There are two types of atoms that engage a chemical reaction. They are anions and cations, or in one word – ions. To understand the principles by which separate atoms bound into the compounds we will need to know the terms the atomic radius and the ionic radius. The atomic radius is the distance halfway across the individual atom and the ionic radius is the distance half way across the individual ion. The size of the atom influences the atom’s tendency to give up or maintain the electron from the outer shell. That is why the distance from the nucleus the most important factor. The bigger the atomic radius, the weaker are the attractive tendencies in the outer shell, and vice versa, the smaller the atomic radius, the stronger are the forces maintaining the electrons in the outer shell.
The electrons from the outermost shell are engaged in transfer or sharing of an electron. For example, the atom of sodium has one electron in the outermost shell. When this electron is removed, the positive ion is produced. Sometimes the atom can have its electron stripped from its outermost orbit. Such elements that lack one or several electrons are called cations. They acquire a positive charge. On the other part, some elements can gain one or several electrons that make them negatively charged. Such elements are called anions.
We can easily learn how many electrons are in the outer shell by looking at the top of the periodic table. Each group of elements has the same valence numbers and types of outer electrons. The periodic table provides us with the information about chemical bound of each element. A compound is a substance in which two or more elements are combined in a fixed proportion. These elements attach together to form a molecule. The attachment is an attraction that contains moderate to strong forces that hold the molecule together. This attachment or force between the elements is called the chemical bond.