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 air water earth Aristotle 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. Dalton’s “Billiard ball” model (1800-1900) Atoms are solid and indivisible. Thompson “Plum pudding” model (1900) Negative electrons in a positive framework. The Rutherford model (around 1910) Atoms are mostly empty space. Negative electrons orbit a positive nucleus.

Thomson Model He proposed a model of the atom that is sometimes called the “Plum Pudding” model. Atoms were made from a positively charged substance with negatively charged electrons scattered about, like raisins in a pudding. 5

Thomson’s Experiment Thomson studied the passage of an electric current through a gas. As the current passed through the gas, it gave off rays of negatively charged particles. Video Link 6

Thomson concluded that the negative charges came from within the atom. A particle smaller than an atom had to exist. The atom was divisible! Thomson called the negatively charged “corpuscles,” today known as electrons. Since the gas was known to be neutral, having no charge, he reasoned that there must be positively charged particles in the atom. But he could never find them. 7

The Gold Foil Experiment http://chemmovies.unl.edu/ChemAnime/RUTHERFD/RUTHERFD.html http://chemmovies.unl.edu/ChemAnime/RUTHERFD/RUTHERFD.html 8

Ernest Rutherford Rutherford shot alpha () particles at gold foil. Zinc sulfide screen Thin gold foil Lead block 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 atom’s mass.

The Gold Foil Experiment This could only mean that the gold atoms in the sheet were mostly open space. Atoms were not a pudding filled with a positively charged material. Rutherford concluded that an atom had a small, dense, positively charged center that repelled his positively charged “bullets.” He called the center of the atom the “nucleus” The nucleus is tiny compared to the atom as a whole. Video Link for Discovery of Elementary Particles 10

Bohr Model In 1913, the Danish scientist Niels Bohr proposed an improvement. In his model, he placed each electron in a specific energy level. 11

Bohr’s model Electrons orbit the nucleus in “shells” Electrons can be bumped up to a higher shell if hit by an electron or a photon of light. There are 2 types of spectra: continuous spectra & line spectra. It’s 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).

Wave Model 13

The Wave Model In fact, it is impossible to determine the exact location of an electron. The probable location of an electron is based on how much energy the electron has. According to the modern atomic model, an atom has a small positively charged nucleus surrounded by a large region in which there are enough electrons to make an atom neutral. 14

Electron Cloud: A space in which electrons are likely to be found. Electrons whirl about the nucleus billions of times in one second. However, They are NOT moving around in random patterns. Location of electrons depends upon how much energy the electron has. 15

Electron Cloud: Depending on their energy they are locked into a certain area in the cloud. Electrons with the lowest energy are found in the energy level closest to the nucleus Electrons with the highest energy are found in the outermost energy levels, farther from the nucleus. 16

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