Modern Atomic Theory:
Models
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In
1913, Neils Bohr, a student of Rutherford's, developed a new model of the
atom. He proposed that electrons are arranged in concentric circular orbits
around the nucleus. This model is patterned on the solar system and is known as
the planetary model. The Bohr model can be summarized by the following four
principles:
- Electrons occupy only certain
orbits around the nucleus. Those orbits are stable and are called
"stationary" orbits.
2.
Each orbit has an energy
associated with it. The orbit nearest the nucleus has an energy of E1, the next
orbit E2, etc.
3.
Energy is absorbed when
an electron jumps from a lower orbit to a higher one and energy is emitted when
an electron falls from a higher orbit to a lower orbit.
4.
The energy and frequency
of light emitted or absorbed can be calculated by using the difference between
the two orbital energies.
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In
1926 Erwin Schrödinger, an Austrian physicist, took the Bohr atom model one
step further. Schrödinger used mathematical equations to describe the
likelihood of finding an electron in a certain position. This atomic model is
known as the quantum mechanical model of the atom. Unlike the Bohr model, the
quantum mechanical model does not define the exact path of an electron, but
rather, predicts the odds of the location of the electron. This model can be
portrayed as a nucleus surrounded by an electron cloud. Where the cloud is most
dense, the probability of finding the electron is greatest, and conversely, the
electron is less likely to be in a less dense area of the cloud. Thus, this
model introduced the concept of sub-energy levels.
Until
1932, the atom was believed to be composed of a positively charged nucleus
surrounded by negatively charged electrons. In 1932, James Chadwick bombarded
beryllium atoms with alpha particles. An unknown radiation was produced.
Chadwick interpreted this radiation as being composed of particles with a
neutral electrical charge and the approximate mass of a proton. This particle
became known as the neutron. With the discovery of the neutron, an adequate model of the atom
became available to chemists.
Since
1932, through continued experimentation, many additional particles have been
discovered in the atom. Also, new elements have been created by bombarding
existing nuclei with various subatomic particles. The atomic theory has been
further enhanced by the concept that protons and neutrons are made of even
smaller units called quarks. The quarks themselves are in turn made of vibrating strings of
energy. The theory of the composition of the atom continues to be an ongoing
and exciting adventure.