The Atom

Unit 3, Presentation 1

History of the atom

Not the history of atom, but the idea of the atom– The atom was not “discovered” until recently

Original idea Ancient Greece (400 B.C.)– Proposed by lesser known scientists

History of Atom

The looked at a beach– Made of sand

If you cut sand – smaller sand

What is the smallest possible piece?What is the smallest possible piece? Atomos - not to be cutAtomos - not to be cut

Another Greek

Aristotle - Famous philosopher All substances are made of 4 elements

– Fire – Hot, dry– Air - light– Earth - cool, heavy– Water - wet

These are blended in different proportions to get all substances

Who Was Right?

Greek society class based Thought to be beneath famous scientists to work

with hands– Aristotle did not experiment

Greeks settled disagreements by argument– Aristotle was more famous– He won

His ideas carried through middle ages.– Alchemists change lead to gold

Who’s Next?

Late 1700’s - John Dalton- England He was a teacher

– summarized results of his experiments and those of other’s

In Dalton’s Atomic Theory– Combined ideas of elements with that of atoms

Dalton’s Atomic Theory

All matter is made of tiny indivisible particles called atoms.

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

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

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

Law of Definite Proportions

Each compound has a specific ratio of elements

It is a ratio by mass Water is always 8 grams of oxygen for each

gram of hydrogen

Law of Multiple Proportions

If two elements form more that one compound, the ratio of the second element that combines with 1 gram of the first element in each is a simple whole number.

What?

Water is 8 grams of oxygen per gram of hydrogen.

Hydrogen Peroxide is 16 grams of oxygen per gram of hydrogen.

2

8 X2Y16 X 8 Y+

Parts of Atoms

J. J. Thomson - English physicist. 1897 Made a piece of equipment called a cathode

ray tube. It is a vacuum tube

Thomson’s Experiment

Voltage source

+-

Vacuum tube

Metal Disks

Thomson’s Experiment

Voltage source

+-

Thomson’s Experiment

Voltage source

+-

Thomson’s ExperimentThomson’s Experiment

Voltage source

+-

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

Thomson’s ExperimentThomson’s Experiment

Voltage source

+-

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

Thomson’s ExperimentThomson’s Experiment

Voltage source

+-

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

Thomson’s ExperimentThomson’s Experiment

Voltage source

+-

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

Thomson’s ExperimentThomson’s Experiment

Voltage source

+-

Voltage source

Thomson’s Experiment

By adding an electric field

+

-

Voltage source

Thomson’s ExperimentThomson’s Experiment

By adding an electric field By adding an electric field

+

-

Voltage source

Thomson’s ExperimentThomson’s Experiment

By adding an electric field By adding an electric field

+

-

Voltage source

Thomson’s ExperimentThomson’s Experiment

By adding an electric field By adding an electric field

+

-

Voltage source

Thomson’s ExperimentThomson’s Experiment

By adding an electric field By adding an electric field

+

-

Voltage source

Thomson’s ExperimentThomson’s Experiment

By adding an electric field By adding an electric field

+

-

Voltage source

Thomson’s ExperimentThomson’s Experiment

By adding an electric field he found that the By adding an electric field he found that the moving pieces were negative moving pieces were negative

+

-

Cathode Ray Tube

Thomson’s Model

Found the electron But he couldn’t find

positive (for a while) Said the atom was like

plum pudding A bunch of positive

stuff, with the electrons able to be removed

Other pieces

Proton - positively charged pieces 1840 times heavier than the electron

Neutron - no charge but the same mass as a proton.

Where are the pieces?

Rutherford’s experiment

Ernest Rutherford English physicist. (1910) Believed in the plum pudding model of the atom. Wanted to see how big they are Used radioactivity Alpha particles - positively charged pieces given off

by uranium Shot them at gold foil which can be made a few

atoms thick

Lead block

Uranium

Gold Foil

Fluorescent Screen

When alpha particles strike a fluorescent screen, it glows.

1. atoms positive charge is concentrated in the nucleus2. proton (p) has opposite (+) charge of electron (-)3. mass of p is 1840 x mass of e- (1.67 x 10-24 g)

particle velocity ~ 1.4 x 107 m/s(~5% speed of light)

(1908 Nobel Prize in Chemistry)

He Expected

The alpha particles to pass through without changing direction very much

Why?– The positive charges were spread out evenly.

Alone they were not enough to stop the alpha particles

What he expected

Because

Because, he thought the mass was evenly distributed in the atom

Because, he thought the mass was evenly distributed in the atom

What he got

How he explained it

+

Atom is mostly empty Small dense,

positive piece at center

Alpha particles are Alpha particles are deflected by nucleus deflected by nucleus if they get close if they get close enoughenough

+

Modern View

The atom is mostly empty space

Two regions Nucleus- protons and

neutrons Electron cloud-

region where you might find an electron

Density and the Atom

Since most of the particles went through, it was mostly empty.

Because the pieces turned so much, the positive pieces were heavy.

Small volume, big mass, big density This small dense positive area is the nucleus

atomic radius ~ 100 pm = 1 x 10-10 m

nuclear radius ~ 5 x 10-3 pm = 5 x 10-15 m

Rutherford’s Model of the Atom

“If the atom is the Houston Astrodome, then the nucleus is a marble on the 50-yard line.”