Proton
| I | INTRODUCTION |
Proton, elementary particle that carries a positive
electric charge and, along with the electron and the neutron, is one of the
building blocks of all atoms. Elementary particles are the smallest parts of
matter that scientists can isolate. The proton is one of the few elementary
particles that is stable—that is, it can exist by itself for a long period of
time. Protons and neutrons are the building blocks of the atomic nucleus, the
center of the atom. Electrons form the outer part of the atom. Protons have a
positive electrical charge of 1.602 x 10-19 coulomb. This charge is
equal but opposite to the negative charge of the electron. Neutrons have no
electrical charge. Protons have a mass of 1.67 x 10-27 kg and, along
with neutrons, they account for most of the mass in atoms. Atoms contain an
equal number of protons and electrons so that every atom has an overall charge
of zero.(See also Atom and Electricity)
The number of protons in the nucleus of an
atom determines what kind of chemical element it is. All
substances in nature are made up of combinations of the 92 different chemical
elements, substances that cannot be broken into simpler substances by chemical
processes. The atom is the smallest part of a chemical element that still
retains the properties of the element. The number of protons in each atom can
range from one in the hydrogen atom to 92 in the uranium atom, the heaviest
naturally occurring element. (In the laboratory, scientists have created
elements with as many as 116 protons in each nucleus.) The atomic number of an
element is equal to the number of protons in each atom’s nucleus. The number of
electrons in an uncharged atom must be equal to the number of protons, and the
arrangement of these electrons determines the chemical properties of the
atom.
| II | STRUCTURE AND CHARACTERISTICS |
The proton is 1,836 times as heavy as the
electron. For an atom of hydrogen, which contains one electron and one proton,
the proton provides 99.95 percent of the mass. The neutron weighs a little more
than the proton. Elements heavier than hydrogen usually contain about the same
number of protons and neutrons in their nuclei, so the atomic mass, or the mass
of one atom, is usually about twice the atomic number.
Protons are affected by all four of the
fundamental forces that govern all interactions between particles and energy in
the universe. The electromagnetic force arises from matter carrying an
electrical charge. It causes positively charged protons to attract negatively
charged electrons and holds them in orbit around the nucleus of the atom. This
force also makes the closely packed protons within the atomic nucleus repel each
other with a force that is 100 million times stronger than the electrical
attraction that binds the electrons. This repulsion is overcome, however, by the
strong nuclear force, which binds the protons and neutrons together into
a compact nucleus. The other two fundamental forces, gravitation and the
weak nuclear force, also affect the proton. Gravitation is a force that
attracts anything with mass (such as the proton) to every other thing in the
universe that has mass. It is weak when the masses are small, but can become
very large when the masses are great. The weak nuclear force is a feeble force
that occurs between certain types of elementary particles, including the proton,
and governs how some elementary particles break up into other particles.
The proton was long thought to be a pointlike,
indivisible particle, like the electron. In the 1950s, however, scientists used
beams of electrons to probe the proton and found that it has a definite shape
and size. These experiments showed that, rather than being an indivisible point,
the proton has an outer diameter of about 10-13 cm, with a cloudlike
shell surrounding a dense center.
Beginning in 1947, physicists discovered more
and more elementary particles in addition to the proton, neutron, and electron.
These particles appeared to be related to protons and neutrons and to each
other. Two different elementary particles had one property, such as an electric
charge, that was identical, while another two particles were related by having
the exact opposite property. These relationships suggested that protons and
other elementary particles might be made up of smaller building blocks, which
scientists called quarks. In 1967 physicists used high-powered electron beams to
probe deep inside the proton and discovered evidence that quarks exist. Three
quarks join together to form a proton. The strong nuclear force is actually a
force that attracts quarks to each other to make a proton or neutron. The quarks
of a neutron or proton will also attract the quarks of another neutron or
proton, thus holding a nucleus together.
Protons originally formed about a thousandth
of a second after the Big Bang, the explosion that scientists believe occurred
at the beginning of the universe (see Big Bang Theory). In that short
time, the temperature of the early universe dropped sufficiently for energetic
quarks to join together. It is possible that protons may break up again, but
this type of event, called proton decay, would be extremely rare. Experiments
have shown that the average lifetime of the proton is at least 1035
years (the number 1035 means a 1 followed by 35 zeros). This may
appear to be an odd answer, since the age of the universe is only about 15 x
109 years. Some protons live for a much shorter time than the average
value, however, and scientists are constructing large experiments with thousands
of tons of material, hoping to see a proton decay.
| III | HISTORY AND CURRENT RESEARCH |
Once the electron was discovered in 1898,
physicists knew that atoms also had to contain positively charged particles that
are much heavier than electrons. These particles would account for some or most
of the mass of the atom and, since atoms have zero electrical charge, they would
balance the charge of the electrons. In 1919 British physicist Ernest Rutherford
reported that he had discovered protons at the Cavendish Laboratory in
Cambridge, England. Rutherford had previously discovered that when the atoms in
some elements broke apart, or radioactively decayed, the part that split off was
a helium nucleus (Radioactivity). This form of radioactivity is called alpha
radiation. Rutherford fired alpha radiation into a container filled with
nitrogen gas. Nitrogen nuclei (each carrying seven positive charges) hit by the
helium nuclei (each carrying two positive charges) changed into oxygen nuclei
(each carrying eight positive charges) and hydrogen nuclei—protons (each
carrying one positive charge). It was the first time that a person had changed
an atom’s nucleus, causing a nuclear reaction.
Scientists later learned to speed up a beam
of protons, using a device called a see particle accelerator. The
high-speed, high-energy protons produced by an accelerator can cause violent
reactions that break up the nucleus of an atom. Looking at the remains,
researchers can discover what elementary particles are inside the nucleus and
how they form the nuclear structure. In 1932 physicists John Cockcroft and
Ernest Walton used a beam of high-energy protons to induce the first nuclear
reaction with artificially accelerated particles. They bombarded lithium atoms
with protons, and the atoms split into two helium nuclei. Proton accelerators
have developed into ring-shaped machines several kilometers in diameter. Because
these huge devices can speed the protons up more and more during each rotation,
they can accelerate protons to extremely high speeds and energies.
In the early 1960s scientists discovered
that the proton has an internal structure. Since then, physicists have used
high-energy beams from particle accelerators to break up the proton to study its
composition. These experiments show that the proton contains quarks, and, to a
lesser extent, antiquarks (particles that are nearly identical to quarks), and
gluons (particles that flit between quarks and hold-or glue—them together,
providing the strong nuclear interaction forces between them). As scientists
probe deeper into the proton, they reveal more of these additional particles,
creating a more complex picture of the proton.
Microsoft ® Encarta ® 2008. © 1993-2007 Microsoft
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