12.4 Atomic Absorption and Emission Spectra

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Phyic Tool box 12.4 Atoic Abortion and iion Sectra A continuou ectru given off by a heated olid i caued by the interaction between neighbouring ato or olecule.

1 Phyic Tool box 12.4 Atoic Abortion and iion Sectra A continuou ectru given off by a heated olid i caued by the interaction between neighbouring ato or olecule. An eiion ectru or line ectru i eitted fro electrically excited gae. An abortion ectru occur when oe of the light fro a continuou ectru i aborbed uon aing through a ga. Ato aborb light of the ae frequencie that they eit. An ato i norally in it ground tate. The excited tate or energy level are given by the dicrete aount of energy that ato can internally aborb. Ionization energy i the axiu energy that can be aborbed internally by an ato, without triggering the lo of an electron. In the eiion ectru, the energy of the eitted hoton equal the change in the internal energy level: h l. When a hoton i aborbed, it energy i equal to the difference between the internal energy level: h l. Ato can receive energy two way: by colliion with high eed article, uch a electron, and by aborbing a hoton. Once raided to an excited tate, an ato can eit hoton either through ontaneou eiion or through tiulated eiion. It ha been long known that atter, when heated, give off light. Solid, liquid, and very dene gae gave off light with a continuou ectru of wavelength. But when heated (in the for of a high voltage) wa given to a rarefied ga, the light ectru wa quite different. It wa dicovered that the heated olid eitted a continuou ectru fro the interaction between each ato or olecule and it cloet neighbour. Wherea, in a hot, rarefied ga, the ato are far enough aart to enure that any light eitted coe fro the individual, iolated ato. A early a the beginning of the nineteenth century, it wa known that the radiation fro electrically excited gae wa dicrete rather than continuou. That i, an excited ga only give off ecific frequencie of light. When thi light i aed through a ectrocoe, a bright-line eiion ectru i oberved. ach ga eit it own ecific et of characteritic frequencie (like a fingerrint), called it eiion ectru. By atching thi ectru to the eleent, we have a very accurate ethod for identifying eleent. It wa alo hown that when white light i allowed to a through a ga, and the

ectru of the tranitted light i analyzed with a ectrocoe, dark line of iing light are aborbed in the continuou ectru at exactly the ae frequencie a the light in the correonding eiion ectru.

ae Frank and Gutav Hertz (In 1914) erfored an exerient with Mercury and dicovered the underlying rincile of the eiion (and abortion) ectru.

2 ectru of the tranitted light i analyzed with a ectrocoe, dark line of iing light are aborbed in the continuou ectru at exactly the ae frequencie a the light in the correonding eiion ectru. Thi i the abortion ectru of the eleent a light i aborbed by the eleent. ae Frank and Gutav Hertz (In 1914) erfored an exerient with Mercury and dicovered the underlying rincile of the eiion (and abortion) ectru. They dicovered that a ercury ato that ha not aborbed any extra internal energy a a reult of a colliion with an electron i norally found in it ground tate. When it ha aborbed 4.9, the allet aount of energy it i caable of aborbing, it firt excitation energy, we ay that the ato i in it firt excited tate. Greater aount of internal energy are the econd, third and o on excitation energie. There are any exciting level. It wa dicovered that above the ionization level, the electron are no longer bound to the nucleu of the ato. Thi ionization energy (the energy required to liberate an electron fro an ato) for ercury deontrate that when ercury aborb energy 10.4 or greater, the tructure of the ercury ato change itelf of internal energy i too great to be aborbed without the ejection of an electron (leaving a oitive ground-tate ercury ion behind). The ejected electron will carry away any exce kinetic energy not needed for it releae, thu allowing the ato to aborb ore energy that 10.4

3 xale lectron with kinetic energy 13.0 collide with ato of a unknown ga. After the colliion the electron are found to eerge with an energy of 5.0 and 8.0. What are the firt and econd excitation level of the ga. Solution: n k, initial k, final The aller value i the firt excitation energy ince thi i the allet aount of energy an ato i caable of aborbing. In analying their data, it wa dicovered that in general, for the eiion ectru of any eleent, we can write: Where level, and i the energy of the eitted hoton, i i the energy of the higher energy f i the energy of the lower energy level. In ter of the wavelength of the ectral line i f xale An unknown ubtance ha firt and econd excitation level of 3.45 and 6.12, reectively. Deterine the energy and wavelength of each hoton found in it eiion ectru when the ato are bobarded with electron of kinetic enegy: a) 3.2 b) 5.0 b) 7.0 Solution: a) Since the firt excitation energy i 3.45, no energy i aborbed, and thu there i no eiion ectru. b) Since the incident electron have energie above both only the firt excitation level, then only the uward tranition to the firt excitation level are oible.

4 Now for the wavelength We need our nergy in oule c) With incident electron of energy 7.0, excitation to both firt and econd energy level are oible. Thu the ae fit value a in art b) along with and The wavelength would then reectively be: And

AP CHEM WKST KEY: Atomic Structure Unit Review p. 1

AP CHEM WKST KEY: Atomic Structure Unit Review p. 1 AP CHEM WKST KEY: Atoic Structure Unit Review p. 1 1) a) ΔE = 2.178 x 10 18 J 1 2 nf 1 n 2i = 2.178 x 10 18 1 1 J 2 2 6 2 = 4.840 x 10 19 J b) E = λ hc λ = E hc = (6.626 x 10 34 J )(2.9979 x 10 4.840 x