6. Periodic Table

118 different chemical elements are known to modern chemistry. 92 of these elements can be found in nature and the others can only be made in laboratories. The last natural element discovered was uranium in 1789. The first man-made element was technetium in 1937. Chemical elements are commonly arranged in the periodic table according to their atomic number.

Dmitri Mendeleev, a Russian chemist, published the first periodic table in 1869. He showed that when the elements were ordered according to atomic weight, a pattern resulted where similar properties for elements recurred periodically. Based on the work of physicist Henry Moseley, the periodic table was reorganized on the basis of increasing atomic number rather than on atomic weight. The formulation of the periodic law states that the chemical properties of the elements are dependent on their atomic numbers. The periodic table lists elements by atomic number. In the periodic table the elements are arranged into periods, groups and block.

• Period

A row of elements across the table is called a period. Each period has a number; from 1 to 8. Period 1 has only 2 elements in it: hydrogen and helium. Period 2 and Period 3 both have 8 elements. Other periods are longer. Elements in a period have consecutive atomic numbers. Elements in the same period show trends in atomic radius, ionization energy, electron affinity, and electro-negativity.

• Group

A column of elements down the table is called a group. There are 18 groups in the standard periodic table. Each group has a number: from 1 to 18. Elements in a group have electrons arranged in similar ways, according to the number of valency electrons, which gives them similar chemical properties (they behave in similar ways). Elements in the same group tend to show patterns in atomic radius, ionization energy, and electro-negativity.

• Block

Specific regions of the periodic table can be referred to as blocks in recognition of the sequence in which the electron shells of the elements are filled. Elements are assigned to blocks by what orbitals their valence electrons or vacancies lie in. The s-block comprises the first two groups (alkali metals and alkaline earth metals) as well as hydrogen and helium. The p-block comprises the last six groups, which are groups 13 to 18 and contains, among other elements, all of the metalloids. The d-block comprises groups 3 to 12 and contains all of the transition metals. The f-block, often offset below the rest of the periodic table, has no group numbers and comprises most of the lanthanides and actinides.

• Valence Electron

A valence electron is an outer shell electron that is associated with an atom, and that can participate in the formation of a chemical bond if the outer shell is not closed; in a single covalent bond, both atoms in the bond contribute one valence electron in order to form a shared pair. For a main group element, a valence electron can exist only in the outermost electron shell; in a transition metal, a valence electron can also be in an inner shell. The number of valence electrons is also important for determining the group of an element in the Periodic table, because an element's number of valence electron is the same as the number of its group in the periodic table. 

Methane (CH₄) used Four covalent bonds. Carbon has four valence electrons and here a valence of four. Each hydrogen atom has one valence electron and is univalent.