The Development The Development of Atomic Theoryof Atomic Theory

I. Early Models of I. Early Models of Atomic StructureAtomic Structure

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

Democritus, Democritus, "The Laughing Philosopher""The Laughing Philosopher"

Democritus was a pre-Democritus was a pre-

Socratic philosopher Socratic philosopher who said that all who said that all matter is made up of matter is made up of various indivisible various indivisible elements which he elements which he called called atoma, atoma, from from which we get the which we get the English word English word atomatom. .

John Dalton’s John Dalton’s Model of the AtomModel of the Atom

John Dalton John Dalton developed developed his atomic his atomic theory in theory in 18031803

It worked well, It worked well, until until subatomic subatomic particles were particles were discovered.discovered.

Dalton’s atomic theoryDalton’s atomic theory

1.1. All matter is made of All matter is made of atomsatoms, which , which are indivisible and are indivisible and indestructible indestructible particlesparticles..

2.2. All atoms of an element are all All atoms of an element are all identical in massidentical in mass and properties. and properties.

3.3. Atoms of different elements have Atoms of different elements have different masses and properties.different masses and properties.

4.4. Compounds are formed by atoms Compounds are formed by atoms combining in combining in small whole number small whole number ratiosratios..

J.J. Thomson’s Model of the J.J. Thomson’s Model of the AtomAtom

J.J. ThomsonJ.J. Thomson discovered discovered the electron the electron while studying while studying cathode ray cathode ray tubestubes inin 1897.1897.

He received He received the Nobel the Nobel Prize in 1906.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 His discovery of the electron…electron…

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

Thomson’s “Thomson’s “Plum Pudding” Plum Pudding” ModelModel of the Atom of the Atom

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

Rutherford’s Model of the Rutherford’s Model of the AtomAtom

In 1894, In 1894, Ernest RutherfordErnest Rutherford was awarded a scholarship was awarded a scholarship to be a research student at to be a research student at the Cavendish Laboratory the Cavendish Laboratory under J.J. Thomson. He under J.J. Thomson. He received the Nobel prize in received the Nobel prize in 1908.1908.

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

The Gold Foil ExperimentThe Gold Foil Experiment

Rutherford fired a Rutherford fired a beam of beam of positively positively charged particles charged particles (called (called alphaalpha particles) at a particles) at a sheet of gold foil sheet of gold foil a few particles a few particles thick. thick.

The Gold Foil ExperimentThe Gold Foil Experiment

Rutherford was Rutherford was expecting results expecting results in line with in line with Thomson's model, Thomson's model, with the stream with the stream of positive of positive particles passing particles passing through the foil.through the foil.

The Gold Foil ExperimentThe Gold Foil Experiment

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

The Gold Foil ExperimentThe Gold Foil Experiment

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

A dense, very positive A dense, very positive charge was condensed charge was condensed into one place, called into one place, called the the nucleusnucleus..

The rest of the atom had to be made up The rest of the atom had to be made up of mostly empty space. He described the of mostly empty space. He described the electrons as “buzzing around the nucleus electrons as “buzzing around the nucleus like bees buzz around a hive”.like bees buzz around a hive”.

One particularly memorable quote One particularly memorable quote attributed to Rutherford is "All science is attributed to Rutherford is "All science is either physics or stamp collecting“.either physics or stamp collecting“.

Rutherford’s Model of the Atom

II. Understanding II. Understanding the basics…the basics…

Let’s look at some basic ideas of physics Let’s look at some basic ideas of physics to better understand the more recent to better understand the more recent models of the atom. We need to models of the atom. We need to understand wave properties, and light understand wave properties, and light energy.energy.

Wave PropertiesWave Properties

–Wavelength, λ, is the distance between two like points on a wave [the unit of wavelength is the meter (m)]

Wave PropertiesWave Properties– Frequency, υ, describes the number

of wave cycles per second.

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

Wave PropertiesWave Properties Amplitude is the maximum

height of a wave, measured from the origin line of the wave.

[A wave has zero amplitude at certain intervals along the wave, called nodes.]

The Electromagnetic The Electromagnetic SpectrumSpectrum

Short wavelength…………………….long wavelengthShort wavelength…………………….long wavelength

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

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

The Electromagnetic The Electromagnetic SpectrumSpectrum

All eAll electromagnetic radiation lectromagnetic radiation (including visible light) travels (including visible light) travels at the at the samesame speed. speed.

The speed of light (The speed of light (cc) )

= 3.0 x 10= 3.0 x 1088 m/s m/s

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 rays?light or infrared rays?

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

CC = = Speed of lightSpeed of lightwavelength x frequencywavelength x frequency

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

= = c/c/c/c/

Question:Question: Orange light has a wavelength of Orange light has a wavelength of

620 nm. What is the wavelength 620 nm. What is the wavelength in meters? What is the in meters? What is the frequency?frequency?

620 nm (10620 nm (10-9 -9 m/1 nm) = 6.2 x 10m/1 nm) = 6.2 x 10-7-7 m m c/ 3.0 x 10 m/s / 6.2 x 106.2 x 10-7-7 m m 4.8 x 1014 s-1