Warm-Up •Draw a picture of an atom.

Be as specific as you can!

History of the Atom

Democritus (400 B.C.) • Greek philosopher

• Coined the word atom, meaning

indestructible

• Atoms are solid, homogeneous, and

indivisible

• Different types of matter existed due to

changes in the atoms that composed them

• Problem: what holds the atom together?

John Dalton (19th century) • English schoolteacher

• Modern Atomic Theory:

1) All matter is composed of atoms

2) All atoms of the same element have identical size, mass & chemical properties

3) Different atoms combine in specific ratios to form compounds

4) In chemical reactions, atoms are separated, combined or rearranged

Law of Multiple Proportions

•2 H2 + O2 2 H2O • H2 + O2 2 H2O2

Cathode Ray Tube

Sir William Crookes (1800s) • Experimented with the CRD:

• No charged objects ray is straight and flows

from cathode to anode

• Positive charge near the tube ray is deflected

toward the charge

• Negative charge near the tube ray is

deflected away from the charge

• Conclusion: particles must be negatively charged

J.J. Thomson (1890s)

• English physicist

• Used CRT to determine charge-to-

mass ratio of the electron

• Mass was much lighter than Hydrogen,

the lightest atom

• Conclusion: Atoms must be divisible!

Plum Pudding Model

• Negative electrons evenly

distributed throughout a

uniform positive charge

• “Chocolate chip cookie

model”

Robert Millikan (1909)

• American physicist

• Determined charge & mass of an

electron:

• Charge = 1-

• Mass = 9.1 x 10-28 g = 𝟏

𝟏𝟖𝟒𝟎 mass of H atom

Ernest Rutherford (1911) • Gold Foil Experiment

• Prediction: particles would

pass straight through (almost)

• Results: a few particles were

deflected at large angles,

some even straight back

towards the source!

Nuclear Atomic Model

• Atom is made of mostly empty

space

• Electrons move through this

• Tiny, dense region in the center

called the nucleus

• Positively charged

• Almost all of the atom’s mass

James Chadwick (1932)

•English physicist

•Nucleus also contains neutrons:

• Neutral (no charge)

• Same mass as a proton

Subatomic Particles

Particle Symbol Location

Relative

Electrical

Charge

Relative

Mass Actual Mass (g)