PowerPoint - Models of the Atom - A Historical Perspective
Models of the Atom a Historical Perspective Early Greek Theories Democritus 400 B.C. - Democritus thought matter could not be divided indefinitely. This led to the idea of atoms in a void. fire earth
Aristotle air 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. John Dalton
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).
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. These rays have a small mass and are negative. Thompson noted that these negative subatomic particles were a fundamental part of all atoms. 1) Daltons 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. Sir J.J. Thomson (Late 1800s) http://www.aip.org/history/mod/fission/fission1/01.html Sir J.J. Thomson (Late 1800s)
Evidence: When cathode rays traveled through an electric field they were repelled from the negative side of the field and attracted to the positive side. From measuring the deflection of the cathode rays in combinations of electric and magnetic fields, Thomson was able to determine that cathode rays had charge and mass. The same results were obtained no matter what the cathode was made from. Electrons Claim:
Cathode rays were composed of a stream of negatively charged particles. These particles were fundamental to all atoms. Electrons Reasoning: The particles were negative because they were repelled by negative charges and attracted towards positive charges. The particles were fundamental to all atoms because the same results were found no matter what material was used to produce the electrons.
Electrons These particles were given the name electrons. Later Robert Millikan, in the USA, determined the charge on an electron. His results, when combined with Thomsons, allowed scientists to calculate the mass of an electron. Evidence: Thomson had determined that atoms contained negative particles called electrons. However, atoms were electrically neutral. Thomsons Plum Pudding Model
Claim: (Early 20th century) Atoms contain an equal amount of positive charge as negative charge Thomson suggested that the electrons in an atom were embedded in a positively charged, diffuse sphere.
Thomsons Plum Pudding Model (Early 20th century) Reasoning: Thomson had no experimental evidence for his model but built on the idea that positive charge in an atom was spread evenly and diffusely throughout a spherical shape. Ernest Rutherford Rutherford shot alpha () particles at gold foil.
Zinc sulfide screen Lead block Thin gold foil Radioactive substance path of invisible -particles Most particles passed through. So, atoms are mostly empty. Some positive -particles deflected or bounced back!
Thus, a nucleus is positive & holds most of an atoms mass. Bohrs model Electrons orbit the nucleus in shells Electrons can be bumped up to a higher shell i hit by an electron or a photon of light. There are 2 types of spectra: continuous spectra & line spectra. Its when electrons fall back down that they release a photon. These jumps down from
shell to shell account for the line spectra seen in gas discharge tubes (through spectroscopes).
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