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Transcript
Where are the electrons ? • Rutherford found the nucleus to be in the center. • He determined that the atom was mostly empty space. • So, how are the electrons arranged in that space? • This was the shortcoming, of Rutherford's model of the atom, the electron position could not be explained. What is the electromagnetic spectrum? • brainstorm Electromagnetic radiation • Is a form of energy that exhibits wavelike behavior as it travels through space. • Electromagnetic spectrum – all the forms of electromagnetic radiation. Electromagnetic spectrum Electromagnetic spectrum • Each line on the spectrum represents a certain wave frequency of radiation. • Each wave frequency is associated with a certain amount of energy. Electromagnetic spectrum The elecromagnetic spectrum includes all forms of radiation, one of which is visible light -- the radiation to which our eyes are sensitive. Gamma-rays, X-rays, ultraviolet, infrared and radio waves are also forms of radiation. We divide the spectrum up according to the wavelength of the radiation. Electromagnetic spectrum Electromagnetic spectrum A wave is described by • Wavelength - the distance between corresponding points on adjacent waves • Frequency- the number of waves that pass a given point in a specific time, usually one second. Unit is the hertz, (Hz). • http://www.colorado.edu/physics/2000/wav es_particles/ Wave description • Mathematically – c= speed = 3 x108 m/s – Speed of light= frequency x wavelength – Since c is constant – Frequency is inversely proportional to wavelength – All three, speed, wavelength, frequency are related example • What is the frequency of a wave that has a wavelength of 200 nm? • 3 x 108/ 200 nm = 1900’s • Wave model of light was accepted Early 1900’s Photoelectric experiments • Experiment - Planck • Photoelectric effect – refers to the emission of electrons from a metal when light shines on the metal. • Shined light on a metal varying the frequency of the light. Below a certain frequency the electrons were not emitted. • When the frequency was high enough there was enough energy to knock electrons loose from the metal. Quantum • Planck suggested that an object emits energy in little packets of energy called a quantum. • Quantum – minimum quantity of energy that can be lost or gained by an electron. • Energy that is given off is related to wave frequency. Go to • http://www.colorado.edu/physics/2000/quan tumzone/lines2.html • http://www.colorado.edu/physics/2000/wav es_particles/ Planck in other words. • Proposed that there is a fundamental restriction on the amount of energy that an object emits or absorbs; each piece of energy is a quantum. • E= hv • h = 6.6262 x 10-34 Js • v= the frequency of the wave Dual wave-particle nature of light • Einstein expanded on Planck’s theory and said that electromagnetic radiation has a dual wave-particle nature. • While light exhibits wave like properties, it can also be thought of as a stream of particles. • Each particle carries a quantum of energy – these particles are called photons. Vocab • Photon – particle of electromagnetic radiation having zero mass and carrying a quantum of energy. Ephoton= hv • Quantum – the minimum quantity of energy that can be gained or lost by an electron. Photoelectric effect explained • Electromagnetic radiation is only absorbed in whole number of photons. • In order for an electron to be bumped off, it must be struck by a photon of a certain minimal amount of energy. • The minimal energy corresponds to a minimal frequency continued • Different metals require energy at different frequencies to exhibit a photoelectric effect. • Each metal has a certain required minimal level of energy required for the electrons to be knocked loose. Ground state • The lowest possible energy level • Close to the nucleus • The lowest energy state of an atom Excited state • When an atom absorbs energy and the electrons move to a higher energy level. • An atom has a higher potential energy than its ground state. Like a ladder • Energy levels are like the rungs on a ladder, you can not step between the rungs. Electrons must jump from level to level, they can not reside between the levels • electrons • When an electron gains enough energy it jumps up an energy level ( rung on a ladder), and becomes excited. • It then immediately returns to its ground state. • The energy is released as a particular wavelength that corresponds to a particular color. A photon of radiation. Line emission spectra • The spectra given off by the electrons of a certain element. • Each spectra is unique to the element or compound. • Line spectra are used to identify elements and/or compounds Line emission spectra • The fact that specific frequencies are emitted indicates that the energy differences between an atoms energy states are fixed • Each line on the spectra represents a photon of energy. Bohr model of atom • The electron circles the nucleus in an orbital. • When the electron gains enough energy ( a certain photon) it jumps to a higher level orbital. • When it returns to ground state it emits the energy (photon). The frequency of the photon is seen in the spectra. Bohr successfully calculated the hydrogen line spectra • Bohrs model of the hydrogen atom accounted mathematically for the energy of each of the transitions of the Lyman, Balmer and Paschen spectral series. • Bottom line – Bohrs model of the H atom, with the line spectra, could be mathematically supported. • summary and beyond