Outside the Nucleus Help
Surrounding the nucleus of an atom are particles having electric charge opposite from the charge of the protons. These are electrons . Physicists arbitrarily call the electron charge negative and the proton charge positive.
An electron has exactly the same charge quantity as a proton but with opposite polarity. Electrons are far less massive than protons, however. It would take about 2,000 electrons to have the same mass as a single proton.
One of the earliest theories concerning the structure of the atom pictured the electrons embedded in the nucleus like raisins in a cake. Later, the electrons were imagined as orbiting the nucleus, making every atom like a miniature star system with the electrons as the planets (Fig. 9-1). Still later, this view was modified further. In today’s model of the atom, the electrons are fast-moving, and they describe patterns so complex that it is impossible to pinpoint any individual particle at any given instant of time. All that can be done is to say that an electron just as likely will be inside a certain sphere as outside. These spheres are known as electron shells . The centers of the shells correspond to the position of the atomic nucleus. The greater a shell’s radius, the more energy the electron has. Figure 9-2 is a greatly simplified drawing of what happens when an electron gains just enough energy to “jump” from one shell to another shell representing more energy.
Fig. 9-1 . An early model of the atom, developed around the year 1900.
Fig. 9-2 . Electrons exist at defined levels, each level corresponding to a specific, fixed energy state.
Electrons can move rather easily from one atom to another in some materials. These substances are electrical conductors . In other substances, it is difficult to get electrons to move. These are called electrical insulators . In any case, however, it is far easier to move electrons than it is to move protons. Electricity almost always results, in some way, from the motions of electrons in a material.
Generally, the number of electrons in an atom is the same as the number of protons. The negative charges therefore exactly cancel out the positive ones, and the atom is electrically neutral. Under some conditions, however, there can be an excess or shortage of electrons. High levels of radiant energy, extreme heat, or the presence of an electrical field (to be discussed later) can “knock” electrons loose from atoms, upsetting the balance.
If an atom has more or less electrons than protons, that atom acquires an electric charge. A shortage of electrons results in positive charge; an excess of electrons gives a negative charge. An element’s identity remains the same, no matter how great the excess or shortage of electrons. In the extreme case, all the electrons may be removed from an atom, leaving only the nucleus. This will still represent the same element, however, as it would if it had all its electrons. A electrically charged atom is called an ion . When a substance contains many ions, the material is said to be ionized . If an atom has more electrons than protons, it is a negative ion . If it has fewer electrons than protons, it is a positive ion . If the number of electrons and protons is the same, then the atom is electrically neutral.
Ionization can take place when substances are heated to high temperatures or when they are placed in intense electrical fields. Ionization also can occur in a substance as a result of exposure to ultraviolet light, x-rays, gamma rays, or high-speed subatomic particles such as neutrons, protons, helium nuclei, or electrons. So-called ionizing radiation , more often called radioactivity , ionizes the atoms in living tissue and can cause illness, death, and genetic mutations.
Lightning is the result of ionization of the air. An electric spark is caused by a large buildup of charges, resulting in forces on the electrons in the intervening medium. These forces pull the electrons away from individual atoms. Ionized atoms generally conduct electric currents with greater ease than electrically neutral atoms. The ionization, caused by a powerful electrical field, occurs along a jagged, narrow channel , as you have surely seen. After the lightning flash, the nuclei of the atoms quickly attract stray electrons back, and the air becomes electrically neutral again.
An element may be both an ion and an isotope different from the usual isotope. For example, an atom of carbon may have eight neutrons rather than the usual six, thus being the isotope 14 C, and it may have been stripped of an electron, giving it a positive unit electric charge and making it an ion.
The atmosphere of our planet becomes less dense with increasing altitude. Because of this, the amount of ultraviolet and x-ray energy received from the Sun gets greater as we go higher. At certain altitudes, the gases in the atmosphere become ionized by solar radiation. These regions comprise the ionosphere of the Earth. The ionosphere has a significant effect on the propagation of radio waves at certain frequencies. The ionized layers absorb or refract the waves. This makes long-distance communication possible on the so-called shortwave radio bands.
Outside the Nucleus Practice Problems
Suppose that the nucleus of an oxygen atom is split exactly in two. Neglect any energy that might be involved in the reaction. Suppose that the original oxygen atom is electrically neutral and that no electrons are gained or lost during the reaction. Is it possible for both the resulting atoms to be electrically neutral?
Yes. The original oxygen atom must have eight electrons in order to be electrically neutral. If these eight electrons are equally divided between the two beryllium atoms, each of which has four protons in its nucleus, then both beryllium atoms will have four electrons, and both will be electrically neutral.
Consider the preceding scenario in which the oxygen atom has been stripped of two of its electrons so that it is a positive ion. Can the resulting two beryllium atoms be electrically neutral?
In this case, no. There must be eight electrons, in total, for both the beryllium atoms to end up neutral. It is possible for one of the beryllium atoms to be neutral, but at least one of them must be an ion.
Practice problems of these concepts can be found at: Particles of Matter Practice Test
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