Models of the Atom a Historical Perspective. fire. Democritus. earth. air. water. Aristotle. Early Greek Theories. 400 B.C. - Democritus thought matter could not be divided indefinitely. This led to the idea of atoms in a void.
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AristotleEarly Greek Theories
based on experimentation not on pure reason.
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.
Atoms are solid and indivisible.
He proposed a model of the atom that is sometimes called the “PlumPudding” model.
Atoms were made from a positively chargedsubstance with negatively charged electrons scattered about, like raisins in a pudding.
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.
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.
Thin gold foil
path of invisible -particlesErnest Rutherford
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.
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.
In 1913, the Danish scientist Niels Bohr proposed an improvement. In his model, he placed each electron in a specific energy level.
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).
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, at atom has a small positively charged nucleus surrounded by a large region in which there are enough electrons to make an atom neutral.
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.
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.