Atomic Theory and How we know what atoms are made of

Atomic Structure

The thing that makes up everything

The First recorded time someone came up with an atomic theory

Democritus

• Believed that matter was made up of tiny unseen particles that could not be divided

• Gave the small particles the name "Atomos" meaning indivisible is ancient Greek.

• Thought that the tiny particles resembled what they created. So cheese atoms looked like super tiny pieces of cheese.

Aristotle

• Thought that matter was composed of four "pure" elements Earth, Fire, Air, Water.

• Thought matter could be divided into smaller pieces forever

• Was more popular the Democritus

Multiple Choice

Who's view of what matter is made up of was somewhat correct?

• Democritus

• Aristotle

Open Ended

What do you think about the fact that someone so long ago, even before the birth of what we know as science, could think of a fairly accurate view of an atom?

Foundations of Modern Atomic Theory

In the 1790's the advancement of equipment and a new emphasis on quantitative chemistry led to new discoveries.

• Three Laws

• Law of the Conservation of Mass

• Law of Definite Proportions

• Law of Multiple Proportions

Law of Conservation of Mass

The law states that mass cannot be created nor destroyed by chemical reaction or physical changes. If you collect all of the new products created by a chemical reaction is should have the same mass as the chemicals you started with.

Law of Definite Proportions

A chemical compound contains the same elements in exactly the same proportions by mass regardless of the size of the sample or it's source.

Law of Multiple Proportions

If two or more different compounds are composed of the same two elements, then the ratio of the masses of the second element combined with a certain mass of the first element is always a ratio of small whole numbers. Meaning that no matter how atoms combine to make a compound they are always in whole number segments. You can't have half an atom.

Multiple Choice

Which is the Law of the Conservation of Mass

• The law states that mass cannot be created nor destroyed by chemical reaction or physical changes. If you collect all of the new products created by a chemical reaction is should have the same mass as the chemicals you started with.

• The law states that mass can not be created nor destroyed by chemical reaction or physical changes. If you collect all of the new products created by a chemical reaction is should have the same mass as the chemicals you started with.

• The law states that mass can not be created nor destroyed by chemical reaction or physical changes. If you collect all of the new products created by a chemical reaction is should have the same mass as the chemicals you started with.

Multiple Choice

Which is the Law of Multiple Proportions

• The law states that mass cannot be created nor destroyed by chemical reaction or physical changes. If you collect all of the new products created by a chemical reaction is should have the same mass as the chemicals you started with.

• The law states that mass can not be created nor destroyed by chemical reaction or physical changes. If you collect all of the new products created by a chemical reaction is should have the same mass as the chemicals you started with.

• The law states that mass can not be created nor destroyed by chemical reaction or physical changes. If you collect all of the new products created by a chemical reaction is should have the same mass as the chemicals you started with.

Multiple Choice

Which is the Law of Definite Proportions

• The law states that mass cannot be created nor destroyed by chemical reaction or physical changes. If you collect all of the new products created by a chemical reaction is should have the same mass as the chemicals you started with.

• The law states that mass can not be created nor destroyed by chemical reaction or physical changes. If you collect all of the new products created by a chemical reaction is should have the same mass as the chemicals you started with.

• The law states that mass can not be created nor destroyed by chemical reaction or physical changes. If you collect all of the new products created by a chemical reaction is should have the same mass as the chemicals you started with.

These three laws started scientists down a long path to understand the structure of matter

The First Modern Atomic Theory

John Dalton

• In 1808, after the laws have become wide spread, John an English school teacher made a proposals that would explain all the behavior seen in chemical compounds

Dalton's Atomic Theory

• All matter is composed of extremely small particles called atoms.

• Atoms of a given element are identical in size, mass, and other properties.

• Atoms cannot be subdivided, created, or destroyed

• Atoms of different elements combine tin simple whole-number ratios to form chemical compounds.

• In chemical reactions, atoms are combined, separated, or rearranged.

Through experimentation based on Dalton's atomic theory we have developed a modern atomic theory. Dalton was right in a lot of ways.

It also set scientists out to discover the structure of the atom.

Modern Atomic Theory

• All matter is composed of atoms. Each atom is made up of smaller subatomic particles: Protons, neutrons, and electrons

• The atoms of one element cannot be converted into the atoms of any other element by a chemical reaction

• Atoms of one element have the same properties, such as average mass and size. These properties are different from the properties of the atoms of any other element

• Atoms of different elements combine in specific proportions to form compounds.

Multiple Choice

Based on Modern atomic theory: Can atoms be broken down into smaller parts?

• Yes

• No

Multiple Choice

Based on modern atomic theory can atoms of two different elements have the same properties of mass and size?

• Yes

• No

There has been many models of the atom. The quantum Mechanical Model being the most modern.

How do we know what makes up an atom?

Dalton's Model

Atoms are small solid balls that can not be divided.

J.J. Thomson

• Used a device, similar to the one to the right, to apply high voltages of electricity to metal in a vacuum.

• Created what was called Cathode Rays

• Thomson placed two charged plates on both sides of the tube.

• He noticed that the cathode ray would bend away from the negative charged plate

J.J. Thomson

• He noticed that the rays were pushed away by a magnetic field in the same manner as a wire carrying a negative electric charge.

• With both these pieces of evidence it leads to the idea that the ray was composed of negative particles.

• He ran the same experiment with different pieces of metal. The same thing happens.

• This allows him to conclude that all atoms have a negative charge part to them.

Thomson's Model

• Called plum pudding model (he is British, think of it like a blueberry muffin or a watermelon).

• Thomson thought of it as a large positive field with little pieces of negative charged particles spread throughout. Like the seeds of a watermelon or blueberries in a muffin.

• Watch the following video if you want to know more

Millikan

• Watch the video about how millikan discovered the charge and the mass of electrons.

The Neutral Atom

Scientists knew that atoms were neutral (no charge) because of this known fact and that Thomson and Millikan proved there was a negative charge part of the atom they determined that there has to be a positive portion too. As seen in Thomson's model of the atom.

This pushed scientists to develop new experiments to find that positive portion.

Rutherford and the discovery of the nucleus

• Watch the video to find out how we discovered that Thomson's model was wrong and that the nucleus is a small positively charge area.

Chadwick discovers the neutron

• Working off the experiments of Frédéric and Irène Joliot-Curie Chadwick would shoot alpha particles (a type of radiation) at Beryllium which would then release its own particles thought to be a different type of radiation.

• Those particles off of the Beryllium would knock protons off of samples of matter such as a sheet of paraffin wax.

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Chadwick discovers the neutron

• Chadwick concluded that in order to knock a proton off an atom the "new" radiation actually had to be something with the same mass as a proton.

• By experimenting with things other then paraffin wax he was sure that they were neutrons (a part of an atom first introduced by his boss Rutherford)

• The neutron was later confirm to be neutral (no charge) and at about the same mass as a proton

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This is how scientists came up with the three fundamental particles of an atom

To see how the model of an atom has changed over time watch the video on the next slide.