| Structure of the Atom |
Structure of the Atom
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Building on Mendeleyev�s work, scientists sought
to explain the periodic law by understanding the structure of the atom. Through
various experiments, scientists discovered that atoms consist of three types of
subatomic particles�electrons,
protons, and
neutrons. Electrons are small, negatively charged particles that orbit a
dense core in the atom called the
nucleus. The nucleus is composed of the larger, positively charged protons
and neutral neutrons. The attractive force between the oppositely charged
electrons and protons holds the orbiting electrons around the nucleus.
Ordinarily, atoms contain an equal number of protons and electrons, creating
electrically neutral atoms.
Electrons
British physicist
Joseph Thomson discovered the electron in 1898 by experimenting with
cathode rays�unexplained rays or beams produced by conducting electricity
through a
vacuum tube. Thomson used magnetic and electric fields to bend the path of
the beam inside the vacuum tube. By adjusting the strength of these fields, he
was able to control the deflection of the beam. From these measurements, Thomson
determined that the cathode ray particles carried a negative charge, and he was
able to calculate the charge-to-mass ratio of the particles. Thomson accurately
hypothesized that these negatively charged particles, which later became known
as electrons, are part of all matter found in nature.
In 1909 American physicist
Robert Millikan determined the charge and mass of individual electrons by
measuring the rate that oil drops laden with electrons fell between two
electrically charged plates (positively charged top plate and negatively charged
bottom plate). By measuring the difference in how fast these electron-laden oil
drops fell when the metal plates were charged and uncharged, Millikan was able
to calculate the total charge on each oil drop. Because each measurement was a
whole number multiple of -1.60 � 10-19
coulombs, Millikan concluded this was the charge carried by a single
electron. Using Thomson�s electron charge-to-mass ratio, Millikan then
calculated the mass of a single electron to be approximately 9.109 � 10-28
grams.
In 1913 Danish physicist
Niels Bohr developed a theoretical model of the hydrogen atom. Bohr proposed
that electrons moving around the nucleus remain in certain quantifiable orbits
called orbitals. These orbitals are similar to the paths of the planets orbiting
around the sun. Bohr�s research further revealed that the electron orbitals
correspond to fixed energy levels, or shells, similar to the layers of an onion.
Each energy level may include several different orbitals
In 1925 Austrian-born physicist
Wolfgang Pauli proposed his
exclusion principle, lending considerable understanding to the complex
behavior of electrons in the atom. Pauli�s exclusion principle states that each
orbital within an energy shell can hold a maximum of two electrons, and that
when two electrons occupy the same orbital, these electrons will have opposite
spins about their own axis. Spin is a property of angular momentum that all
electrons possess.
In 1926 Austrian physicist Erwin Schr�dinger
applied the wave properties of matter to the arrangement of electrons within the
atom. This work, known as
quantum theory, models the configuration and the increasing number of
orbitals contained in each successive shell moving away from the nucleus. In
general, electrons fill the lowest-energy shells first. Once a lower-energy
shell is filled, electrons begin filling the next highest energy level.
Protons
In the early 1900s, Thomson also proved that
positively charged particles are a fundamental part of the atom. Thomson used a
modified cathode-ray tube filled with hydrogen gas. By passing a spark through
the gas, he was able to bump the electrons off of the hydrogen atoms, leaving
particles known as
ions. Thomson accelerated the hydrogen ions through an electric field and
observed that the ions deflected toward the negatively charged
electrode (electric conductor). As a result, Thomson correctly concluded
that hydrogen ions contain positively charged particles; these particles are now
referred to as protons.
Experiments using an instrument known as a
mass spectrometer revealed that protons have a
mass roughly 1800 times greater than that of electrons. The mass
spectrometer also showed that each element is differentiated by the number of
protons it contains (known as the atomic number). The elements are arranged in
the periodic table by increasing atomic number. For example, hydrogen has one
proton, helium has two, and lithium has three.
Nucleus and Neutrons
In the early 1900s, British physicist
Ernest Rutherford discovered both the nucleus of the atom and neutrons. He
conducted experiments that shot positively charged subatomic particles through
metal foil. Rutherford observed that nearly all the subatomic particles passed
straight through the foil, while a few were deflected at large angles. From
these observations, Rutherford concluded that each atom in a metal foil must
have an extremely dense core, or nucleus, deflecting the few particles that come
near it. This core is surrounded by a much greater volume of empty space, which
allows most particles to pass through. From the large angles of deflection,
Rutherford concluded that the nucleus was positively charged and that it
repelled the subatomic particles. By measuring these angles, he was also able to
estimate the number of protons in the nucleus. However, because the mass of the
protons accounted only for half the weight of the nucleus, Rutherford
hypothesized that an equal number of neutrally charged particles must also
compose the nucleus. These particles were later named neutrons.
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