The Structure of an Atom

Chapter 3

Early Theories

• Greek Philosophers– 4 Elements

• Air

• Fire

• Wind

• Water

– Democritus• Atoms make up matter

– Aristotle• Refuted Democritus

and atom’s existence forgotten

Deomcritus

Atoms

Differences in atoms

Dalton’s Atomic Theory

• All matter is composed of extremely small particles called atoms.

• All atoms of a given element are identical.• Atoms cannot be created, divided into smaller

particles, or distroyed.• Different atoms combine in simple whole number

ratios to form compounds.• In a chemical reaction, atoms are separated,

combined or rearranged.

Dalton’s Atomic Theory

• All matter is composed of extremely small particles called atoms.

• All atoms of a given element are identical.• Atoms cannot be created, divided into smaller

particles, or destroyed. (This part proven wrong)• Different atoms combine in simple whole number

ratios to form compounds.• In a chemical reaction, atoms are separated,

combined or rearranged.

Deomcritus

Atoms

Differences in atoms

Dalton

•Atoms

•Sameness

•Created/destroyed

•Combination

•Rearragement

Subatomic Particles and the Atom

• Cathode ray tube– Stream of charged

particles (electrons).

– See pg 60 for picture

• J. J. Thompson– Used cathode ray tube

to prove existence of electron.

– Proposed “Plum Pudding Model”

Subatomic Particles and the Atom

• Plum Pudding• J. J. Thompson• Plum Pudding Model

Deomcritus

Atoms

Differences in atoms

Dalton

•Atoms

•Sameness

•Created/destroyed

•Combination

•Rearragement

Thompson

•Atoms composed of electrons

Ernest Rutherford

• Gold Foil experiment– Used to prove the

existence of a positively charged core (Nucleus)

• The results were “like firing a large artillery shell at a sheet of paper and having the shell come back and hit you!”

Ernest Rutherford• What should have

happened • What DID happened

Deomcritus

Atoms

Differences in atoms

Dalton

•Atoms

•Sameness

•Created/destroyed

•Combination

•Rearragement

Thompson

•Atoms composed of electrons

Rutherford

•Positively Charged Nucleus

Subatomic Particles and the Atom

• Chadwick– Worked with Rutherford.– Noted there was energy in the nucleus, but wasn’t the

protons.– Concluded that neutral particles must aslo exist in

nucleus.

• Mosley– Assigned atomic number to atoms

• Bohr– Designed “Bohr Model” of the atom.

Deomcritus

Atoms

Differences in atoms

Dalton

•Atoms

•Sameness

•Created/destroyed

•Combination

•Rearragement

Thompson

•Atoms composed of electrons

Rutherford

•Positively Charged Nucleus

Chadwick

•Neutrons exist in Nucleus

Subatomic Particles and the Atom

• Three main particles: (see chart on pg 97)– Proton

• Positive• In nucleus

– Neutrons• Neutral• In nucleus

– Electrons• Negative• Orbiting the nucleus (not inside)

Atomic Differences

• Atomic Number– Distinguishes one atom

from another.

– = #p+ (& #e- for a neutral atom)

• Isotopes– Differ in mass number

• Mass # = #p+ + #no

– Change is in # no

• Atomic Mass Unit

– Defined as 1/12 mass of a Carbon-12 atom

• p+ = 1 amu

• no = 1 amu

• e- = 0 amu

Atomic Differences

• Mass on the periodic table is based on the percent abundance of each isotope of that element.

• Therefore it is a decimal.• Ex: Chlorine exist naturally as Cl-37 and Cl-

35. Cl-37 is about 75% and Cl-35 is about 25%. So, (37 X .75) + (35 X .25) = 36.5 amu.

• See Ex problem on pg 103 for more ideas.

Questions?

• Ask now, or forever come in the morning for them!