The Atom!

(not this guy)

The Atom!

As people get more data, they have come up with models to explain what an atom is, and how it works.

Everything you have learned about atoms before now, and everything you will learn in this class, is a model. It is not 'the truth'; it is an explanation that best fits the available data.

Democritus (460 BC to 370 BC) actually described something that sounds a lot like atoms...2000 years before anybody else.

But there were two problems:

1. Aristotle was super important at that time, and didn't like Democritus's theory. He preferred the “earth/wind/fire/water” type elements.

(plus Aristotle had a fabulous beard that you can't argue with) →

2. Democritus had no way to test any of his theories, which makes them scientifically about as useful as a bicycle for a fish.

Up until Antoine Lavoisier (1743-1794), everybody was using silly elements.Alchemists thought that everything was made of earth, fire, water, and air.

Sometimes aether or phlogiston was added.

Some systems used sound or light as elements.

If something burned, it was thought to have lots of fire element. Hard things had lots of earth. So hard, flammable things like wood would be a mix of fire and earth.

This didn’t do a very good job of explaining things, but nobody had much of a better idea.

Fortunately, Lavoisier was a complete genius.

Using little more than a scale, he showed that air was partly made of oxygen which was needed for life, and that it was a pure substance (an “element”).

He also showed that you could combine hydrogen and oxygen to make water, and:

1.They always combined in the same ratio2.You always got out the same mass you put in.

He also helped create the metric system.

Unfortunately, the French Reign of Terror happened, and they guillotined him, along with a lot of other people.

The discovery of elements basically created the field of chemistry, but nobody really knew what they were made of, or why one element was different from the next.

This is John Dalton (1766-1844)

He provided some of the early theories into what exactly atoms were, and how they behave.

(Dalton also wore incredibly stylish round glasses)

1. Elements are made of tiny particles called atoms.

2. All atoms of a given element are identical.

3. The atoms of a given element are different from those of any other element; the atoms of different elements have different weights. 4. Atoms of one element can combine with atoms of other elements to form chemical compounds; a given compound always has the same relative numbers of types of atoms. 5. Atoms cannot be created, divided into smaller particles, nor destroyed in a chemical process; a chemical reaction simply changes the way atoms are grouped together.

Back to Dalton: Dalton's (Mostly Correct) Theory of Atoms (~1800-1810)

1. Elements are made of tiny indivisible particles called atoms.

2. All atoms of a given element are identical.

3. The atoms of a given element are different from those of any other element; the atoms of different elements have different weights. 4. Atoms of one element can combine with atoms of other elements to form chemical compounds; a given compound always has the same relative numbers of types of atoms. 5. Atoms cannot be created, divided into smaller particles, nor destroyed in a chemical process; a chemical reaction simply changes the way atoms are grouped together.

Back to Dalton: Dalton's (Mostly Correct) Theory of Atoms (~1800-1810)

True, except for one word

true

true

Close enough to true that he couldn't have discovered otherwise with the tools of the time

In practical terms, Dalton is the first person to show that you could have two different compounds, both made of the same elements/atoms, but the ratio of the two is what makes them different:

H2O H

2O

2

Water. Delicious. Hydrogen peroxide. Not tasty.Necessary for life. Used as rocket fuel at high

concentrations

Back to Dalton: Dalton's (Mostly Correct) Theory of Atoms (~1800-1810)

1810 -1896:

War of 1812 (Washington D.C. Burned down. Star Spangled Banner written)

Civil War (“War of Northern Aggression” for certain types of southerners)

Gunfight at the O.K. Corral

And some other stuff that can besafely left to your history class.

Virgil Earp

(Actually, a lot of brilliant chemistry was done during this time, but not much affecting what we really knew about atoms)

1897: J.J. Thomson discovers the electron (but he doesn't know it yet)

He was playing around with Cathode Ray Tubes (electrified tubes with gas in them) and discovered that the stuff coming out was attracted to charged objects.

This meant the stuff was charged.

It was also the same, no matter what element was in the tube, and the mass was tiny—lighter than the smallest atom.

1897: J.J. Thomson discovers the electron (but he doesn't know it yet)

He figured they were tiny bits of an atom.

This meant atoms could be split into smaller things (take that, Dalton).

He called them “corpuscles”

Everyone else decided to call them “electrons” instead.

The “Plum Pudding” Model of the AtomThomson decided that atoms were a sea of positive charge, with electrons floating in it.

Like raisins in a plum pudding, apparently.

I guess people eat this?

1909-1911: Ernest Rutherford (Lord Nelson) Blasts His Way into History

When you're nobility, nobody will make fun of you for a mustache like that.

Rutherford shot tiny particles at a thin piece of gold foil, with detectors on the other side and all around it.

1909-1911: Ernest Rutherford (Lord Nelson) Blasts His Way into History

If Dalton was right, the particles should all bounce off the hard little atoms.

If Thomson was right, they should all just fly right through the soupy atoms (which leads to some interesting questions about why you can’t walk through walls)

Instead, almost all the radiation went straight through, but some bounced off.

So atoms were mostly empty, but must have something hard that could deflect particles.

Rutherford's model has small, hard nucleus in the center, with protons in it (neutrons hadn’t been discovered yet), and electrons somewhere around it. He did predict that neutrons might exist, but didn't find any evidence.

Niels Bohr is coming to get you!

Niels Bohr is coming to get you!

Rutherford's model really said nothing about where exactly the electrons were—just that they were somewhere around the nucleus.

1913: Niels Bohr comes up with a crazy theory...that works.

Let's start with a staircase, and have somebody go up 4 steps:

1913: Niels Bohr comes up with a crazy theory...that works.

And now down 2 steps.

No problem

1913: Niels Bohr comes up with a crazy theory...that works.

But what if....

Still easy

1913: Niels Bohr comes up with a crazy theory...that works.

GO DOWN 2.5 STEPS!

?????

In Bohr's model, the electrons orbit the nucleus, but can only be at certain distances—never halfway in between. They are 'quantized'.

Bohr's model works perfectly...for hydrogen. Not for any other element. However, the idea of quantized electrons is still important to today's much more complicated model.

Summary of it all, in one word associations:Lavoisier – elements

Dalton – atoms

JJ Thomson – electrons

Rutherford – nucleus

Bohr – quantum

Some important names that don't fit in the story here, but are equally important:

Marie Curie – radioactivity

James Chadwick – neutrons

Henry Moseley – atomic number