Chapter 4Atomic Structure

4.1 Studying Atoms

Early Chemists and Atomic Models

• Who are the early chemists that shaped atomic theory?

• What were the accepted models of the atom?

• How have the models changed over time?

• What experiments were performed to change the models?

Ancient Greek Models

• Democritus was one of first philosophers on record• Believed that matter was made of

extremely small particles• Different particles had different

properties

Ancient Greek Models

• Aristotle was another Greek that described atomic structure• His model was that of the 4

elements :–Fire -Air–Water -Earth

Dalton’s Atomic Theory

• Dalton studied the behavior of gases in air• Discovered that gases consist of

individual particles instead of “one substance”

Evidence for Atoms

• Measured mass of elements when they formed compounds• Showed that the ratio of masses

was always the same

Example• 16 g of Oxygen gas reacts with 24

g of Magnesium • 1.6 g of Oxygen gas reacts with

2.4 g of Magnesium• Ratios are always the same

Dalton’s Theory• All elements are composed of atoms• All atoms of the same element have

the same mass• Compounds contain atoms of more

than one element• Atoms always combine the same way

in each compound

Dalton’s Theory

• Atoms are represented by spheres• Atoms of different elements

would be different size spheres

Thomson’s Model

• Thomson noticed that materials attract or repel other materials when rubbed• Like charges repel, unlike attract• Some charged particles can flow

from one location to another

The Experiment

Thomson used a sealed glass tube with an electric currentA glowing beam is generated between the metal plates at the ends

Video

The Experiment

• Thomson placed charged particles on either side of the glass tube• The stream of particles would

bend toward or away from the charged plates

Evidence for Subatomic Particles

• The particles were negative because they were repelled by negative charged plates• All metal disks reacted the same way, so

he assumed the particles came from inside atoms• Their mass was 1/2000 the mass of

Hydrogen atoms

The Conclusions

• Thomson concluded that Dalton was wrong about atoms being the smallest particles• His experiments showed that

atoms were made of smaller things with charges

Thomson’s Model• Referred to as the “Plum Pudding”

model• Since atoms are neutral, the charges

needed to be spread out• Negative charges were evenly

distributed among the positive particles of the atom

Earnest Rutherford

• He lived from 1871-1937• He was testing some of the ideas

of Thomas with gold foil• He was working with Uranium,

which emits a fast moving particle with a positive charge

The Hypothesis• Rutherford believed that the mass

and charge of particles in gold would be too small to deflect the alpha particles of Uranium• The particles would pass straight

through the gold or only move slightly

The Experiment• A stream of alpha particles was

aimed at gold foil• The particles would hit a screen that

was placed around the foil and would flash when hit• The location of the flash was studied

The Result

• One out of every 20,000 particles would deflect by more than 90˚• Some particles bounced straight

back

The Conclusion

• Something was making the particles deflect• The charge was not evenly

distributed• The positive charged particles

were concentrated in one place

The Nucleus

• The nucleus is the location of all of the positive charges in the atom• Most of an atom is empty space

where the electrons are located