Atomic Models

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 TheoriesJOHN DALTON - 1808

Revised early Greek ideasinto testable scientific theory

Based his Atomic Theory onthree important concepts:1. Law of Conservation of Mass2. Law of Multiple Proportions3. Law of Definite Proportions

Atomic TheoriesLaw of Conservation of MassStates that mass cannot be created or

destroyed the mass of the reactants equals the mass of the

products

Atomic TheoriesDalton’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 TheoriesDalton’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 Theories

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

subatomic particles!(Protons, Neutrons, Electrons)

Atomic ModelsThe Adventures of J.J. Thomson,

Plum Pudding and the Electron! MMMMM…..Th

at plum pudding looks

delicious!

Atomic ModelsJJ Thomson:

English Physicist Experimented with “cathode

rays” Was able to determine that

the mass of the particles in the ray was much smaller than the mass of a hydrogen atom

The particle must be a smaller than an atom!

I LOVE plum

pudding!

Atomic ModelsJJ Thomson’s Cathode Ray Tube (CRT)

Anode – attached to thepositive terminal of thevoltage source

Cathode – attached to theNegative end of the voltagesource

Atomic ModelsA magnet placed near the tube deflected the

beam, proving it was negatively charged.

A small paddlewheel in the tube turned when hit by the beam, meaning the particles had mass.

Atomic ModelsThompson called the negatively charged

particles in the beam “corpuscles”

This name was later changed to “electrons” by one of Thompson’s associates.

Atomic ModelsThompson’s Conclusion:

An atom is a sphere of positive charge with electrons embedded in it

“Plum-Pudding” Model of the

Atom

Atomic ModelsIn 1909, one of his students, Ernest

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

Atomic Models

Rutherford’s experiment

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

Atomic Models• What conclusions were drawn from

Rutherford’s Experiment?

Atomic ModelsRutherford’s Model of the Atom:

Electrons orbit the nucleus just as planetsorbit the sun

Did not explain why the negatively charged electrons did not crash into the

positively charged nucleus.

Atomic ModelsTwo years later, Danish physicist, Niels Bohr,

proposed the Bohr Model of the atom

Atomic ModelsBohr’s Model Electrons are

located certain distances from the nucleus

Each distance is a certain quantity of energy that the electron can have

Atomic Models

Electrons closest to the nucleus have the lowest energy, while the ones further away are in higher energy levels

Atomic ModelsThe difference

between two energy levels is called a quantum of energy.

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

Atomic ModelsElectrons and Light (pg. 92)

Basic Information:Light travels in wavesEach wave has a certain wavelength

(distance between two consecutive peaks of a wave)

Atomic ModelsEach wavelength hasa certain frequency (the # of waves thatpass through a specific point in onesecond)

Atomic ModelLight and the Visible Electromagnetic Spectrum

We can only see a small amount of the electromagnetic spectrum

Bohr found that if you pass a high voltage through a gas and look at it through a prism, it will have a distinctive pattern of colored lines called “line-emission spectrum”

Atomic ModelsBohr calculated that the line-emission

spectrum corresponded to energy levels in the atom (which give off a different wavelength of visible light)The colors are caused when an electron falls

from its excited state to its ground state and gives off energy in the form of light! (pg. 94)

Atomic ModelsGreat video on line-emission spectrum

http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/linesp16.swf

Spectrum of a Fluorescent Light

Spectrum