This is the familiar table found on the wall of almost every chemistry classroom in the world. Most chemistry textbooks feature a periodic table printed on the inside front cover of the book. Since like charges repulse each other with great force, and any charge on an atom would be repeated on every atom, a pure sample of an element with any charge at all would explode violently. All other elements react with other elements to form compounds. In the special case where the reaction produces fixed numbers of the same types of atoms in exactly the same configuration, we say the elements have formed a molecule .
Molecules are the simplest possible samples of new substances called molecular compounds .
Usually, molecules form so that the atoms within them share electrons and thereby attain a magic number of electrons, with enhanced stability similar to the noble gases. Although the orbitals that are occupied by the electrons in an atom are quite complicated to describe exactly, we can think of them as concentric shells around the nucleus. Each successive shell starting from the nucleus and working outward can contain only so many electrons. Once a shell is filled, additional electrons must occupy shells further out from the nucleus. The first shell can take only two electrons, the second 8, the third 8, the fourth 18, the fifth 18, and the sixth 32.