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The Evolution of Early Atomic Models

Early Models of Atomic Structure

The work of Thomson, Rutherford and Bohr

Thomsons ModelDiscovered the electron while studying cathode ray tubes in 1897.

He received the Nobel Prize in 1906

Thomsons cathode ray tube (a vacuum tube with 2 electrodes)

His discovery of the electronThomson found that the cathode ray was a beam of negative particles (electrons) and so atoms were not indivisible.

Thomsons Plum Pudding Model of the AtomHe described his atomic model as negative charged electrons scattered in a lump of positively charged material, like raisins scattered in plum pudding ( a popular dessert at the time).

Rutherfords Model of the AtomRutherfords Gold Foil experiment led to the development of his atomic model in 1911He received the Nobel prize in 1908.

The Gold Foil ExperimentRutherford fired a beam of positively charged particles (called alpha particles) at a sheet of gold foil a few particles thick.

The Gold Foil ExperimentRutherford was expecting results in line with Thomsons model, with the stream of positive particles passing through the foil.

The Gold Foil ExperimentInstead, he observed that some of the alpha particles were repelled, while most went through the foil unchanged.

There was only one explanationA dense, very positive charge was condensed into one place, called the nucleusThe rest of the atom had to be mostly empty space

Rutherfords Model of the Atom

Understanding the basics of Modern Atomic TheoryWe need to understand wave properties, and light energy

Wave PropertiesWavelength , is the distance between two like points on a wave

Wave Properties

Frequency, v, describes the number of wave cycles per second.The unit of frequency is cycles/second (s-1), or the Hertz (Hz)

Wave PropertiesAmplitude is the maximum height of a wave, measured from the origin of the waveA wave has zero amplitude at certain intervals along the wave,called nodes

The Electromagnetic SpectrumShort wavelengthlong wavelengthHigh frequencylow frequencyHigh energylow energy

How are wavelength and frequency related?How are frequency and energy related?What type of electromagnetic radiation has the lowest frequency?

The Electromagnetic SpectrumAll electromagnetic radiation (including visible light) travels at the same speed.The speed of light(c) = 3.0 x 108m/s 3.0 x 1010cm/s 3.0 x 1017nm/sWhat travels faster, x- rays or visible light?

Questions:Which has the highest frequency, red light or green light?Which has the longest wavelength, x-rays or microwaves?Which has the highest energy, yellow light or infrared?

Questions:What mathematical relationship can you draw about wavelength and frequency?

The wavelength and frequency of light are inversely related. C = vSpeed of light = wavelength x frequency

( Since c = 3.0 x 108 m/s, you will be asked to solve for wavelength or frequency.)

= c/v v= c/

The Bohr ModelIn 1912, Niels Bohr adapted Rutherfords model to Plancks quantum theory and so developed his theory of atomic structure

Atoms can give off lightBohrs model explained the atomic emission spectrum of hydrogen. For this he received the Nobel Prize in 1922. His atomic model is based on these ideas.The atomic emission spectrum of an element is emission of particular frequencies (colors) of light by energized atoms of that elementEach atoms emission spectrum is unique

The emission spectrum of hydrogenThe most prominent spectral lines are violet, blue, blue-green, and red.

Which of the lines has the lowest frequency?Which of the lines has the shortest wavelength?

Atomic Emmision Spectra

Atoms can give off lightFirework colorantsRed: strontium, lithiumorange: calciumGold: ironGreen: bariumBlue: copperPurple: strontium + copperSilver: magnesium

BohrsModelIn 1913, Bohr proposed his model of the atom. He determined that electrons can be located in certain discrete energy states, called energy levels

Bohr related his model to a ladderAs a person can stand on one rung of a ladder or the next, yet is is impossible for a person to stand between the rungs an electron can be found in one energy level or the next, but not between levels.The only way for the electron to jump to the next level is for it to have a quantum leap, which is the leap from one energy level to another.

The energy of the electron has a definite value in a stationary orbit. The electron can jump from one stationary orbit to another.It it jumps from an orbit of lower energy E1 to an orbit of higher energy E2 , it aborbs a photon.If it jumps from an orbit of higher energy E2 to an orbit of lower energy E1 , it emits a photon.

Which jump would produce the highest energy light?

The Energy Levels of HydrogenQuantum- the amount of energy required to move an electron from one level to another

The Bohr Model of the AtomQuantized energy levelsElectron moves in a circular orbitElectron jumps between levels by absorbing or emitting photon of a particular wavelength

Quantized Energy LevelsSince only certain energy changes occur for each type of element an atom must contain discrete energy levels.

Bohrs atomic model was ultimately not successfulBohrs model considered the electron as a particle, and classical physics shows that a charged particle accelerating around a circular path would lose energy, and so the electrons would fall into the nucleus.

The modern model of the atom considers the electron, not as a particle, but as a matter-wave.