An electron is one of the three subatomic particles that make up an atom. The other two subatomic particles are protons and neutrons, which are found in the nucleus (center) of the atom. Electrons are found in the area outside the nucleus of an atom. They have a negative electrical charge (as opposed to protons, which are positively charged, and neutrons, which have no charge) and are quite small. An electron is only about 1/2,000 the size of a proton or a neutron. Protons and neutrons have masses of approximately 1 atomic mass unit (amu) each, whereas electrons only have a mass of .0006 amu (9.11 x 10- 28 g). The atomic number of an element is equal to the number of protons in one atom of the element. In a neutral atom, the number of electrons is equal to the number of protons, therefore, the atomic number also indicates the number of electrons in an atom. An atom can gain or lose electrons at which point it gains an electric charge and is called an ion.
Exactly where the electrons in an atom are located has been the topic of much research and debate. Until the early 1800s the idea that elements were made up of smaller particles called atoms was unknown. English chemist John Dalton performed various experiments that led him to develop his atomic theory. This theory stated that all elements are made up of atoms. According to Dalton, atoms were the smallest particles possible and could not be divided into smaller particles. Atoms of the same element were exactly alike, and atoms of different atoms were different. Atoms of different elements combined to form compounds. Protons, neutrons, and electrons were still undiscovered.
In 1897, English physicist Joseph John Thomson was experimenting with passing an electric current through a gas. The gas was made of uncharged atoms, but when an electric current passed through it, negatively charged particles in the form of rays were given off. Thomson was faced with the problem of explaining where these negatively charged particles originated. He reasoned that the only place these particles could have come from was the individual atoms in the gas. Therefore, the atom must be made up of even smaller particles. He used the term corpuscles to describe the negatively charged particles that we now call electrons.
Now that Thomson had identified electrons, the problem he faced was determining the placement of these particles inside the atom. Thomson developed a model of electrons being scattered throughout a positively charged material, much like plums would be scattered throughout plum pudding (this model of the atom is often referred to as the "plum pudding" model). In 1911, British physicist Ernest Rutherford proposed that there are positively charged particles in an atom called protons. The protons are located in the nucleus of an atom, and the electrons are scattered outside of the nucleus around the edge of the atom. The negatively charged electrons are held in place by the force that results from the attraction toward the positively charged protons. This force is called the electromagnetic force. This model explained further the idea of subatomic particles, but still could not describe the exact location of the electrons in an atom.
A few years later, in 1913, Danish physicist Niels Bohr improved upon the Rutherford model. He developed the idea of energy levels in an electron. According to this new atomic model, the electrons were placed in definite orbits--called energy levels--around the nucle us. Each energy level is a certain distance away from the nucleus. This model likens electrons orbiting the nucleus to planets orbiting the Sun. Today, scientists realize that this is not exactly correct. Electrons do not move in a definite orbit, always a certain dis tance from the nucleus. In fact, the exact location of the electrons in an atom cannot be determined. Only the approximate location where an electron is likely to be found can be predicted.
The electrons in an atom take up a particular amount of space as they move around the nucleus at a rate of billions of time per second. This space is referred to as the electron cloud. The electron cloud is not a definite shape or size, rather it is simply the space where electrons are li kely to be located. Each electron appears to stay in a certain area within the cloud. The area of the cloud in which any particular electron is likely to be found depends upon how much energy the electron has. This area is an energy level within the electron cloud. Electrons with low energies are in the lowest energy level, closest to the nucleus. Electrons with higher energies are in higher energy levels, increasing in distance from the nucleus as energy increases. Each energy level can hold a certain number of electrons. The lowest energy level can hold only two electrons; the second level, eight; and the third level, 18.
How the energy levels are filled in an atom determines the properties of the different elements. This electron arrangement is important in predicting the chemical properties of an element. One of the most important properties determined by electron arrangement is the bonding characteristics of an element. Some elements bond readily to other elements and others hardly bond at all. This is determined by the number of electrons--called valence electrons--present in the outermost energy shell. An atom is most stable when its valence electrons satisfy what is called the octet rule. This means that an atom with eight electrons in its outermost energy level is very stable. An atom will bond with another in order to satisfy the octet rule. When two atoms combine, they can gain or lose electrons. When one atom transfers electrons to another, it is called ionic bonding. When electrons are shared between two atoms, it is called covalent bonding.
The number of valence electrons of an atom can also be used to group elements together into families on the periodic table. Elements within a family share many properties. Metals have one, two, three, or four valence electrons. These electrons are in the outer energy level quite weakly, so they are lost easily. This is the reason metals react readily with water or other atmospheric elements in a reaction known as corrosion. Elements that are nonmetals have five, six, seven, or eight valence electrons. Nonmetals tend to gain electrons during chemical reactions. Nonmetals with eight valence electrons are chemically unreactive.
The first family on the periodic table is the alkali metals. Atoms of these elements have only one valence electron, so they tend to bond easily. In fact, they are so reactive that they are rarely found in nature not combined with other elements. Family 2 on the periodic table is the alkaline earth metals with two valence electrons. They lose these electrons easily. Since they have two electrons that they must lose, they are not quite as reactive as the alkali metals. The transition elements have one or two valence electrons, which they lose when they react. They can also lose an electron from the energy level that is next to the highest. Atoms of these elements can also share electrons when they react with other elements. Family 13, the boron family, has three valence electrons. Family 14 is known as carbon family, and atoms of these elements have four valence electrons. Family 15, the nitrogen family, has five valence electrons. Atoms of these elements tend to gain or acquire electrons when they react with other atoms. Family 16 is called the oxygen family. Atoms of these elements have six valence electrons and tend to gain or acquire electrons in chemical reactions. Members of the halogen family, family 17, have seven valence electrons. A halogen atom needs only to gain one electron to fill its outer electron shell. This property makes elements in family 17 the most active nonmetals. These elements are also rarely found uncombined in nature. Halogens tend to react with the alkali metals rather easily. The last family, family 18, contains the noble gases. These are the elements with eight valence electrons, or complete energy levels. These elements are unreactive.
Electrons not only determine an element's reactivity, they also are the cause of various phenomena, for example, beta radiation. Some electrons can be formed inside of the nucleus of an atom when a neutron breaks apart. When this occurs, the electron shoots out of the atom and is called a beta particle, a type of radioactivity. Another example of an electron-caused phenomenon is static electricity. Sometimes the electrons in the outermost energy levels are easily lost from an atom. When an atom loses an electron, it becomes positively charged. When it gains an electron, it becomes negatively charged. An object can also become charged if the atoms of which it is composed gain or lose electrons. An example of an entire object becoming charged is when a balloon is rubbed with a piece of cloth. The cloth loses electrons that are transferred to the balloon, and the balloon acquires a negative charge. When the balloon is held up to a wall, the negative charge causes the electrons in the wall to move away from the area. That area of the wall then becomes positively charged. The negative balloon is attracted to the positive wall, and will be held on the wall by the electromagnetic force.
This example illustrates the phenomenon of induction—an electrical charge built up due to the rearrangement of atoms. The wall became charged because of induction. When electrons flow from one object to another it is called conduction. Certain materials--called conductors--allow for this electron flow better than others. Metals are good conductors because the electrons in these atoms are loosely held and free to move. Materials that do not allow for electrons to flow are called insulators. Insulators do not conduct electricity well because the electrons are tightly held and are not free to move.
Another well-known phenomenon that occurs because of electrons is lightning. Clouds can build up a negative charge, and if they pass near the surface of Earth, Earth can become positively charged because of induction. Electrons then are attracted to the positively charged Earth and jump from the cloud. As they move through the air, they produce a great quantity of light and heat. The light is the lightning bolt we see. The heat causes the air around it to expand quickly, which is the thunder we hear.
Many electronic devices and equipment are made possible because of electrons. Televisions, computer monitors, and video games all use a device called a cathode-ray tube. The cathode-ray tube is a vacuum tube that emits a beam of electrons onto a screen. As the electrons hit the screen, which is coated with a fluorescent material, they cause the screen to glow, producing an image. Photocells in cameras are based on a phenomenon called the photoelectric effect. When light shines on a metal, the loosely held electrons are shot off of the surface of the metal. For any particular metal, a particular frequency of light must be used for the photoelectric effect to take place.
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