Development of the MODEL of the ATOM

Way back – to the time of the Greeks

Democritus (460-370 BC)stated that all matter is made of tiny particles.

He used the word “atomos” whichmeans “indivisible” or “uncuttable”

Democritus’ idea was prettymuch lost for the next 2000 years.

Then in the late 1700’s, amateurscientists in Europe began to makediscoveries that marked the startof the science of chemistry. Mostimportantly – they discovered thefirst elements.

In 1803, an English doctor, John Dalton, was the first to explain experimental results in terms of ATOMS.

Dalton’s element symbols

1. All matter is made of indivisible and indestructible atoms.

2. All atoms of an element have identical physical and chemical properties.

3. Atoms of different elements have different properties.

4. Atoms of different elements combine in simple whole-number ratios to form compounds. (for example - NO, NO2, N2O; but not N1.5O2.8)

5. Chemical reactions consist of the combination, separation, or rearrangement of atoms.

Dalton’s Atomic Theory

So, Dalton’s model of the atom was

a simple SPHERE

Dalton even thought about how atoms could combine to form compounds

Then, in 1897EnglishmanJ.J. Thompsonmade an importantdiscovery – atomswere composed ofsmaller particles.

He made this discovery by working with “cathode ray tubes”. He was able to get materials to emit a beam of particles. He investigated how these particles were affected by a magnetic field.

Thompson’s results:

The particles were MUCH SMALLER than a complete atom.

They had a NEGATIVE electrical charge.

He had discovered ELECTRONS.

JJ talking about the tiny size of electrons.(in 1934)

"Could anything at first sight seem more impractical than a body which is so small that its mass is an insignificant fraction of the mass of an atom of hydrogen? --which itself is so small that a crowd of these atoms equal in number to the population of the whole world would be too small to have been detected by any means then known to science."

“cloud” of positive charge

Negatively-chargedelectrons

Thompson proposed a NEW MODELfor the atom. It wasn’t a simple sphereas Dalton believed. His model was called the “Plum-pudding model”, but I call it the “chocolate-chip cookie doughmodel”, with the electrons being the chocolate chips

In 1910 – New ZealanderErnest Rutherford discoveredthat atoms have a NUCLEUS..

http://highered.mcgraw-hill.com/sites/0072402334/student_view0/animations.html#

He made this discovery by doing a clever experiment called the “Gold Foil Experiment”

Thompson’s model could not explain the results from the Gold Foil Experiment.

A NEW MODEL was needed.

Rutherford’s model has apositively-charged nucleus at the centerwith negative electrons whizzing around it. Most of the atom is actually emptyspace!

1913 – Neils Bohr (from Denmark) discovered that the electrons can only be in certain “energy levels”.

A NEW MODEL was again needed!

Bohr analyzed the lightemitted by atoms to make

his big discovery.

In the Bohr Model, the positive nucleusis surrounded by electrons in energylevels. The levels increase in energy asthey get further from the nucleus.

There were still changesto come, but we will use the Bohr Model for muchof our discussion of atoms.

This is often how science works.

Hypotheses are made.Experiments are done to test them.If needed, new hypotheses are formed.

Models are used to help us visualize things we cannot see.

Think about this statement:

“ALL MODELS ARE WRONG……SOME ARE USEFUL.”