Periodic Table

The first step in being able to use the information contained in the periodic table is to understand how it is arranged. Most periodic tables are similar to one another but to lessen confusion the periodic table shown in Figure 1 will be used. One of the first things that stands out is that the table is composed of metals, nonmetals, and metalloids.

The metallic elements are familiar to us all through our everyday lives. From experience we know that metals are shiny, conduct heat and electricity very well (think about electrical wires and pots and pans), can be formed into many different shapes (in other words, they are malleable), and can be drawn into wires (are ductile). The only metal that is not a solid at room temperature is mercury, which exists as a liquid and is often used in thermometers. The nonmetal elements familiar to us include the atmospheric gases nitrogen and oxygen (O). Other important nonmetals, especially for the maintenance of life, are carbon (C), hydrogen (H), sulfur (S), and phosphorus (P). Most nonmetals are either gases or solids at room temperature and have properties opposite those of the metals.

The placement of metals and nonmetals in the table, it should be noticed, is not random. The nonmetals all Figure 1. The periodic table. Elements 113, 115, and 117 have not yet been discovered. The table above shows only their expected positions. The addition of elements 114, 116, and 118 have only been reported near the beginning of the twenty-first century. Illustration by EIG. Courtesy of Gale Group. occur on the right hand side of the table, while the metals occur on the left hand side. Moving across the table from metals to nonmetals we encounter the metalloids (or semimetals), which include boron (B), silicon (Si), germanium, arsenic (As), antimony (Sb), tellurium, and astatine (At). The properties of metalloids fall in between those of metals and nonmetals. A familiar metalloid is silicon, the major material of which computer memory chips are made.

The periods, or horizontal rows, of the table are numbered on the left hand side from 1 to 7. The first period contains two elements, hydrogen and helium (He), the second period and third period each contain eight elements, while the fourth and fifth periods each contain 18 elements.

The numbering of groups (the vertical columns, also known as families) follows two different conventions, both of which should be familiar. In the system commonly used in North America, Roman numerals and letters are used to denote the various groups. The alternate system, devised by the International Union of Pure and Applied Chemistry Convention (IUPAC—the same group responsible for certifying atomic masses and element names) in 1985, numbers the Groups from 1 through 18. The IUPAC system is the system to which most countries are turning and this is the system used in this text. The alternative system numbering will be shown in parenthesis when applicable.

In both systems the various groups in the periodic table are placed into families which consist of groups of related elements. These families are given the same name in each system. Groups 1 and 2 (IA, IIA) are called the main group metals. Group 1 individually, is referred to as the alkali metals while Group 2 is called the alkaline earth metals. The Group 1 and 2 metals are both very reactive and readily form positive charged atoms (called cations) by losing electrons. Group 1 metals lose one electron to become +1 cations and Group 2 metals can lose two electrons to become +2 cations.

Groups 3 through 12 (refer to table to see alternate numberings) are referred to as the transition metals. The transition metal family, unlike the main Group 1 and 2 metals, form cations of differing charge (from +1 to +3). Many transition metal compounds are colored. Groups 13 through 18 (lllA-VlllA) are called the main group nonmetals. The inner transition metal family is comprised of two series called the lanthanides and actinides, neither of which are numbered in either system.

As briefly explained before, each box, along with the symbol for the element, represents an individual element. Each element is characterized by a unique atomic number (the number that appears above the elemental symbol), which denotes the number of protons in the nucleus of that atom. One can see that the number of protons determines the element. If an atom has six protons in its nucleus it is a carbon atom, while 34 protons determine a selenium (Se) atom. Protons each carry a charge of +1, while electrons carry a charge of -1. Therefore, neutral elements must have equal numbers of protons and electrons.

Also contained in each box is a number written below each elemental symbol. This number is the atomic mass number, the average atomic mass for a given element. It is an average because not all atoms of a given element have the same mass. While all atoms of the same element must have the same number of protons, as mentioned above, they can differ in the number of neutrons they contain in the nucleus. Neutrons, as the name implies, are neutral particles found in the nucleus. These particles help to stabilize the atom by separating the positively charged protons in the nucleus. Some elements have only one possible combination of protons and neutrons, such as sodium. All sodium (Na) atoms consist of 11 protons and 12 neutrons. Atoms containing the same number of protons but different numbers of neutrons are referred to as isotopes. For example, there are two isotopes of carbon found in nature. Carbon-12 has six protons and six neutrons in its nucleus and a natural abundance of 98.889%, while carbon-13 has six protons and seven neutrons with a natural abundance of 1.111%. By averaging the atomic mass of each isotope of carbon, the average atomic mass of 12.01 atomic mass units, or amu (1 amu = 1.66X10^-27 kilogram), is calculated and appears under carbon in the periodic table. It is important to note that isotopes of the same element are in most instances chemically indistinguishable.