The History of the Atom

Back to Table of Contents Black Boxes Democritus Dalton’s Theory Thomson’s Electron Rutherford’s Nucleus Goldstein’s Protons Chadwick’s Neutron Atomic Symbols Back to Table of Contents

Black Boxes Black Boxes are anything that you cannot see inside Like a cell phone… Or a locked box… Or an atom…

The Beginning of the Atom

The ancient Greeks (back around 500BC) believed there only four elements Earth Air Water Fire

Democritus He also lived in ancient Greece but he believed differently He said that all matter was made up of particles that could not be divided into smaller particles He called these particles – Atoms

The Atomic Theory

Democritus’ idea of the atom was largely ignored until an English schoolteacher did some experiments over 2000 years later, he was… John Dalton (1766-1804) Leading to his atomic theory…

Dalton’s Atomic Theory All matter is made up of atoms Atoms are indestructible and cannot be divided into smaller particles (Atoms are indivisible) All atoms of one element are exactly alike, but they are different from atoms of other elements A given compound always has the same relative numbers and kinds of atoms. Atoms join in whole number ratios. Atoms are neither created nor destroyed in any chemical reaction.

Dalton’s Model of the Atom He believed the atom was a solid sphere An analogy to his atom would be…

Thomson’s Cathode Ray Tube Experiments and the Discovery of the Electron

The Electron JJ Thomson (1856-1940) used the cathode ray tube to prove that the atom was made up of electrons Click here to listen to him talk about it

But, what was that green light? Was it a light? Was it a particle? To test this, he brought a magnet close to the cathode ray tube to see what would happen.

So, the magnet caused the cathode ray to move. What does that tell us? Would a magnet affect a light from a flashlight? (you could try this at home) Probably not. Therefore, the cathode ray must be a particle!

Thomson also noticed… That the cathode ray was coming out of the negative end (cathode) of the tube and going toward the positive end (anode) Therefore, because opposites attract, he concluded that the cathode ray must be negative He called these particles… Negative end Negative terminal Positive terminal Positive end

ELECTRONS! The discovery would alter Dalton’s model of the atom because now there is something inside it But, in addition to the negatively charged electrons, there must be something giving it a positive charge because the overall charge of the atom is neutral (not negative) Let’s look at Thomson’s model of the atom…

Thomson’s Plum Pudding Model of the Atom He believed the atom was made of positively charged stuff with negatively charged particles scattered throughout Why the “plum pudding” model? What is “plum pudding”?

An American analogy to his atom would be… Plum Pudding is an English dish sort of like bread pudding with raisins in it. An American analogy to his atom would be… …Is like… But we’ll still refer to it as the “Plum Pudding” model in class

Rutherford and the Nucleus

Ernest Rutherford In 1908, Rutherford performed the Gold Foil Experiment. (1871-1937) In it, he shot alpha particles (very small, very dense, very fast particles) at a thin layer of gold foil.

He expected all of the alpha particles to go straight through It would be like if you were shooting bullets at a cake…all of the bullets (or alpha particles) would go straight through the cake (or gold foil atoms)

Gold foil Detector screen Alpha particles source Alpha particles Thomson’s Atom Alpha particles

But, what he found was this…

Or…

Did you notice how most of the alpha particles went straight through but a very tiny amount were deflected at odd angles? That could only happen if there was something very tiny in the atom that was dense enough to deflect the alpha particles. Like this…

To recap the Gold Foil Experiment

Rutherford proved that an atom was mostly empty space with a very small, very dense, positively charged nucleus in it.

Rutherford’s Nuclear Model of the Atom He believed the atom was made of a tiny positively charged nucleus with negatively charged particles orbiting it Rutherford’s Nuclear Model of the Atom

Or… If the atom were Paul Brown Stadium, the nucleus would be a pea on the fifty-yard line

A Rutherford Atom Analogy In which the peach pit is the nucleus and the rest of the peach represents the electrons buzzing around

Other important discoveries… In 1886, Goldstein discovered the proton (which is located in the nucleus) In 1932, Chadwick discovered the neutron (also located in the nucleus)

Atomic Symbols Now, we can determine the number of each of these particles if we know the atomic number and mass number. Atomic Number = # of protons Mass Number = # of protons + # of neutrons Charge = # of protons - # of electrons

X Reading symbols Symbols Contain the symbol of the element, the mass number and the atomic number X Mass number Atomic number Charge

F Try it… 19 9 -1 Find the… Atomic number Mass number Charge # of protons # of neutrons # of electrons

F The answers… 19 9 -1 Find the… Atomic number Mass number Charge # of protons (p+) # of neutrons (n0) # of electrons (e-) = 9 = 19 = -1 = 10

How about this one… Ca 41 20 Find the… Atomic number Mass number Ca 20 Find the… Atomic number Mass number Charge # of protons (p+) # of neutrons (n0) # of electrons (e-)

The answers… Ca 41 20 Find the… Atomic number Mass number Charge Ca 20 Find the… Atomic number Mass number Charge # of protons (p+) # of neutrons (n0) # of electrons (e-) = 20 = 41 = 0 = 21