Atomic Theories

Atomic TheoriesAtomic Theory – A Short History

Fifth Century, BCEDemocritus

Believed matter was composed of very small, individual particles that were indestructible

He called them “atomos” (meaning uncuttable)

His ideas persisted for centuries even though there was no experimental proof

Atomic Theories

JOHN DALTON - 1808Revised early Greek ideas

into testable scientific theory

Based his Atomic Theory on

three important concepts:

1. Law of Conservation of Mass

2. Law of Multiple Proportions

3. Law of Definite Proportions

Atomic TheoriesLaw of Conservation of Mass

States that mass cannot be created or destroyed

the mass of the reactants equals the mass of the products

Atomic TheoriesLaw of Multiple Proportions (pg 77)

States that when two elements combine to form two or more compounds, the mass of one element that combines with a given mass of the other is in the ratio of small, whole numbers

Atomic TheoriesLaw of Definite Proportions

States that a chemical compound always contains the same elements in exactly the same proportions by mass

Example: water

Atomic Theories

Dalton’s Principles 1. All matter is composed of extremely small

particles called atoms which cannot be subdivided, created or destroyed

2. Atoms of a given element are identical in their physical and chemical properties Example: all water molecules freeze at 0 deg C

and react with explosively with sodium

Atomic TheoriesDalton’s Principles (continued)

3. All atoms of one element are different from those of any other element

4. Atoms combine in simple, whole-numbered ratios to form compoundsBased on the Laws of Definite and Multiple

Proportions

Atomic Theories

Dalton’s Principles (continued)

5. In chemical reactions, atoms are combined, separated or rearranged but NEVER created, destroyed or changedBased on The Law of Conservation of Mass

Atomic TheoriesDalton’s Principles (continued)

6.Atoms of one element are never changed into atoms of another element in a chemical reaction

Atomic Theories

Dalton, however, did all this work in the early 1800’s without ever knowing about

subatomic particles!

(Protons, Neutrons, Electrons)

AtomsThe Adventures of J.J. Thomson,

Plum Pudding and the Electron! MMMMM…..Th

at plum pudding looks

delicious!

AtomsChapter One: The Discovery of the First Subatomic Particle: The Electron

JJ Thomson was an English Physicist who was studying electricity. He decided to builda glass tube which contained two metal plates at either end. Hethen pumped all the air out of the tube and attached a

voltagesource to either end. A glowing beam came out of the

cathode and struck the anode and the walls of the glass tube. He called this a

cathode ray.

AtomsJJ Thomson’s Cathode Ray Tube (CRT)

Anode – attached to thepositive terminal of thevoltage source

Cathode – attached to theNegative end of the voltagesource

AtomsTo test this, he placed a magnet near the tube

and the beam was deflected away from it, proving it was negatively charged.

He also placed a small paddlewheel in the tube, which turned when hit by the beam. That meant the particles had mass.

The particles are called “electrons.”

AtomsWhere did the beam come from?

Because most of the air had been removed from the tube, and the beam originated at the negatively charged cathode, Thompson reasoned the ray must be negatively charged.

CRT Tube with Paddle Wheel

AtomsAtoms have no charge, yet they gave off

negatively charged electrons. The scientists hypothesized that there must be positive charges also included in the atom.

JJ Thomson proposed the “Plum Pudding” model of the atom, which states that electrons were embedded in a

mass of positively charged matter.

AtomsIn 1909, one of his students, Ernest

Rutherford, disproved the “Plum Pudding” model by doing is famous “Gold Foil” experiment.

BLING!

AtomsAtomic Nuclei

History1911: Ernest Rutherford does his famous experiment

“Gold Foil Experiment”He shot radioactive alpha particles at an extremely

thin gold foil Most particles went right through to the other side

and were detected on the screen behind it.Only 1 in 8000 bounced back.

Atoms

Atoms

FOIL (sideway view)

These particles passthrough the foil to theother side and hit thedetecting screen

This particle hit the dense nucleus and bounced off ofit

Detecting Screen

AtomsRutherford’s experiment

http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/ruther14.swf

AtomsRutherford’s Conclusions:

Most atoms are made of empty space

All of an atom’s positive charge and almost all of its mass is contained in an extremely small area

He called this a “nucleus”

Eventually, other scientists later determined more details about the nuclei of atoms.

AtomsRutherford’s Model of the Atom:

Electrons orbit the nucleus just as planetsorbit the sun. However, he could not explainwhy the negatively charged electrons did notcrash into the positive charged nucleus.

AtomsTwo years later, Danish physicist, Niels Bohr,

proposed the Bohr Model of the atom

AtomsBohr’s Model has the following characteristics: Electrons are located certain distances from the

nucleus Each distance is a certain quantity of energy that

the electron can have Electrons closest to the nucleus have the lowest

energy, while the ones further away are in higher energy levels

The difference between two energy levels is called a quantum of energy.

Electrons can be only in an energy level, NOT between levels.

Electrons do not give off energy while they are in an energy level.

(Page 91)

Atoms• Various Scientists:

– Created the quantum mechanical model:– Electrons are not located in specific, fixed,

circular orbits– Electrons have certain allowed energies and

one can determine how likely it is to find an electron with that particular amount of energy

– Looks at probabilities of locating an electron