explore the emission spectra produced by various atoms

1. Emission Spectra In this presentation you will: • explore the emission spectra produced by various atoms Next >

2. Introduction When a solid material is heated, it becomes incandescent, that is, it emits light.   The same thing happens if gases and vapors are heated, or if their atoms or molecules are excited in an electrical discharge. There is an important difference in the light produced in the two cases. Next >

3. Creating a Spectrum If light is passed through a prism or diffraction grating, it is split up into a spectrum: the light of different colors of which it is composed. The spectra of light emitted by incandescent solids are continuous, with bands of different colors merging into each other. Hot solid Hot gas Light Light Prism Prism Spectrum Spectrum The spectra of gases, however, are in the form of discrete lines or bands. Next >

4. Creating a Spectrum When hydrogen is excited in a gas discharge tube, it emits its spectrum. The spectrum of gaseous iron is very different. These lines, or band spectra, will always be the same for gas atoms or molecules of a given element. Spectra are an important method of identifying elements. Next >

5. Creating a Spectrum An emission spectrum can be produced by heating the gas or using high voltage electricity to excite the gas in a gas discharge tube. Prisms 5 kV Power supply Discharge tube (hot gas) Slit Spectrum The light from the hot gas is passed through a slit and then split by a prism. The emission spectrum will appear on the screen. Next >

6. Gas Discharge Tubes Gas discharge tubes use the emission principle. Different color light is produced depending on the gas used. Gas discharge tube Neon Argon Helium Next >

7. Flame Test Sodium salts give a yellow flame In chemistry, the well known flame test method is used to identify chemicals by placing them in a flame. When small amounts are placed in a flame, the chemical elements are vaporized, producing a colored flame. Copper salts give a green flame The color of the flame relates to the elements present because of their emission spectra. Lithium salts give a pink flame Next >

8. Spectroscope The study of light spectra is called spectroscopy, and the device used to study spectra is called a spectroscope. A simple spectroscope consists of a slit and a prism or diffraction grating. The user points it at the light source to see the spectrum inside. Next >

9. Spectroscope More complex spectroscopy is carried out using computer-controlled sensors, which analyze the light and produce a graph. Next >

10. A New Atomic Model Scientists had developed and refined models of the atom for some 2,500 years after a Greek philosopher, Democritus, proposed the notion that matter could be made up of atoms. None of the models was able to explain the quantum nature of light emission. Next >

11. A New Atomic Model In 1920, a Danish scientist, Niels Bohr, refined the earlier model formed by his tutor, Ernest Rutherford, placing electrons orbiting the atom nucleus in ‘shells’. This model has formed the basis for the theory of quantum physics. Next >

12. Bohr’s Theory Bohr's theory can be summarized in the following two statements: Electrons can only occupy certain orbits, or shells, in an atom. Each orbit represents a definite energy for the electrons in it. Next >

13. Bohr’s Theory Light is emitted by an atom when an electron jumps from one of its allowed orbits to another. These orbits are called energy levels. Each orbit represents definite electron energy, and this electron jump represents a definite energy jump. Next >

14. Bohr’s Theory This change in electron energy from a higher orbit to a lower one leads to the emission of light (photon) of a definite energy or wavelength. The energy ΔE = hf Where: h is Planck’s constant = 6.626 × 10−34 Js f is frequency of the photon in Hz Since the color of the light is determined by its frequency (and wavelength), the color of the light produced will depend on the change in energy as the electron changes orbit. Next >

15. Hydrogen A hydrogen atom only has a single electron and only a few possible energy levels. Therefore, the emission spectrum of hydrogen only has a few bands. Other elements have more electrons and, consequently, more possible energy levels and so produce more bands. Next >

16. Question 1 Hydrogen has a single electron and only a few possible energy levels. Which of the following emission spectra is hydrogen? A) B) Next >

17. Question 1 Hydrogen has a single electron and only a few possible energy levels. Which of the following emission spectra is hydrogen? A) B) Next >

18. Question 2 Which of the following statements describes how an electron will emit a photon of light? A) In an excited state, an electron will orbit faster, giving off energy. B) In an excited state, an electron will jump to a lower energy level (or orbit), emitting a photon. C) In an excited state, an electron will glow, giving off light. D) In an excited state, the electron will spiral into the nucleus and collide with it, giving off energy. Next >

19. Question 2 Which of the following statements describes how an electron will emit a photon of light? A) In an excited state, an electron will orbit faster, giving off energy. B) In an excited state, an electron will jump to a lower energy level (or orbit), emitting a photon. C) In an excited state, an electron will glow, giving off light. D) In an excited state, the electron will spiral into the nucleus and collide with it, giving off energy. Next >

20. Summary In this presentation you have seen: • how gases, when excited, give off light, which can be split into a spectrum that uniquely identifies the element in the gas • that the light spectrum is caused by excited electrons changing between discrete energy levels and emitting a photon as they do so End >