Atomic Models

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Models of the Atom: a Historical Perspective John Dalton Early Greek Theories • 400 B.C. - Democritus thought matter could not be divided indefinitely. • This led to the idea of atoms in a void. fire Democritus

• 1800 -Dalton proposed a modern atomic model based on experimentation not on pure reason. All matter is made of atoms. Atoms of an element are identical. Each element has different atoms. Atoms of different elements combine in constant ratios to form compounds. • Atoms are rearranged in reactions. • His ideas account for the law of conservation of mass (atoms are neither created nor destroyed) and the law of constant composition (elements combine in fixed ratios). • • • •




water • 350 B.C - Aristotle modified an earlier theory that matter was made of four “elements”: earth, fire, water, air. • Aristotle was wrong. However, his theory persisted for 2000 years.

Adding Electrons to the Model
Materials, when rubbed, can develop a charge difference. This electricity is called “cathode rays” when passed through an evacuated tube (demos). These rays have a small mass and are negative. Thompson noted that these negative subatomic particles were a fundamental part of all atoms. 1) Dalton’s “Billiard ball” model (1800-1900) Atoms are solid and indivisible. 2) Thompson “Plum pudding” model (1900) Negative electrons in a positive framework. 3) The Rutherford model (around 1910) Atoms are mostly empty space. Negative electrons orbit a positive nucleus.

Ernest Rutherford (movie: 10 min.)
• Rutherford shot alpha (α) particles at gold foil.
Zinc sulfide screen Lead block Thin gold foil

Radioactive substance path of invisible α-particles

Most particles passed through. So, atoms are mostly empty. Some positive α-particles deflected or bounced back! Thus, a “nucleus” is positive & holds most of an atom’s mass.

• Electrons orbit the nucleus in “shells” • Electrons can be bumped up to a higher shell if hit by an electron or...
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