Oct 3, 2017

Introduction to the Periodic Table



Scientists spend a lot of time organizing information into useful patterns. Before they can organize information, however, they must possess it, and it must be correct. Botanists had enough information about plants to organize their field in the eighteenth century. Because of uncertainties in atomic masses and because many elements remained undiscovered, chemists were not able to organize the elements until a century later.

We can distinguish one element from all others by its particular set of observable physical properties. For example, sodium has low density of 0.971 g/cm3 and a low melting of 97.81 degree Celsius. No other element has this same combination of density and melting point. Potassium, though, also has a low density (0.862 g/cm3) and low melting point (63.65 degree Celsius), much like sodium. Sodium and potassium further resemble each other in that both are good conductors of heat and electricity, and both react vigorously with water to liberate hydrogen gas. Gold, conversely, has a density (19.32 g/cm3) and melting point (1064 degree Celsius) that are very much higher than those of sodium or potassium, and gold does not react with water or even ordinary acids. It does resemble sodium potassium and in its ability to conduct heat and electricity, however. Chlorine is very different still from sodium, potassium, and gold. It is a gas under ordinary conditions, which means that melting point of solid chlorine (-101 degree Celsius) is far below room temperature. Also chlorine is a non-conductor of heat and electricity.

Even from these very limited data, we get an inkling of a useful classification scheme of the elements. If the scheme is to group together elements with similar properties, then sodium and potassium should appear in the same group. And if the classification scheme is in some way to distinguish between elements that are good conductors of heat and electricity and those that are not, chlorine should be set apart from sodium, potassium, and gold.

The classification system we need is the one shown in Figure below (and inside the front cover), known as the periodic table of the elements. In upcoming blog posts, I will describe how the periodic table was formulated, and you will also learn its theoretical basis. For the present, we will consider only a few features of the table.



Features of the Periodic Table
In the periodic table, elements are listed according to increasing atomic number starting at the upper left and arranged in a series of horizontal rows. This arrangement places similar elements in vertical groups, or families. For example, sodium and potassium are found together in a group labeled 1 (called the alkali metals). We should expect other members of the group, such as cesium and rubidium, to have properties similar to sodium and potassium. Chlorine is found at the other end of the table in a group labeled 17.

Some of the groups are given distinctive names, mostly related to an important property of the elements in the group. For example, the group 17 elements are called the halogens, a term derived from Greek, meaning "salt former".



Each element is listed in the periodic table by placing its symbol in the middle of a box in the table. The atomic number (Z) of the element is shown above the symbol, and the weighted-average atomic mass of the element is shown below its symbol. Some periodic tables provide other information, such as density and melting point, but the atomic number and atomic mass are generally sufficient for our needs. Elements with atomic masses in parentheses, such as plutonium, Pu (244), are produced synthetically, and the number shown is the mass number of the most stable isotope.

It is customary also to divide the elements into two broad categories metals and nonmetals. In Figure above, colored backgrounds are used to distinguish the metals (tan) from the nonmetals (blue and pink). Except for mercury, a liquid, metals are solids at room temperature. They are generally malleable (capable of being flattened into thin sheets), ductile (capable of being drawn into fine wires), and good conductors of heat and electricity, and have a lustrous or shiny appearance.
The properties of nonmetals are generally opposite those of metals; for example, nonmetals are poor conductors of heat and electricity. Several of the nonmetals, such as nitrogen, oxygen, and chlorine, are gases at room temperature. Some, such as silicon and sulfur, are brittle solids. One bromine is a liquid.

Two other highlighted categories in Figure are a special group of nonmetals known as the noble gases (pink), and a small group of elements, often called metalloids (green), that have some metallic and some nonmetallic properties.

The horizontal rows of the table are called periods. (The periods are numbered at the extreme left in the periodic table inside the front cover.) The first period of the table consists of just two elements, hydrogen and helium. This is followed by two periods of eight elements each, lithium through neon and sodium through argon. The fourth and fifth periods contain 18 elements each, ranging from potassium through krypton and from rubidium through xenon.

The sixth period is a long one of 32 members. To fit this period in a table that is held to a maximum width of 18 members, 15 members of the period are placed at the bottom of the periodic table. This series of 15 elements start with lanthanum and these elements are called the lanthanides. The seventh and final period is incomplete (some members are yet to be discovered), but it is known to be a long one. A 15-member series is also extracted from the seventh period and placed at the bottom of the table. Because the elements in this series start with actinium they are called the actinides.

The labeling of the groups of the periodic table has been a matter of some debate among chemists. The 1-18 numbering system used in Figure 2-15 is the one most recently adopted. Group labels previously used in the United States consisted of a letter and a number, closely following the method adopted by

Mendeleev, the developer of the periodic table. As seen in Figure 2-15, the A groups 1 and 2 are separated from the remaining A groups (3 to 8) by B groups 1 through 8. The International Union of Pure and Applied Chemistry (IUPAC) recommended the simple 1 to 18 numbering scheme in order to avoid confusion between the American number and letter system and that used in Europe, where some of the A and B designations were switched! Currently, the IUPAC system is officially recommended by the American Chemical Society (ACS) and chemical societies in other nations. Because both numbering systems are in use, we show both in Figure 2-15 and in the periodic table inside the front cover. However, except for an occasional reminder of the earlier system, we will use the IUPAC numbering system in this text.

Useful Relationships from the Periodic Table
The periodic table helps chemists describe and predict the properties of chemical compounds and the outcomes of chemical reactions. Throughout this text, we will use it as an aid to understanding chemical concepts. One application of the table worth mentioning here is how it can be used to predict likely charges on simple monatomic ions.

Main-group elements are those in groups 1, 2, and 13 to 18. When main-group metal atoms in groups 1 and 2 form ions, they lose the same number of electrons as the IUPAC group number. Thus, Na atoms (group 1) lose one electron to become and Ca atoms (group 2) lose two electrons to become Ca 2+. Aluminum in group 13 loses three electrons from Al 3+ (here the charge is “group number minus 10”). The few other metals in groups 13 and higher from more than one possible ion, a matter that we deal with in next blog posts.

When nonmetal atoms form ions, they gain electrons. The number of electrons gained is normally 18 minus the IUPAC group number. Thus, an O atom gains 18 – 16 = 2 electrons to become O2-, and a Cl atoms gains 18 – 17 = 1 electrons to become Cl-. The “18 minus group number” rule suggests that an atom of Ne in group 18 gains no electrons: 18 – 18 = 0. The very limited tendency of the noble gas atoms to form ions is one of several characteristics of this family of elements.

The elements in groups 3 to 12 are the transition elements, and because all of them are metals, they are also called the transition metals. Like the main group metals, the transition metals form positive ions, but the number of electrons lost is not related in any simple way to the group number, mostly because transition metals can form two or more ions of differing charge.

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