His table became known as the periodic table of the elements.

The periodic law: periodic law states that the repeating chemical and physical properties of elements change periodically with the elements’ atomic numbers.

Each element has a chemical symbol.

Each horizontal row of elements (from left to right) on the periodic table is called a period.

Each vertical column of elements (from top to bottom) on the periodic table is called a group.
Elements in the same group often have similar chemical and physical properties. For this reason, a group is also called a family. 

The starting materials in a chemical reaction are reactants.

The substances formed from a reaction are products. 
In this example, carbon and oxygen are reactants. Carbon dioxide is the product. 

If you know the name of the covalent compound, you can often write the chemical formula for that compound. Covalent compounds are usually composed of two nonmetals. The names of many covalent compounds use prefixes. Each prefix represents a number, as shown in Table 1. The prefixes tell you how many atoms of each element are in a formula. Figure 2 shows you how to write a chemical formula from the name of a covalent compound. 

If the name of a compound contains the name of a metal and the name of a nonmetal, the compound is ionic. To write the formula for an ionic compound, make sure the compound’s charge is 0. In other words, the formula must have subscripts that cause the charges of the ions to cancel out. Figure 3 shows you how to write a chemical formula from the name of an ionic compound. 

 Ionic Compounds

To balance an equation, you must use coefficients. A coefficient is a number that is placed in front of a chemical symbol or formula. For example, 2CO represents two carbon monoxide molecules. The number 2 is the coefficient. 

For an equation to be balanced, all atoms must be counted. So, you must multiply the subscript of each element in a formula by the formula’s coefficient. For example, 2H2O contains a total of four hydrogen atoms and two oxygen atoms. Only coefficients—not subscripts—are changed when balancing equations. Changing the subscripts in the formula of a compound would change the compound.