History of Atomic TheoryDemocritus

– 460-371 B.C.– ancient Greek philosopher– believed all matter consisted of

extremely small particles that could not be divided

– atoms, from Greek word atomos, means “uncut” or “indivisible”

Aristotle– believed all matter came from only

four elements—earth, air, fire and water

Who Was Right?

• Greek society was slave based

• No experiments – It was all a thought game

• Settled disagreements by argument

• Aristotle was more famous so he won

• His ideas carried through to the middle ages.

John Dalton (Late 1700’s)

• School teacher in England

• Based his conclusions on experimentation and observations.

• Combined ideas of elements with that of atoms

Dalton’s Atomic Theory

1. All elements are composed of submicroscopic indivisible parts called atoms.

2. Atoms of the same element are identical, those of different atoms are different.

3. Atoms of different elements combine in whole number ratios to form compounds.

4. Chemical reactions involve the rearrangement of atoms. No new atoms are created or destroyed.

Parts of Atoms• Most of Dalton’s theory is accepted today.• Except the part about atoms being indivisible

English physicist

Provided the first evidence that atoms are made of even smaller particles

Description of a cathode ray tube and a short video of how it works:http://www.chem.uiuc.edu/clcwebsite/cathode.html

J.J. Thomson and the Cathode Ray Tube

1897

Thomson’s Experiment

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Thomson’s Experiment

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Thomson’s Experiment

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Thomson’s Experiment

• Passing an electric current makes a beam appear to move from the negative to the positive end.

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Thomson’s Experiment

• Passing an electric current makes a beam appear to move from the negative to the positive end.

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Thomson’s Experiment

• By adding an electric field

Thompson’s Experiment

• By adding an electric field

Thompson’s Experiment

• By adding an electric field

Thompson’s Experiment

• By adding an electric field he found the moving particles were negative

Thompson’s Model• Found the electron

– 1 unit of negative charge– Mass 1/2000 of hydrogen atom– Later refined by Millikan to

1/1840

• Concluded that there must be a positive charge since atom was neutral

• Atom was like plum pudding– A bunch of positive stuff, with

electrons able to be removed.

Other Pieces

• Proton – positively charged pieces 1,840 times heavier than the electron

• Neutron – no charge but the same mass as a proton.

Ernest Rutherford• Former student of J.J.

Thomson– Believed in plum pudding

• Wanted to find out how big they are

• Fired positively charged alpha particles at a piece of gold foil, which can be made a few atoms thick

1871-1937

Rutherford’s Experiment

• When alpha particles hit a flourescent screen it will glow.

• Here’s what it looked like (pg. 90)

• What he expected to see

– Alpha particles should pass through without change in direction

– Positive charges were spread out evenly. Alone they were not enough to stop an alpha particle

• What he got

http://micro.magnet.fsu.edu/electromag/java/rutherford/

• How he explained it– Atom is mostly empty– Small dense, positive piece at the center– Alpha particles are deflected if they get

close enough to positive center

Niels Bohr (1885-1862)• Electrons have orbits about the

nucleus (planetary theory)• Electrons could only exist at given

energy levels• An energy level is where an electron

is likely to be moving• Energy levels were like steps on a

ladder– An electron can only be at any given

step at any given time

Modern Atomic Theory

Bohr Model—shows electrons in orbit around protons and neutrons

Quantum-mechanical model—doesn’t show exact location of electrons, just probable place

Structure of the Atom

• There are two regions– The nucleus

• Protons and neutrons• Positive charge• Almost all of the mass

– Electron cloud• Most of the volume of an atom• Region where electron can be

found

Subatomic particles

Counting the pieces

• Atomic number = number of protons– Same as the number of electrons in a

neutral atom

• Mass number = the number of protons + neutrons

Atomic Mass Unit AMU

• Mass of a proton = 1.67 x 10 -27g– A pretty inconvenient number– New unit referenced to mass of an isotope

of carbon: carbon -12– Carbon-12 has 6 protons and 6 neutrons

• Has a mass of 12.00000 amu – an atomic mass unit

– Therefore 1 proton and 1 neutron has a mass of 1 amu.

So why not whole numbers for atomic masses in periodic table?

• Reported numbers are average atomic mass units, reflecting the abundance of isotopes for any given number.

• In nature most elements occur as a mixture of two or more isotopes

Isotopes• Atoms of the same element can have

different numbers of neutrons

• Different mass numbers

• Called isotopes