Chapter 4 – History of the Atom

Democritus 460 – 370 BC

n  The first to propose the idea of an atom n  Indivisible n  Indestructible n  Atom comes from the Greek

word atomos, which means “indestructible”

n  Everything is made of them

This is my globe. His name is Globey.

First Concept of an Atom

Unfortunately… n  Aristotle: opposed

Democritus and said all matter is a combo of n  Earth n  Air n  Fire

n  Water

n  Why? Because Democritus had no proof.

What about this

Democritus fellow?

Oh, don’t listen

to him, he cray-cray!

Some dude (Plato?) Aristotle

I may be crazy, but I’m right! Time

will tell.

Evidence for a new Atomic Theory:

1.  Law of conservation of mass (matter): Mass is neither created or destroyed in a chemical reaction.

2.  The law of Definite proportions: Chemical

compounds always have the same proportion of elements no matter how much of that compound you have.

2.  Example: water is always 2 parts Hydrogen to 1 part Oxygen

3.  The law of Multiple proportions: When two elements form compounds the ratio of their masses always form small whole numbers.

Example: Carbon and Oxygen combine to form CO or CO2, but never C0.5O1

John Dalton 1766-1844 n  Dalton’s Atomic Theory of Matter:

n  Each element is composed of extremely small particles called atoms that are cannot be divided

n  All the atoms of a given element are identical to each other, but they differ from those of any other element

n  Atoms are neither created nor destroyed in any chemical reaction, just rearranged

n  Atoms bond together to form compounds. A given compound always has the same relative numbers and kinds of atoms.

Do you like my fancy pants? You do? They’re called nickers.

JJ Thompson 1856-1940

•  Discovered electron 1897 – Cathode Ray Experiment

•  Electrons are negatively charged

particles in an atom.

I’m not Teddy Roosevelt.

The Cathode Ray Tube Experiment

video

•  Plum Pudding model 1904

•  Electrons in a soup of positive charges

•  Discovered isotopes 1913

Very interesting, there seems to be both positive and negative forces working

within an atom.

I really need a girlfriend.

JJ Thompson 1871-1937

Plum Pudding Model

Robert Millikan n  In 1909 Robert Millikan

measured the exact electronic charge and mass of an electron in his Oil Drop Experiment.

How did I come up with this device? MAGIC, that’s how.

The Oil Drop Experiment n  Millikan concluded that:

•  The mass of an electron is 9.10939 x 10-28 g

•  Which is 1840

times smaller than that of the Proton n  The electronic

charge on an electron is 1.60 x 10-19 C

•  The relative charge on an electron is -1 (relative to a proton which is +1) video

Ernest Rutherford 1871-1937

n  Nuclear Model 1910 n  Gold Foil Experiment

I’m kinda cool. No, really. Do you want to see my foil collection?

Gold Foil Experiment •  Alpha particles scatter from the gold foil.

Rutherford’s Model (cont)

n  Rutherford concluded that… n The Atom is mostly empty space n Has a region called the nucleus that…

n … is positively charged n … contains most of the mass of the atom

n The electrons are very small and orbit the nucleus in fixed paths.

Rutheford’s Model (cont) This model is called the Nuclear Atom or

Nuclear Model

•  If we drew the atom to scale and made protons and neutrons a centimeter in diameter, then the electrons would be less than the diameter of a hair and the entire atom's diameter would be greater than the length of thirty football fields!

•  99.999999999999% of an atom's volume is just empty space!

“Discovery” of Proton

n  If there are negative charges in the atom, there must be positive charges balancing them out = PROTONS

n Ernest Rutherford gets credit for the proton because he named it.

n Moseley discovered atomic number

Discovery of the Neutron

n  In 1932, the English physicist James Chadwick (1891–1974) confirmed the existence of yet another subatomic particle: the neutron.

n  Neutrons: n Located in nucleus with proton n Have no charge n mass nearly equal to a proton.

NEUTRon. Because It’s NEUTRAL, you see. Neutral… get it?

Niels Bohr 1885-1962 n  Planetary Model 1913

n  Nucleus surrounded by orbiting electrons at different energy levels

n  Electrons have definite orbits

n  Utilized Planck’s Quantum Energy theory

n  Worked on the Manhattan Project (US atomic bomb)

I’m not bohring. Get it? BOHRing?... I’ll just be over here exciting electrons…

Bohr Model for Nitrogen

Ernst Schrödinger 1887-1961

n  Quantum Mechanical Model 1926 n  Schrodinger: Used math to

describe the likely hood that an electron will be found in a particular place. (probability) n Electrons do not follow a fixed path. n Electrons are in electron clouds

called “orbitals”. n The denser parts of the clouds are

where they are more likely to be found.

Can I borrow your cat for a bit?

Quantum Mechanical Theory

n  Uncertainty Principle: n  You cannot know the location

and the momentum of an electron at the same time. n  You cannot know exactly where

an electron will be. n  Just where it might be based on

probability.

Werner Heisenberg 1901-1976

I don’t know how fast I’m going but I know where I am.

Quantum Model of the Atom

400 BC 1803 1904 1910 1913 1926

Niels Bohr

J J Thomson

Democratus

John Dalton

Schroedinger / Heisenberg

Ernest Rutherford

Atomic Theory