat a material in a flame. The energy and temperature of a flame is so large than no chemical bond can survive those conditions.   Therefore, the color of a flame must be caused by individual atoms, not molecules.  Different atoms show certain color prefer­ences based on their unique chemical natures.

 

Fluorescent light bulbs are an example of a third way to produce light.  Atoms in a fluorescent tube also absorb electrical energy, but  the internal volume of a fluorescent bulb is a rari­fied gas.  Electrons are accelerated by high voltage through the bulb and individual electrons collide with individual atoms, transferring a large amount of energy to a gaseous atom as a "packet".  Atoms release this energy spontaneously, and the "white" light of a flourescent is actually the re-emission of the energy by an arsenic-containing powder coating the inside of the glass tube.

 

Whenever an atom releases energy by collisions with nearby neighbors, all colors of light are produced; but when an atom absorbs and releases energy in isolation, only a certain set of discrete frequencies are absorbed and released.  Spec­trosco­py is the study of light frequencies absorbed and released by atoms and molecules, which is the atom's or molecule's characteristic spectrum.

 

In this activity, qualitative observations will be made of atomic spectra using both flame spectroscopy and discharge tubes.  In Part One, metal chlorides will be heated in a Bunsen burner flame.  Testing hydrochloric acid in a flame, HCl, shows a colorless flame; therefore, the color of the flame must be due to the metal ion alone.  In the same way, heating the metal scoop alone which is used to hold the samples shows no color, either.  In Part Two, we will separate the individual colors emitted by the gas using a diffraction grating, and observations of different elemental and molecular gases clearly shows that each chemi­cal sub­stance has a characteristic pattern of light frequen­cies.

 

 

It can be shown that the flame spectrum of an atom, if the colors of light were separated using a diffraction grating, would match the spectrum obtained from a gas discharge tube.

 

 

Instructions

 

In Part One, the instructor will place small amounts of different metal chlorides on the tip of the metal scoop and heat it in the flame of a Bunsen burner.  Record the color of each flame.

 

In Part Two, spectrum boxes and diffraction gratings will be available for the viewing atomic spectra.  Sketch the spectra of helium, mercury, neon, and hydrogen on the report sheet.  Note both the relative positions of the lines and their relative intensities.  Attempt to describe or charac­terize each spectrum briefly with words.

 

 

 

Report Sheet

 

Part One: Flame Spectra

 

The instructor will place small amounts of different metal chlorides on the tip of the metal scoop and heat it in the flame of a Bunsen burner.  Record the color of each flame.

 

            Compound                                                     Color

 

 

lithium chloride         

 

sodium chloride                                                        

 

potassium chloride

 

 

 

magnesium chloride

 

calcium chloride

 

barium chloride

 

 

nickel chloride

 

cobalt chloride

 

copper chloride

 

iron chloride

 

 

Does there appear to be any relationship between the color of the flame and the periodic chart?

 

 

 

 

 

 

 

 

 

Part Two: Line Spectra

 

                          Violet          Blue           Green       Yellow      Orange         Red

 

                            400             450            500              550             600           650

 

 

hydrogen

 

 

 

helium

 

           

 

 

 

neon

 

 

 

mercury

 

 

 

 

Does there appear to be a relationship between the lines of a given element, or between elements based on the periodic chart?