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www.ck12.orgC HAPTERChapter 1. Atomic Emission Spectra1Atomic Emission Spectra Define ground state. Define excited state. Describe how atomic emission spectra are produced.How much energy does it take to shoot an arrow?Archery as a sport or a means of defense has existed for centuries. At rest, there is no tension on the bowstring andno force on the arrow. When the string and arrow are pulled back, we now have a situation where kinetic energy(pulling of the string) has been converted to potential energy (the tension on the string). The archer releases thearrow and the potential energy is translated into kinetic energy as the arrow moves. It turns out that electrons behavethe same way when energy is put into the system or released from the system.Atomic Emission SpectraThe electrons in an atom tend to be arranged in such a way that the energy of the atom is as low as possible. Theground state of an atom is the lowest energy state of the atom. When those atoms are given energy, the electronsabsorb the energy and move to a higher energy level. These energy levels of the electrons in atoms are quantized,meaning again that the electron must move from one energy level to another in discrete steps rather than continuously.An excited state of an atom is a state where its potential energy is higher than the ground state. An atom in theexcited state is not stable. When it returns back to the ground state, it releases the energy that it had previouslygained in the form of electromagnetic radiation.So how do atoms gain energy in the first place? One way is to pass an electric current through an enclosed sampleof a gas at low pressure. Since the electron energy levels are unique for each element, every gas discharge tube willglow with a distinctive color depending on the identity of the gas (see Figure 1.1).“Neon” signs are familiar examples of gas discharge tubes. However, only signs that glow with the red-orange colorseen in the figure are actually filled with neon. Signs of other colors contain different gases or mixtures of gases.1

www.ck12.orgFIGURE 1.1Gas discharge tubes are enclosed glass tubes filled with a gas at low pressure through which an electric currentis passed. Electrons in the gaseous atoms first become excited, and then fall back to lower energy levels, emittinglight of a distinctive color in the process. Shown are gas discharge tubes of helium, neon, argon, krypton, andxenon.Scientists studied the distinctive pink color of the gas discharge created by hydrogen gas. When a narrow beamof this light was viewed through a prism, the light was separated into four lines of very specific wavelengths (andfrequencies since λ and v are inversely related). An atomic emission spectrum is the pattern of lines formed whenlight passes through a prism to separate it into the different frequencies of light it contains. The Figure 1.2 showsthe atomic emission spectrum of hydrogen.FIGURE 1.2When light from a hydrogen gas discharge tube is passed through a prism,the light is split into four visible lines. Eachof these spectral lines corresponds to adifferent electron transition from a higherenergy state to a lower energy state. Every element has a unique atomic emissionspectrum, as shown by the examples ofmercury (Hg) and strontium (Sr).Classical theory was unable to explain the existence of atomic emission spectra, also known as line-emission spectra.According to classical physics, a ground state atom would be able to absorb any amount of energy rather than only2

www.ck12.orgChapter 1. Atomic Emission Spectradiscrete amounts. Likewise, when the atoms relaxed back to a lower energy state, any amount of energy couldbe released. This would result in what is known a continuous spectrum, where all wavelengths and frequenciesare represented. White light viewed through a prism and a rainbow are examples of continuous spectra. Atomicemission spectra were more proof of the quantized nature of light and led to a new model of the atom based onquantum theory.Summary Atomic emission spectra are produced when excited electrons return to ground state. The emitted light corresponds to energies of the specific electrons.PracticeQuestionsUse the link below to answer the following riodic4.html1. How many spectral lines are there for lithium?2. How many spectral lines are there for beryllium?3. Which element would you expect to have more lines: Na or Mg?ReviewQuestions1. What is the ground state of an atom?2. What is an excited state?3. Why do we see emission lines when electrons return to the ground state? atomic emission spectrum: The pattern of lines formed when light passes through a prism to separate it intothe different frequencies of light it contains. continuous spectrum: All wavelengths of light are present. excited state: A state where the potential energy of the atom is higher than the ground state. ground state: The lowest energy state of the atom.References1. Image copyright l i g h t p o e t, 2014. .2. User:Jurii/Wikimedia Commons, Heinrich Pniok (Wikimedia: Alchemist-hp). noble gases.jpg .3. CK-12 Foundation - Christopher Auyeung, using emission spectra available in the public domain. H spectrum: spectrum-H.svg; visible spectrum: visible spectrum.svg; He spectrum: Emission Spectrum.svg; Fe spectrum: spectrum-Fe.svg .3

Atomic Emission Spectra discrete amounts. Likewise, when the atoms relaxed back to a lower energy state, any amount of energy could be released. This would result in what is known a continuous spectrum, where all wavelengths and frequencies are represented. White light viewed through a prism and a rainbow are examples of continuous spectra. Atomic