The periodic table is an arrangment of the chemical elements ordered by atomic number so that chemical periodic properties of the elements (chemical periodicity) are made clear.
There is no one single or best structure for the periodic table but by whatever consensus there is, the standard form used here is very useful and the most common. The periodic table is a masterpiece of organised chemical information and the evolution of chemistry's periodic table into the current form is an astonishing achievement.
The standard form of the periodic table shown here includes periods (shown horizontally) and groups (shown vertically). The chemical properties of elements in the vertical groups are similar in some respects to each other.
The elements can also be classified into the main-group elements (or representative elements) in the columns labeled 1, 2, and 13–18; the transition metals in the columns labeled 3–12; and inner transition metals in the two rows at the bottom of the table (the top-row elements are called lanthanides and the bottom-row elements are actinides.
Image of the standard periodic table of the elementsImage of the standard periodic table of the elements with He identified by electronic configurationImage of the standard periodic table of the elements with H in Group 17Image of the standard periodic table of the elements coloured by IUPAC namesImages of various periodic tables
There are many variants of the periodic table, some useful, others less so. A few are linked shown below. Click on the images below to see images of the periodic table in a variety of styles. Many other periodic table formats are catalogued at Mark Leach's Meta-synthesis web site. A classic book showing many forms of the periodic table in print is: Edward G. Mazurs, Periodic representations of the periodic system during one hundred years, University of Alabama Press USA, 2nd edition, 1974. ISBN: 0-8173-3200-6.
Explore the chemical elements through this (standard) periodic table
The standard form of the periodic table shown here includes periods (shown horizontally) and groups (shown vertically). The properties of elements in groups are similar in some respects to each other.
Learning Objectives
In the 19th century, many previously unknown elements were discovered, and scientists noted that certain sets of elements had similar chemical properties. For example, chlorine, bromine, and iodine react with other elements (such as sodium) to make similar compounds. Likewise, lithium, sodium, and potassium react with other elements (such as oxygen) to make similar compounds. Why is this so?
In 1864, Julius Lothar Meyer, a German chemist, organized the elements by atomic mass and grouped them according to their chemical properties. Later that decade, Dmitri Mendeleev, a Russian chemist, organized all the known elements according to similar properties. He left gaps in his table for what he thought were undiscovered elements, and he made some bold predictions regarding the properties of those undiscovered elements. When elements were later discovered whose properties closely matched Mendeleev’s predictions, his version of the table gained favor in the scientific community. Because certain properties of the elements repeat on a regular basis throughout the table (that is, they are periodic), it became known as the periodic table.
Mendeleev had to list some elements out of the order of their atomic masses to group them with other elements that had similar properties.
The periodic table is one of the cornerstones of chemistry because it organizes all the known elements on the basis of their chemical properties. A modern version is shown in Figure (PageIndex{1}). Most periodic tables provide additional data (such as atomic mass) in a box that contains each element’s symbol. The elements are listed in order of atomic number.
Features of the Periodic Table
Elements that have similar chemical properties are grouped in columns called groups (or families). As well as being numbered, some of these groups have names—for example, alkali metals (the first column of elements), alkaline earth metals (the second column of elements), halogens (the next-to-last column of elements), and noble gases (the last column of elements).
Each row of elements on the periodic table is called a period. Periods have different lengths; the first period has only 2 elements (hydrogen and helium), while the second and third periods have 8 elements each. The fourth and fifth periods have 18 elements each, and later periods are so long that a segment from each is removed and placed beneath the main body of the table.
Metals, Nonmetals and Metalloids
Certain elemental properties become apparent in a survey of the periodic table as a whole. Every element can be classified as either a metal, a nonmetal, or a semimetal, as shown in Figure (PageIndex{2}). A metal is a substance that is shiny, typically (but not always) silvery in color, and an excellent conductor of electricity and heat. Metals are also malleable (they can be beaten into thin sheets) and ductile (they can be drawn into thin wires). A nonmetal is typically dull and a poor conductor of electricity and heat. Solid nonmetals are also very brittle. As shown in Figure (PageIndex{2}), metals occupy the left three-fourths of the periodic table, while nonmetals (except for hydrogen) are clustered in the upper right-hand corner of the periodic table. The elements with properties intermediate between those of metals and nonmetals are called semimetals (or metalloids). Elements adjacent to the bold zigzag line in the right-hand portion of the periodic table have semimetal properties.
Example (PageIndex{1})
Based on its position in the periodic table, do you expect selenium (Se) to be a metal, a nonmetal, or a semimetal?
Solution
The atomic number of selenium is 34, which places it in period 4 and group 16. In Figure 2.7.1, selenium lies above and to the right of the diagonal line marking the boundary between metals and nonmetals, so it should be a nonmetal. Note, however, that because selenium is close to the metal-nonmetal dividing line, it would not be surprising if selenium were similar to a semimetal in some of its properties.
Exercise (PageIndex{1})
Based on its location in the periodic table, do you expect indium (In) to be a nonmetal, a metal, or a semimetal?
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