Have you watched television today? TV sets are common, yet each one is a complex system. The outer case is made mostly of plastic, and the screen is made of glass. Many of the parts that conduct electricity are metals or combinations of metals. Other parts in the interior of the set contain materials that barely conduct electricity. All of the different materials have one thing in common. They are made up of even simpler materials. In fact, if you had the proper equipment, you could separate the plastics, glass, and metals into these simpler materials.
Each element is different and has unique properties. These differences can be described in part by looking at the relationships between the atomic particles in each element. The periodic table contains numbers that describe these relationships.
Number of Protons and Neutrons Look up the element chlorine on the periodic table found on the inside back cover of your book. Cl is the symbol for chlorine but what are the two numbers? The top number is the element’s atomic number. It tells you the number of protons in thenucleus of each atom of that element. Every atom of chlorine, for example, has 17 protons in its nucleus.
Isotopes Although the number of protons changes from element to element, every atom of the same element has the same number of protons. However, the number of neutrons can vary even for one element. For example, some chlorine atoms have 18 neutrons in their nucleus while others have 20. These two types of chlorine atoms are chlorine-35 and chlorine-37. They are called isotopes (Isuh tohps), which are atoms of the same element that have different numbers of neutrons. You can tell someone exactly which isotope you are referring to by using its mass number. An atom’s mass number is the number of protons plus the number of neutrons it contains. The numbers 35 and 37, which were used to refer to chlorine, are mass numbers. Hydrogen has three isotopes with mass numbers of 1, 2, and 3. They are shown in Figure 1. Each hydrogen atom always has one proton, but in each isotope the number of neutrons is different.
Classification of Elements
Elements fall into three general categories—metals, metalloids (ME tuh loydz), and nonmetals. The elements in each category have similar properties. Metals generally have a shiny or metallic luster and are good conductors of heat and electricity.All metals, except mercury, are solids at room temperature.Metals are malleable (MAL yuh bul), which means they can be bent and pounded into various shapes. The beautiful form of the shell-shaped basin in Figure 2 is a result of this characteristic. Metals are also ductile, which means they can be drawn into wires without breaking. If you look at the periodic table, you can see that most of the elements are metals.
Other Elements Nonmetals are elements that are usually dull in appearance. Most are poor conductors of heat and electricity. Many are gases at room temperature, and bromine is a liquid. The solid nonmetals are generally brittle, meaning they cannot change shape easily without breaking. The nonmetals are essential to the chemicals of life.More than 97 percent of your body is made up of various nonmetals, as shown in Figure 3. You can see that, except for hydrogen, the nonmetals are found on the right side of the periodic table. Metalloidsare elements that have characteristics of metals and nonmetals. On the periodic table, metalloids are found between the metals and nonmetals. All metalloids are solids at room temperature. Some metalloids are shiny and many are conductors, but they are not as good at conducting heat and electricity asmetals are. Somemetalloids, such as silicon, are used tomake the electronic circuits in computers, televisions, and other electronic devices.
Source: Glencoe Science-The Nature of Matter-SE_0078617650