The alkali-metal group is group 1 or IA the alkaline-earth group is group 2 or IIA the halogen group is group 17 or VIIA and the noble-gas group is group 18 or VIIIA. But the idea of rows (periods) across which properties of elements vary and columns (groups) of elements with similar properties remains.Įach group (column) is identified by two numbers. Since Mendeleev’s time many more elements have been discovered: there are 118 in the modern periodic table below.

About 150 years ago, Russian chemist Dmitri Mendeleev recognized this periodicity and created a table with horizontal rows of elements in atomic-weight order and groups of similar elements in vertical columns. Notice that the colors repeat periodically-every eight elements. The noble gases, alkali metals, alkaline earth metals, and halogens are color coded. H He Li Be B C N O F Ne Na Mg Al Si P S Cl Ar K Ca Here is a list of the first twenty elements in order of increasing atomic number: (One meaning of “noble” is “unreactive”.) All are gases that undergo almost no chemical reactions, so He, Ne, Ar, Kr, and Xe are called noble gases. Just before the turn of the 20 th century, chemists discovered another group of similar elements: He, Ne, Ar, Kr, and Xe. Be, Mg, Ca, Sr, and Ba are known as alkaline earth metals or alkaline earths. Another group of elements, Be, Mg, Ca, Sr, and Ba also have similar physical properties, react with air and water (but more slowly than alkali metals), and react with halogens to form compounds with formulas like BeF 2, BeCl 2, MgCl 2, etc. ) All halogens consist of diatomic molecules, such as Cl 2. (“Halogen” comes from Greek hals, halo-”salt”. The elements F, Cl, Br, and I are called halogens. The elements Li, Na, K, Rb, and Cs are called alkali metals. (These compounds are collectively referred to as “salts” because their properties are similar to those of table salt, NaCl.) The reaction of Na(s) with Cl 2(g) to form NaCl(s) is shown in this video.

In addition, all these elements react with fluorine, chlorine, bromine, and iodine to form similar compounds: LiF, NaF, KF, RbF, CsF, LiCl, NaCl, etc. As you saw in the videos previously, the elements Li, Na, K, Rb, and Cs have similar properties and react with air and water in similar ways.

Color coded periodic table with gases