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پروتون - Proton

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.


Contributed By:
Gordon Fraser

Microsoft ® Encarta ® 2008. © 1993-2007 Microsoft Corporation. All rights reserved.

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