ABOUT SCIENCE

Elements and the Periodic Table - The terms element and atom are often used in a similar context. You might hear, for example, that gold is an element made of gold atoms. Generally, element is used in reference to an entire macroscopic or microscopic sample, and atom is used when speaking of the submicroscopic particles in the sample. The important distinction is that elements are made of atoms and not the other way around. How many atoms are bound together in an element is shown by an elemental formula. For elements in which the basic units are individual atoms, the elemental formula is simply the chemical symbol: Au is the elemental formula for gold, and Li is the elemental formula for lithium, to name just two examples. For elements in which the basic units are two or more atoms bonded into molecules, the elemental formula is the chemical symbol followed by a sub script indicating the number of atoms in each molecule. For example, elemental nitrogen, commonly consists of molecules containing two nitrogen atoms per molecule. Thus N2 is the usual elemental formula given for nitrogen. Similarly, O2 is the elemental formula for oxygen (two oxygen atoms per molecule), and S8 is the elemental formula for sulfur (eight atoms per molecule). 

The periodic table is a listing of all the known elements with their atomic masses, atomic numbers, and chemical symbols. Recall from our introduction to the atom in Chapter 9that the total mass of an atom is called its atomicmass. This is the sum of the masses of all the atom’s components (electrons, protons, and neutrons). A special unit is used for atomic masses. This is the atomic mass unit, amu, where 1 atomic mass unit is equal to 1.661 x 10-24 g , which is slightly less than the mass of a single proton. Also recall from Chapter 9that the atomic number of an element is the number of protons in an atom of that element. It is also equal to the number of electrons in the neutral atom. Besides atomic numbers, atomic masses, and chemical symbols, there is much more information about the elements in the periodic table (Figure 1). The way the table is organized in groups tells you a lot about the elements’ structures and how they behave. Look carefully at Figure 2. It shows that metals make up most elements.

Metals are defined as those elements that are shiny, opaque, and good conductors of electricity and heat. Metals are malleable, which means they can be hammered into different shapes or bent without breaking. They are also  ductile, which means they can be drawn into wires. All but a few metals are solid atroom temperature. The exceptions include mercury, Hg; gallium, Ga; cesium, Cs; and francium, Fr. These metals are all liquids at a warm room temperature of 30°C (86°F). Another interesting exception is hydrogen, H. Hydrogen acquires the properties of a liquid metal only at very high pressures (Figure 3).

Under normal conditions, hydrogen behaves as a nonmetallic gas. The nonmetallic elements, with the exception of hydrogen, are on the right of the periodic table. Nonmetals are very poor conductors of electricity and heat, and they may also be transparent. Solid nonmetals are neither malleable nor ductile. Rather, they are brittle and shatter when hammered. At 30°C (86°F), some nonmetals are solid (such as carbon, C). Other nonmetallic elements are liquid (such as bromine, Br). Still other nonmetals are gaseous (like helium, He). 

Six elements are classified as metalloids: boron, B; silicon, Si; germanium, Ge; arsenic, As; tin, Sn; and antimony, Sb. You’ll see them between the metals and the nonmetals in the periodic table. The metalloids have both metallic and nonmetallic characteristics. For example, they are weak conductors of electricity. This makes them useful as semiconductors in the integrated circuits of computers. Note in the periodic table how germanium, Ge (number 32), is closer to the metals than to the nonmetals. Because of this positioning, we can tell that germanium has more metallic properties than silicon, Si (number 14), and is a slightly better conductor of electricity. So we find that integrated circuits fabricated with germanium operate faster than those fabricated with silicon. Because silicon is much more abundant and less expensive to obtain, however, silicon computer chips remain the industry standard.