Download Electromagnetic Spectrum and Atomic Emission and more Study notes Chemistry in PDF only on Docsity!
EXPERIMENT 7
Continuous Spectrum and Atomic Emission Spectra
Continuous Spectrum:
In 1670, physicist Isaac Newton observed continuous rainbows of colors when shine sunlight passed through a prism. Around 1801, Thomas Young conclusively demonstrated that light was a wave. Lately, scientist discovered that light itself is a type of electromagnetic wave. The behavior of electromagnetic wave depends on its wavelength. With the discovery of gamma rays in 1900, all wavelength ranges have discovered so that electromagnetic spectrum was completed filled. Generally, they are classified by wavelength into radio wave, microwave, infrared, visible light, ultraviolet, X-rays and gamma rays. Areas of light that possess short wavelengths are located on the left of the spectrum. The colors displayed are within the visible spectrum (between 380 - 75 0 nm). Figure 1 : The distribution of electromagnetic waves with respect to frequency and wavelength, highlighting visible part of the electromagnetic spectrum. (Philip Ronan, Gringer) Electromagnetic waves are typically described by the frequency and wavelength. Since an infinite range of wavelength of electromagnetic waves travels at the speed of light (c), the relation is illustrated by the following equation:
where c = 3.0 x 10^8 m/s; λ is the wavelength; f is the frequency. Note – frequency and wavelength are inversely proportional to each other.
Atomic Emission Spectra: In 1826, physicist John Herschel observed monochromatic lines of pure color on a dark background when the shine light from heated gas passed through a spectroscope. Herschel's discovery was studied extensively in the 1800's. It was realized that a heated gas emits a unique combination of colors, called emission spectrum, depending on its composition. The nucleus of the atom contains protons and neutrons. Atoms also possess electrons, which are located outside the nucleus. In a neutral atom there is the same number of electrons as protons. For example, the silver atom has 47 protons and 47 electrons. An electron is a negatively charged particle with a mass that's about 1800 times smaller than that of the proton or the neutron. Protons are positively charged and neutrons are neutral. The arrangement of the electrons in an atom is called its electronic structure. For simplicity, let's consider the simplest of the atoms, hydrogen. There is only one proton and one electron in a hydrogen atom. We can't see electrons in an atom so we have to study them indirectly. One piece of evidence about the arrangement of electrons is the electromagnetic spectrum. For example, the spectrum of hydrogen is an important piece of evidence that light interacts with matter through the absorption and emission of discrete packets of energy, called quanta. We now call these quanta "photons." Energy In the figures above both emission and absorption are shown. On the right side of the figure an electron moves from the n = 4 energy level to the n = 2 energy level. A photon is emitted from the atom with an energy equal to the difference between n = 4 and n = 2 energy levels. On the left side of the figure the absorption of a photon by an atom is represented. When energy is absorbed by the atom, the electron is promoted from the n = 1 energy level to the n = 4 energy level. For this to take place, the energy must match the energy difference n= n= n= n= e- Absorption Emission of Photon n= n= n= n= n=
Absorption Emission Ground state Excited state e- e-
- A discharge tube filled with hydrogen gas is placed into the power supply which can apply high voltage.
- Turn on the power supply. The hydrogen gas discharge tube produces a signature reddish cast color.
- Place a diffraction grating (25,400 lines/inch) in front of the discharge tube; it will split the different wavelengths. Three most prominent color lines should be observed on the diffraction grating.
- Write down your observation in the space below:
3. The Rydberg Equation
In 1885, Johann Balmer derived a mathematical formula to calculate the visible wavelengths of the hydrogen spectrum displayed. The visible spectrum of light from hydrogen displays four wavelengths, 410 nm, 434 nm, 486 nm, and 656 nm. In 1888, Johannes Rydberg generalized the Balmer’s equation for all transitions of hydrogen: =
1 𝑛𝑓^2
1 𝑛𝑖^2 where λ is the wavelength; n is equal to the energy level (initial and final) Use the Rydberg equation to calculate the wavelengths of emission photons in different electron transitions of hydrogen atoms and then identify the electromagnetic regions. Also, you should be able to convert wavelength to frequency. Electron Transition Wavelength (nm) Frequency (Hz) Electromagnetic region n=5 to n=1 95 3.15 x 10^15 Ultraviolet n=5 to n= n=5 to n= n=4 to n= n=3 to n= n=4 to n=
4. Determining Elements by Using the Quantitative Analysis Spectroscope
As we discuss before, the separation of light can be achieved by using a diffracting grating. Quantitative analysis spectroscope, a device uses diffracting grating with other components which can easily and quickly determine the emitted light of definite wavelengths of an element in its gaseous form. Figure 2 : The quantitative analysis spectroscope (Werner Schulz) Procedure:
- You will break into groups by the instructor. About 4-5 students per group.
- Turn on the light source. Ask your instructor/technician for help if necessary.
- Point the slit of the spectroscope directly at the discharge tube.
- View the emission lines with the scale that appear on the right hand side, and record the color and wavelength for each line in the date table. The number on the scale represents 1000 Å or 100 nm unit. *** The visibility of the spectrum can be improved by holding the hand round the narrow end of the spectroscope using the thumb and the index finger to keep stray light from around the eye.**
- Identify the element in the discharge tube by comparing observed lines to the standard spectra.
- Repeat for the reaming discharge tubes.
