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Electrons in Atoms. I. Waves and Particles. A. A New Atomic Model. Rutherford model of the atom Dense positively charged nucleus Mostly empty space Did not explain where the electrons are located around the nucleus
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Electrons in Atoms I. Waves and Particles
A. A New Atomic Model • Rutherford model of the atom • Dense positively charged nucleus • Mostly empty space • Did not explain where the electrons are located around the nucleus • Did not explain why negative electrons did not move to the p+ in the nucleus and collapse atoms
A. A New Atomic Model • Prior to 1900 : • Electrons are particles (concentration of energy and other properties in space and time) • Light is waves (energy spread out over a larger region of space and time; an oscillation that moves outward from a source.
A. A New Atomic Model • New atomic model emerged as a result of experimentation with absorption and emission of light by matter. • Studies revealed a relationship between light and an atom’s electrons. • Light was shown to exhibit both particle and wave-like behaviors. • Electrons were also shown to exhibit dual wave-particle nature.
B. Waves • Velocity (c) – Unit: m/s • Amplitude (A) - distance from the origin to the trough or crest. Unit will be length (m, …) • Wavelength () - length of one complete wave ; peak-to-peak distance. Unit: m, nm… • Frequency ( or f ) - # of waves that pass a point during a certain time period • hertz (Hz) = 1/s or s-1
crest A A origin trough B. Waves greater amplitude (intensity)
B. Waves • Frequency & wavelength are inversely proportional c = c: speed of light (3.00 108 m/s) : wavelength (m, nm, etc.) : frequency (Hz)
WORK: = c = 3.00 108 m/s 2.73 x 1016 s-1 B. Waves • EX: Light near the middle of the ultraviolet region of the EM spectrum as a frequency of 2.73 x 1016 s-1. Calculate its wavelength. GIVEN: = 2.73 x 1016 s-1 = ? c = 3.00 108 m/s = 1.10 x 10-8 m
WORK: = c = 3.00 108 m/s 4.34 10-7 m B. Waves • EX: Find the frequency of light with a wavelength of 434 nm. GIVEN: = ? = 434 nm = 4.34 10-7 m c = 3.00 108 m/s = 6.91 1014 Hz
C. Electromagnetic Spectrum • Electromagnetic radiation – a form of energy that exhibits wavelike behavior as it travels through space • Electromagnetic spectrum – all forms of electromagnetic radiation together
R O Y G. B I V red orange yellow green blue indigo violet C. EM Spectrum HIGH ENERGY LOW ENERGY
C. EM Spectrum HIGH ENERGY LOW ENERGY
C. EM Spectrum • Spectroscopy – branch of science that studies the interaction of light and atoms. • Spectrum – pattern of or when electromagnetic radiation is separated into its parts. • Spectroscope – instrument used to measure the wavelength of light.
C. EM Spectrum • Continuous spectra – use a diffraction grating to separate the wavelengths into the visible light spectrum. Shows all wavelengths in a given range. • Example : • visible light (400 nm – 700 nm)
C. EM Spectrum • Emission (bright line) spectra • Each line = different wavelength of light. • Lines represent energy emitted as e- fall from excited state to lower/ground state (from high energy levels to lower energy levels) • Usage – analyze substances for elements present. Each emits its own color. (Stars/astronomy)
C. EM Spectrum • Absorption (black line) spectra – shows the fraction of incident radiation absorbed by the material over a range of frequencies.
C. EM Spectrum • Ground state vs. excited state • Ground state – Electrons in the atom are in the lowest energy levels. • Excited state – Electrons in the atom are in higher energy levels. Electrons become “excited” when they gain enough energy to jump to a higher energy level in the atom. • Example: flames, spectral tubes
D. Waves vs Particles • Waves can bend around small obstacles. • Waves can fan out from pinholes. • Particles effuse (trickle out) from pinholes. • Ex. Balloon gradually deflates.
E. Particle Behavior of Light • Max Planck (1900) • Observed - emission of light from hot objects • Concluded - energy is emitted in small, specific amounts (quanta = bundle of energy)
Classical Theory Quantum Theory E. Particle Behavior of Light • Planck (1900) vs.
E. Particle Behavior of Light • Einstein (1905) – observed the photoelectric effect. Electromagnetic radiation strikes the surface of the metal, ejecting electrons from the metal, creating an electrical current.
E. Particle Behavior of Light • Einstein (1905) • Concluded - light has properties of both waves and particles “wave-particle duality” • Light moves like a wave, but transfers energy like a stream of particles.
E. Particle Behavior of Light • Photon – bundle of energy given off as light. A particle of electromagnetic radiation having zero mass and carrying a quantum of energy. • Quantum - minimum amount of energy that can be lost or gained by an atom
E. Particle Behavior of Light • The energy of a quantum of energy is proportional to its frequency. E: energy (J, joules) h: Planck’s constant (6.6262 10-34 J·s) : frequency (Hz) E = h
E. Particle Behavior of Light • EX: Find the energy of a red photon with a frequency of 4.57 1014 Hz. GIVEN: E = ? = 4.57 1014 Hz h =6.6262 10-34 J·s WORK: E = h E = (6.6262 10-34 J·s) (4.57 1014 s-1) E = 3.03 10-19 J