Development of Atomic Theory

Democritus• GREECE around 400 b.c. Democritus said:

– 47  The material cause of all things that exist is the coming together of atoms and void. Atoms are too small to be perceived by the senses. They are eternal and have many different shapes, and they can cluster together to create things that are perceivable. Differences in shape, arrangement, and position of atoms produce different things. By aggregation they provide bulky objects that we can perceive with our sight and other senses.

– 48  We see changes in things because of the rearrangement of atoms, but atoms themselves are eternal. Words such as ‘nothing’, ‘the void’, and ‘the infinite’ describe space. Individual atoms are describable as ‘not nothing’, ‘being’, and ‘the compact’. There is no void in atoms, so they cannot be divided. I hold the same view as Leucippus regarding atoms and space: atoms are always in motion in space. 

Adapted from  The Presocratic Philosophers, by G. S. Kirk and J. E. Raven. The University Press, Cambridge, England, 1962. Selections from Early Greek Philosophy by Milton C. Nahm.

War of the Minds

• Democritus said:• If you break a piece of

matter in half, then in half again, and again, how many times do you have to break it until it can’t be broken anymore?

• There is a stopping point of matter, that is the atom.

• Artistotle said:• There are only four

elements: earth, wind, fire, and water.

• NO ATOMS, Matter cannot be separated.

• I WIN!!!!

Jump ahead to the 1800’s

• John Dalton (1764-1844)– Hey, that Democritus

guy was on to something.

Dalton’s Atomic Theory

Elements are composed of atoms– tiny, hard, unbreakable, spheres

All atoms of a given element are identical– all carbon atoms have the same chemical and

physical properties

Atoms of a given element are different from those of any other element– carbon atoms have different chemical and physical

properties than sulfur atoms

Dalton’s Atomic Theory Cont.Atoms of one element combine with atoms of

other elements to form compounds. – Law of Constant Composition

• all samples of a compound contain the same proportions (by mass) of the elements

– Chemical FormulasAtoms are indivisible in a chemical process.

– all atoms present at beginning are present at the end

– atoms are not created or destroyed, just rearranged

– atoms of one element cannot change into atoms of another element

• cannot turn Lead into Gold by a chemical reaction

Unbreakable? Really?

• J.J. Thomson investigated a beam called a cathode ray

• he determined that the ray was made of tiny negatively charged particles we call electrons

• electrons were smaller than a hydrogen atom

• if electrons are smaller than atoms, they must be pieces of atoms

•if atoms have pieces, they must be breakable•Thomson also found that atoms of different elements all produced these same electrons

Cathode Ray

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mmm…pudding

Atom breakable!! Atom has structure Electrons suspended in a

positively charged electric field– must have positive charge

to balance negative charge of electrons and make the atom neutral

mass of atom due to electrons atom mostly “empty” space

– compared size of electron to size of atom

Plum Pudding Model

OK, Prove it.• How can you prove something is empty?• put something through it

– use large target atoms• use very thin sheets of target so do not absorb “bullet”

– use very small particle as bullet with very high energy • but not so small that electrons will affect it

• bullet = alpha particles, target atoms = gold foil– particles have a mass of 4 amu & charge of +2 c.u.– gold has a mass of 197 amu & is very malleable

The Gold Foil Experiment

• Experiment performed by Ernest Rutherford• “It was a result similar to shooting a gun at a

piece of paper and have the bullet bounce back.”

What’s Happening

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The atom contains a tiny dense center called the nucleus– the volume is about 1/10 trillionth the

volume of the atomThe nucleus is essentially the entire mass of

the atomThe nucleus is positively charged

– the amount of positive charge of the nucleus balances the negative charge of the electrons

The electrons move around in the empty space of the atom surrounding the nucleus (responsible for chemical reactions)

The Nuclear Atom

All about the nucleus• The nucleus was found to be composed of two

kinds of particles• Some of these particles are called protons

– charge = +1– mass is about the same as a hydrogen atom

• Since protons and electrons have the same amount of charge, for the atom to be neutral there must be equal numbers of protons and electrons

• The other particle is called a neutron– has no charge– has a mass slightly more than a proton

That’s all for now…

• Modern Atomic Theory• Big Questions Remain: • How do the electrons

travel in the atom?• Why do the electrons

stay out of the nucleus?• If all atoms are just a

combination of protons, neutrons, and electrons, what makes them different from each other?