Unit: Atomic

Structure

History of the Atom

Important Experiments Leading to Atomic

Theory

Democritus (400 B.C.)•A Greek philosopher

•Was the first person to think about an atom’s existence.•Believed that matter was composed of tiny indivisible particles called atoms.

•He had no experimental evidence to support his thoughts.

Hmmm… atoms…

“atomos”

John Dalton (1766-1844)

•A meteorologist•Unlike Democritus, he had experimental evidence to support his theory.•Dalton had four major points (postulates) to his theory.

Dalton’s Theory1.) All elements are composed of indivisible particles called atoms.

2.) Atoms of the same element are identical. The atoms of any one element are different from those of another.

Dalton’s Theory3.) Atoms of different elements mix or combine in whole number ratios.Example: Oxygen combines with hydrogen to form water in a 2:1 ratio.

4.) Chemical reactions occur when atoms separate, join, or rearrange. In a chemical reaction, atoms of one element NEVER change into another.

Joseph John (J.J.) Thompson (1859-1940)

• In the tube was an inert gas, and two plates, a positive and a negative.

• The particles in the gas were attracted to the positive plate.

• Therefore, the particles MUST have a negative charge. (Opposites attract.)

Cathode Ray Tube Experiment

J.J. Thompson• Discovered the electron.• From his experimental

evidence, he believed that the atom was a solid positive sphere with electrons shoved into the sides of it.

• His model was said to resemble a popular English dessert called Plum Pudding, and so his model was deemed the “Plum Pudding Model.”

Ernest Rutherford click*• Used the gold foil experiment to

discover the nucleus.• Shot high energy beam of alpha

particles into gold foil.

Gold Foil Experiment

Ernest Rutherford Conclusions

Observation ConclusionMost of the alpha

particles went through he concluded…

Few particles were deflected at small

angles he concluded…

Very rarely particles were deflected at large angles he concluded…

The atom is mostly empty

space

The alpha particles hit a small, very dense,

and positively charged center (nucleus)

The alpha particle came close to something small and positive

(nucleus)

Eugene Goldstein (1850-1930)

•Goldstein discovered the proton.

James Chadwick (1891-1974)

•Chadwick discovered the neutron.

Side note… Not all of Dalton’s postulates were correct.

•We now know that atoms are indeed divisible – atoms can be broken down into their subatomic particles, protons, neutrons, and electrons (and these too can be broken down even further!).•We also know that not all atoms of the same element are identical. Isotopes exist for different elements. (We’ll talk about this later.)

Properties of Subatomic Particles

ParticleSymbo

lCharge

Relative Mass

e- -1 1/1840

p+ +1 1

1 0 n

Electron

Proton

Neutron

Location of Particles•Protons – In the nucleus•Neutrons – In the nucleus•Electrons – In the electron

cloud outside the nucleus in energy levels

Neutral Atom

An atom is considered neutral when it has the same number of electrons and protons.(p+ = e-)