Unit 3 - Atomic Theory

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The Atomic TheoryUnit 3 – Honors Chemistry

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• Democritus (~400 BC)• Greek philosopher • Thought that all matter

was made of smallparticles.

• Called them “ATOMS”

which were indivisibleand indestructible.

Discovering the Atoms



• Antoine Lavoisier (1789)• Law of Conservation of Mass• Mass can neither be created

nor destroyed!• Joseph Proust (1797)

• Law of Definite Proportions• Compounds always contain

the same proportion of

elements by mass.

Discovering the Atoms



• John Dalton (1803)• English Scientist who created

the first Atomic Theory:1. All matter is made of atoms.2.

Atoms of an element areidentical in size and mass.3. Atoms of different elements

can mix physically or combine chemically.

4. In reactions, atomscombine, separate or rearrange.

5. Atoms cannot be divided,created or destroyed.

Discovering the Atom

John Dalton’s

Atom



• JJ Thomson (1897)• Performed the cathode

ray experiments• Discovered electrons• Said electrons areembedded throughoutthe atom.

• Plum Pudding Model•

The atoms containedpositively-chargedmaterial, withnegatively-chargedelectrons in it.




• R.A. Millikan (1909)• The Oil Drop Experiment• Calculated the specific

mass and charge of theelectron.

• Mass = 9.109 x 10 -28 kg




• Ernest Rutherford (1911)• The Gold Foil Experiment

• Fired alpha rays (+) ata sheet of gold foil.

• Most of the rays passedthrough, but somebounced back!

• Conclusions•

The atom is mostlyempty space• A dense, positively-charged nucleus is incenter of each atom




• Eugene Goldstein (1886)• Observed a Carthode Ray

Tube• Some particles travelled to

the negative end.• Discovered the proton!

• 1840 times bigger thanan electron.




• Neils Bohr (1922)• Planetary Model of the

Atom• Electrons move indefinite orbits aroundthe nucleus

• Each energy level islocated at a certaindistance from thenucleus




• James Chadwick (1932)• Discovered the neutron.• No Charge•

Same mass as the proton.




• Enrico Fermi (1942)• Successfully split the

atomic nucleus• Paved the way for nuclear

power and weapons.






• EVERYTHING is made ofATOMS!

• Understanding the atomhelps us understand howeverything works.

• Understanding thestructure of the atom helpsus understand how it doeswhat it does.

Atomic Structure



• The Atom is mostlyempty space.

• A tiny, positively chargednucleus in the center.

• Even tinier, negativelycharged electronswhirling around it.

• Atomic Structure



• The Nucleus• Makes up 99.99% of the

atom’s mass. • About 1 fm wide (10 -15 )• Composed of Protons

and Neutrons.




• Protons (p +)• Positively-Charged

Particle• 1 amu (10 -27 kg)• Determines the type of

element.• Neutrons (n 0)

• Carries NO CHARGE!• 1 amu (10 -27 kg)• Holds the nucleus

together with strongforces.




• Electron Clouds• Shows the probable

location of an electron.• Lowest energy level is

closest to the nucleus.• Electrons (e -)

• Negatively-ChargedParticle

•

0 amu (10 -31 kg)• Determines the chemical

properties of the element.

Atomic Structure



• Atomic Number (Z)• The Number of Protons in

an Element• Every element has a

different Atomic Number!• In a neutral atom, the

number of protons equalsthe number of electrons.

•

Mass Number (A)• The number of Protons +

the number of Neutrons

Identifying Elements



Identifying Elements

Z = #p + = #e -

A = Z + n0

n 0 = A - Z



• Elements• Different numbers of protons.

• Ions•

Different numbers ofelectrons.• Lose an e- Positive Ion• Gain an e- Negative Ion

• Isotopes• Different numbers of neutrons.• Hydrogen has Three Isotopes• Unstable isotopes are

radioactive!

Changing Elements



• Accounts for theabundance of eachisotope.

•

The number found on theperiodic table.

Average Atomic Mass

Atomic Mass = (0.7890)(24 amu) + (0.1000)(25 amu) + (0.1110)(26 amu)







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Unit 3 - Atomic Theory