Atomic Spectra Introduction

Transcription

1 Atomic Spectra Introduction: Light and all other electromagnetic radiation is energy that is emitted in the form of waves. Thus light behaves like a wave, and the energy of light varies with the wavelength (the length of the wave) and frequency of the light (number of waves that pass a point in a second). The shorter the wavelength of light (the higher the frequency) the higher the energy. Gamma rays and x-rays are high-energy radiation, radio waves are low-energy radiation, and visible light is intermediate in energy (wavelength falls between 350 and 650 nm). Matter can either absorb or emit light, but when it does this, the matter gains or loses the exact amount of energy that corresponds to the energy of that light. This emission of light by matter is the basis for several common light sources, including neon lights and sodium-vapor lights. Atoms normally exist in a lowest energy, most stable state. If an atom gains energy (absorption of light is one way to do this), it becomes less stable and emits light to return to the more stable state. The atoms of each element emit very specific light. In fact, the emission of light by the elements is so distinctive that light is often used to determine which elements are present in a sample. For example, when hydrogen gas is zapped by an electric current, a pink color is given off. If this light is passed through a prism, it is separated into four colors: violet, blue, green, and red. The spectrum is referred to as discrete since only certain colors are seen and the spectrum is not continuous. In 1913, Niels Bohr explained the discrete spectrum of hydrogen by relating it to the electron. Normally the electron in the hydrogen atom is located in the first energy-level (shell). When a hydrogen atom atoms gains energy, the electron moves from a lower energy-level to one of higher energy. The energy gained by the atom is exactly the amount of energy needed to move the electron from the lower energy-level to the higher energy-level. With its electron in a higher energy-level, the atom is now in an unstable, higher energy, excited state. The tendency is for electrons to occupy the lowest level available. So shortly after gaining the energy, the electron returns to a lower energy-level. Energy must be given up when this occurs, and the energy is lost as light. Each line in the emitted light of hydrogen represents the movement of an electron from a specific outer (higher energy) level to a specific inner (lower energy) one. The red glow of a neon sign is caused by neon atoms excited by electricity. The many colors in fireworks are the result of excited metal atoms.

2 2 infrared n = n= 6 n= 5 n = 4 n = 3 visible n = 2 ultraviolet Electron transitions in the hydrogen atom n = 1 This lab will allow you to view the emissions from spectrum tubes containing elements in the gaseous phase. You will also test various salts in a hot Bunsen burner to learn the expected color given off by an excited atom. Both the spectrum tubes and flames will be viewed through a spectroscope to allow you to see the visible spectrum generated by the electrons as they give off energy and move between energy levels in the atoms. Procedure: Solid metal salts in Bursen burner. The solid salts are on the counter in small snap containers. Each salt has a large card near it with its name on it. Just behind the salt is a glass container or bottle containing several nichrome wires with labels for that particular salt. Use the appropriate wire with its own salt. Place a drop of distilled water on the loop of the wire and scoop up some solid to be tested. Hold the solid in the hottest part of the flame. Observe the color produced and record it on your report form. Next, while one person burns the salt, the other should try to observe the flame using rainbow glasses. Report any lines observed on the report form. Repeat the above steps for all the solids available on the lab cart.

3 Gaseous elements in spectrum tubes. Gas spectrum tubes are placed throughout the lab area. Make sure you turn off the power supply when you have finished with each tube. Observe the color produced when each gas is first turned on and record it on your report form. Next, observe the bright line spectrum for each of the gases using your rainbow glasses. It will be necessary to lower the room lights to see the lines from the spectrum tubes clearly. Record the bright-line spectrum for each gas on your report form. 3

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5 5 Name Date Report for Atomic Spectra Experiment: Data: Ions in salt samples: Compound Color Line Spectrum LiCl SrCl2 BaCl2 CuCl2 NaCl KCl CaCl2 Gases in spectrum tubes: Gas Color Line Spectrum Questions: 1. A July 4 th fireworks display is red when ignited. What element(s) is/are probably present in the fireworks?

6 2. Suppose you are cooking spaghetti in water you have salted with NaCl. You notice that when the water boils over, it causes the flame of the gas burner to turn bright orange. How do you explain the appearance of the colored flame? 6