Slide 1 / 155 Slide 2 / 155 Chemistry Atomic Structure 2015-08-29 www.njctl.org Slide 3 / 155 Slide 4 / 155 Table of Contents: Atomic Structure Click on the topic to go to that section · The Bohr Model The Bohr Model · Quantum Mechanics · The Quantum Model · Electron Configurations Return to Table of Contents Slide 5 / 155 Slide 6 / 155 Evolution of Atomic Theory The Problem with the Nuclear Atom Dalton Thomson Rutherford Democritus So far we have established: Nucleus containing Volume occupied 1803 1897 1912 460 BC 1. Atoms are composed of protons and by electrons protons, neutrons, and neutrons electrons. ? 2. The protons and neutrons comprise the vast majority of the mass of an atom and are found together in the small, 10 -4 A o dense nucleus. 3. The electrons are found outside the nucleus and o 1-5A Atomos occupy the vast majority of Dalton's Plum Nuclear the volume. Pudding Model Postulates Model
Slide 7 / 155 Slide 7 (Answer) / 155 The Problem with the Nuclear Atom The Problem with the Nuclear Atom 1 1 Students type their answers here Students type their answers here Nucleus containing Volume occupied Nucleus containing Volume occupied protons and by electrons protons and by electrons neutrons neutrons Positive and negative charges Answer attract, so why aren't the electrons and protons together? 10 -4 A 10 -4 A o o Why don't the electrons "fall" into the nucleus? o o 1-5A 1-5A [This object is a pull tab] Question: What are some physical problems with this model? Question: What are some physical problems with this model? Slide 8 / 155 Slide 9 / 155 The Problem with the Nuclear Model The Problem with the Nuclear Atom Perhaps electrons orbit the nucleus...like planets orbit the sun. If this were the The nucleus of an atom is small, 1/10,000 the size of the atom. case, electrons would constantly be The electrons are outside the nucleus, moving freely within the Death spiral of accelerating as they travel in a circle: vast empty atom. The nucleus is positive; the electron is the electron. negative. a = v 2 /r - There is an electric force, F E = kq 1 q 2 /r 2 , pulling However, an accelerating charge the electrons towards the nucleus. radiates electromagnetic energy...light. + There is no other force acting on the electrons; they feel a net force towards the As a charge radiates light it loses nucleus. energy. All the kinetic energy would be radiated away in about a billionth of a second...then the electron would fall into the nucleus. All the atoms in the Why don't the electrons fall in... universe would collapse. why doesn't the atom collapse into its nucleus? https://phet.colorado.edu/sims/radiating-charge/radiating-charge_en.html Slide 10 / 155 Slide 11 / 155 The Problem with the Nuclear Model The Problem with the Nuclear Model If the Rutherford model of the atom were correct, the atom Our observations tell us the nuclear model is insufficient should emit energy as the orbit of the electron decays. 1. Most atoms are stable and do not release energy at all. Since the electron would speed up as it decays, the amount of If electrons were continuously orbiting the nucleus in uniform circular energy released should be of an increasingly higher frequency. motion, they would be accelerating, and accelerating charges release When light, a form of energy, passes through a prism, it is energy. This is not observed. shown to be made up light waves of many different frequencies and energies that make up a continuous spectrum. Increasing frequency and energy
Slide 12 / 155 Slide 13 / 155 The Problem with the Nuclear Model The Problem with the Nuclear Model When electricity is passed through gases (made up of atoms), the If electrons in atoms were constantly releasing energy at increasing atoms become energized but appear to emit energy in very unique frequencies, we would see this emission of energy at increasingly patterns. high frequency. This would create what is called a continuous spectrum representing all frequencies of light. e- emits energy continuous spectrum Slide 14 / 155 Slide 15 / 155 The Problem with the Nuclear Model 2 An accelerating charge emits light energy. 2. When energized atoms do emit energy, a continuous spectrum is True not produced; instead, an emission spectrum is produced displaying emitted light at specific wavelengths and frequencies. False e- e- light energy External energy added emitted (electricity, light, etc.) Emission Spectrum nucleus Slide 15 (Answer) / 155 Slide 16 / 155 3 When hydrogen atoms are energized by electricity, what 2 An accelerating charge emits light energy. is observed? True False Answer True A A continuous spectrum of light [This object is a pull tab] B An emission spectrum of specific colors only. C Neither a nor b
Slide 16 (Answer) / 155 Slide 17 / 155 3 When hydrogen atoms are energized by electricity, what 4 Why was the Nuclear Model insufficient? is observed? A It could not explain the existence of emission spectra B It could not account for the stability of the atom Answer B C It required the electrons to be in the nucleus and the protons in orbit around the nucleus A A continuous spectrum of light D A and B B An emission spectrum of specific colors only. [This object is a pull tab] C Neither a nor b Slide 17 (Answer) / 155 Slide 18 / 155 4 Why was the Nuclear Model insufficient? Emission Spectra and the Bohr Model A scientist named Niels Bohr A It could not explain the existence of emission interpreted these observations spectra and created a new model of the Answer atom that explained the existence B It could not account for the stability of the atom D of emission spectra and provided a framework for where the C It required the electrons to be in the nucleus and electrons can exist around the the protons in orbit around the nucleus nucleus. D A and B [This object is a pull tab] Slide 19 / 155 Slide 20 / 155 Emission Spectra and the Bohr Model Emission Spectra and the Bohr Model Bohr knew that the wavelengths seen in the emission spectra of Each of these patterns include the variable "n" but no one knew hydrogen had a regular pattern. Each series was named after the what "n" was. Bohr proposed that "n" referred to a particular orbit scientist who observed these particular spectral lines. around the nucleus where an electron could be. Lyman Series Bohr proposed that (spectral lines in the UV range) electrons could orbit the nucleus, like planets orbit the sun...but only in certain specific orbits. Balmer Series (spectral lines in the visible and UV range) He then said that in these orbits, they wouldn't radiate energy, as would be expected normally of an Paschen Series accelerating charge. (spectral lines in the infrared range) These stable orbits would somehow violate that rule.
Slide 21 / 155 Slide 22 / 155 Emission Spectra and the Bohr Model The Bohr Atom The lowest energy level is called the ground state; Each orbit would correspond to a different energy level for the the others are excited states. electron. n = 3 n Increasing energy = 2 n n = 1 5 4 3 + 2 1 Slide 23 / 155 Slide 24 / 155 Emission Spectra and the Bohr Model Emission Spectra and the Bohr Model Bohr reasoned that each spectral line was being produced by an electron "decaying" from a high energy Bohr orbit to These possible energy states for atomic electrons were quantized – only certain values were possible. The spectrum could be a lower energy Bohr orbit. explained as transitions from one level to another. Hydrogen atom Electrons would only radiate when they moved between orbits, n = 4 n = 3 not when they stayed in one orbit. n = 2 n = 1 e- upper + upper e- lower lower Since only certain frequencies of light were produced, only certain orbits must be possible. Slide 25 / 155 Slide 25 (Answer) / 155 5 According to Bohr, "n" stands for... 5 According to Bohr, "n" stands for... A the number of cycles A the number of cycles B the number of electrons B the number of electrons Answer C C the energy level of the orbit C the energy level of the orbit D the number of orbits D the number of orbits [This object is a pull tab]
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