The Evolution of Early Atomic Models

Early Models of Atomic Structure The work of Thomson, Rutherford and Bohr…

Thomson’s Model Discovered the electron while studying cathode ray tubes in He received the Nobel Prize in 1906

Thomson’s cathode ray tube (a vacuum tube with 2 electrodes)

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

Thomson’s “Plum Pudding” Model of the Atom He 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).

Rutherford’s Model of the Atom Rutherford’s Gold Foil experiment led to the development of his atomic model in 1911 He received the Nobel prize in 1908.

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

The Gold Foil Experiment Rutherford was expecting results in line with Thomson’s model, with the stream of positive particles passing through the foil.

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

There was only one explanation… A dense, very positive charge was condensed into one place, called the nucleus The rest of the atom had to be mostly empty space

Rutherford’s Model of the Atom

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

Wave Properties Wavelength – λ, 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 Properties Amplitude is the maximum height of a wave, measured from the origin of the wave A wave has zero amplitude at certain intervals along the wave,called nodes

The Electromagnetic Spectrum Short wavelength………long wavelength High frequency……………low frequency High energy……………………low energy

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

The Electromagnetic Spectrum All electromagnetic radiation (including visible light) travels at the same speed. The speed of light(c) = 3.0 x 10 8 m/s 3.0 x cm/s 3.0 x cm/s 3.0 x nm/s 3.0 x nm/s What 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 = λv C = λv Speed of light = wavelength x frequency ( Since c = 3.0 x 108 m/s, you will be asked to solve for wavelength or frequency.) λ = c/v v= c/λ λ = c/v v= c/λ

The Bohr Model In 1912, Niels Bohr adapted Rutherford’s model to Planck’s quantum theory and so developed his theory of atomic structure

Atoms can give off light Bohr’s model explained the atomic emission spectrum of hydrogen. For this he received the Nobel Prize in 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 element Each atom’s emission spectrum is unique

The emission spectrum of hydrogen The 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 light Firework colorants Red: strontium, lithium orange: calcium Gold: iron Green: barium Blue: copper Purple: strontium + copper Silver: magnesium

Bohr’sModel In 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 ladder As 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 E 1 to an orbit of higher energy E 2, it aborbs a photon. If it jumps from an orbit of higher energy E 2 to an orbit of lower energy E 1, it emits a photon.

Which jump would produce the highest energy light?

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

The Bohr Model of the Atom Quantized energy levels Electron moves in a circular orbit Electron jumps between levels by absorbing or emitting photon of a particular wavelength

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

Bohr’s atomic model was ultimately not successful Bohr’s 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.