Ch. 4 Arrangement of Electrons in Atoms 4.1 THE DEVELOPMENT OF A NEW ATOMIC MODEL Light Before 1900, scientists thought that light behaved only as wave discovered that also has particle-like characteristics Light as a Wave electromagnetic radiation: form of energy that acts as a wave as it travels includes: visible light, X rays, ultraviolet and infrared light, microwaves, and radio waves All forms are combined to form electromagnetic spectrum Light as a Wave Light as a Wave all form of EM radiation travel at a speed of 3.0 x 108 m/s in a vacuum it has a repetitive motion wavelength: (λ) distance between points on adjacent waves; in nm (109nm = 1m) frequency: (ν) number of waves that passes a point in a second, in waves/second Inversely proportional! c Photoelectric Effect when light is shone on a piece of metal, electrons can be emitted no electrons were emitted if the light’s frequency was below a certain value scientists could not explain this with their classical theories of light Ex: coin-operated soft drink machine Photoelectric Effect Max Planck: a German physicist suggested that an object emits energy in the form of small packets of energy called quanta quantum- the minimum amount of energy that can be gained or lost by an atom Planck’s constant (h): 6.626 x 10-34 J*s E h Photoelectric Effect Einstein added on to Planck’s theory in 1905 suggested that light can be viewed as stream of particles photon- particle of EM radiation having no mass and carrying one quantum of energy energy of photon depends on frequency Photoelectric Effect EM radiation can only be absorbed by matter in whole numbers of photons when metal is hit by light, an electron must absorb a certain minimum amount of energy to knock the electron loose this minimum energy is created by a minimum frequency since electrons in different metal atoms are bound more or less tightly, then they require more or less energy H Line-Emission Spectrum ground state- lowest energy state of an atom excited state- when an atom has higher potential energy than it has at ground state line-emission spectrum- series of wavelengths of light created when visible portion of light from excited atoms is shined through a prism H Line-Emission Spectrum scientists using classical theory expected atoms to be excited by whatever energy they absorbed continuous spectrum- emission of continuous range of frequencies of EM radiation H Line-Emission Spectrum Why had hydrogen atoms only given off specific frequencies of light? current Quantum Theory attempts to explain this using a new theory of atom H Line-Emission Spectrum when an excited atom falls back to ground state, it emits photon of radiation the photon is equal to the difference in energy of the original and final states of atom since only certain frequencies are emitted, the differences between the states must be constant Bohr Model created by Niels Bohr (Danish physicist) in 1913 linked atom’s electron with emission spectrum electron can circle nucleus in certain paths, in which it has a certain amount of energy Bohr Model Can gain energy by moving to a higher rung on ladder Can lose energy by moving to lower rung on ladder Cannot gain or lose while on same rung of ladder Bohr Model a photon is released that has an energy equal to the difference between the initial and final energy orbits Bohr Model problems: did not work for other atoms did not explain chemical behavior of atoms