The Evolution of Early Atomic The Evolution of Early Atomic ModelsModels

Early Models of Atomic StructureEarly Models of Atomic Structure

The work of Thomson, The work of Thomson, Rutherford and Bohr…Rutherford and Bohr…

Thomson’s ModelThomson’s ModelDiscovered the Discovered the electron while electron while studying cathode studying cathode ray tubes in 1897.ray tubes in 1897.

He received the He received the Nobel Prize in 1906Nobel Prize in 1906

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

His discovery of the electron…His discovery of the electron…

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

Thomson’s “Plum Pudding” Model Thomson’s “Plum Pudding” Model of the Atomof the Atom

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

Rutherford’s Model of the AtomRutherford’s Model of the AtomRutherford’s Gold Rutherford’s Gold Foil experiment led Foil experiment led to the development to the development of his atomic model of his atomic model in 1911in 1911

He received the He received the Nobel prize in Nobel prize in 1908.1908.

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

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

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

There was only one explanation…There was only one explanation…

A dense, very A dense, very positive charge positive charge was condensed was condensed into one place, into one place, called the nucleuscalled the nucleus

The rest of the The rest of the atom had to be atom had to be mostly empty mostly empty spacespace

Rutherford’s Model of the AtomRutherford’s Model of the Atom

Understanding the basics of Understanding the basics of Modern Atomic TheoryModern Atomic Theory

We need to understand wave We need to understand wave properties, and light energyproperties, and light energy

Wave PropertiesWave Properties

Wavelength – Wavelength – λλ, is the distance , is the distance between two like points on a wavebetween two like points on a wave

Wave PropertiesWave Properties

Frequency, v, describes the number Frequency, v, describes the number of wave cycles per second.of wave cycles per second.

The unit of frequency is The unit of frequency is cycles/second (scycles/second (s-1-1), or the Hertz (Hz)), or the Hertz (Hz)

Wave PropertiesWave Properties

Amplitude is the maximum height of Amplitude is the maximum height of a wave, measured from the origin of a wave, measured from the origin of the wavethe wave

A wave has zero amplitude at certain A wave has zero amplitude at certain intervals along the wave,called intervals along the wave,called nodesnodes

The Electromagnetic SpectrumThe Electromagnetic Spectrum

Short wavelength………long Short wavelength………long wavelengthwavelength

High frequency……………low frequencyHigh frequency……………low frequency

High energy……………………low energyHigh energy……………………low energy

How are wavelength and frequency How are wavelength and frequency related?related?

How are frequency and energy How are frequency and energy related?related?

What type of electromagnetic radiation What type of electromagnetic radiation has the lowest frequency?has the lowest frequency?

The Electromagnetic SpectrumThe Electromagnetic Spectrum

All electromagnetic radiation All electromagnetic radiation (including visible light) travels at the (including visible light) travels at the same speed.same speed.

The speed of light(c) = 3.0 x 10The speed of light(c) = 3.0 x 1088m/sm/s

3.0 x 103.0 x 101010cm/scm/s

3.0 x 103.0 x 101717nm/snm/s

What travels faster, x- rays or visible What travels faster, x- rays or visible light?light?

Questions:Questions:

Which has the highest frequency, red Which has the highest frequency, red light or green light?light or green light?

Which has the longest wavelength, x-Which has the longest wavelength, x-rays or microwaves?rays or microwaves?

Which has the highest energy, yellow Which has the highest energy, yellow light or infrared?light or infrared?

Questions:Questions:

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

The wavelength and frequency of The wavelength and frequency of light are inversely related.light are inversely related.

C = C = λλvvSpeed of light = wavelength x Speed of light = wavelength x

frequencyfrequency

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

λλ = c/v v= c/ = c/v v= c/λλ

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

Atoms can give off lightAtoms can give off lightBohr’s model explained the atomic Bohr’s model explained the atomic emission spectrum of hydrogen. For this emission spectrum of hydrogen. For this he received the Nobel Prize in 1922. His he received the Nobel Prize in 1922. His atomic model is based on these ideas.atomic model is based on these ideas.

The atomic emission spectrum of an The atomic emission spectrum of an element is emission of particular element is emission of particular frequencies (colors) of light by energized frequencies (colors) of light by energized atoms of that elementatoms of that element

Each atom’s emission spectrum is uniqueEach atom’s emission spectrum is unique

The emission spectrum of The emission spectrum of hydrogenhydrogen

The most prominent spectral lines are The most prominent spectral lines are violet, blue, blue-green, and red.violet, blue, blue-green, and red.

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

Atomic Emmision SpectraAtomic Emmision Spectra

Atoms can give off lightAtoms can give off lightFirework colorantsFirework colorants

Red: strontium, Red: strontium, lithiumlithium

orange: calciumorange: calcium

Gold: ironGold: iron

Green: bariumGreen: barium

Blue: copperBlue: copper

Purple: strontium + Purple: strontium + coppercopper

Silver: magnesiumSilver: magnesium

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

Bohr related his model to a ladderBohr related his model to a ladder

As a person can stand on one rung of a As a person can stand on one rung of a ladder or the next, yet is is impossible for ladder or the next, yet is is impossible for a person to stand between the rungs… a person to stand between the rungs… an an electron can be found in one energy level electron can be found in one energy level or the next, but not between levels.or the next, but not between levels.

The only way for the electron to jump to The only way for the electron to jump to the next level is for it to have a quantum the next level is for it to have a quantum leap, which is the leap from one energy leap, which is the leap from one energy level to another.level to another.

The energy of the electron has a definite The energy of the electron has a definite value in a stationary orbit. The electron value in a stationary orbit. The electron can jump from one stationary orbit to can jump from one stationary orbit to another.another.

It it jumps from an orbit of lower energy EIt it jumps from an orbit of lower energy E1 1

to an orbit of higher energy Eto an orbit of higher energy E22 , it aborbs a , it aborbs a photon.photon.If it jumps from an orbit of higher energy If it jumps from an orbit of higher energy EE22 to an orbit of lower energy E to an orbit of lower energy E11 , it emits a , it emits a photon.photon.

Which jump would produce the Which jump would produce the highest energy light?highest energy light?

The Energy Levels of HydrogenThe Energy Levels of Hydrogen

Quantum- Quantum- the amount of energy the amount of energy required to move an electron from required to move an electron from one level to anotherone level to another

The Bohr Model of the AtomThe Bohr Model of the AtomQuantized energy Quantized energy levelslevelsElectron moves in Electron moves in a circular orbita circular orbitElectron jumps Electron jumps between levels by between levels by absorbing or absorbing or emitting photon of emitting photon of a particular a particular wavelengthwavelength

Quantized Energy LevelsQuantized 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 Bohr’s atomic model was ultimately not successfulnot successful

Bohr’s model considered the electron as a Bohr’s model considered the electron as a particle, and classical physics shows that a particle, and classical physics shows that a charged particle accelerating around a charged particle accelerating around a circular path would lose energy, and so circular path would lose energy, and so the electrons would fall into the nucleus.the electrons would fall into the nucleus.

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