There are 88 naturally occurring elements. Chemistry would be a nearly impossible subject to study if there were not some relationships between the elements. Chemists several centuries ago noticed that the properties of elements seem to reoccur periodically. At first it was though that this periodicity was a function of atomic weight but modern scientists have shown that it varies based on atomic number (#of protons ) . As a result they formulated the Periodic Law.
The properties of an element are a periodic (repeating) function of their atomic numbers.
Mendeleev was one of the first to use such a law (although he thought the properties varied due to atomic weight) to create a table which organized the elements. He placed elements that had similar properties underneath each other in columns. These columns are known as chemical families.
We understand today that the properties vary according to atomic number and the reason for the similar chemical and physical properties is due to the electron structure of the atom . Therefore elements in the same column in the periodic table have both similar properties and electron arrangement.
One of the chemical properties that is similar is the formula of the compounds created with other elements.
For Example :
beaker Calcium combines with Chlorine to form Calcium Chloride: The formula is CaCl2
Magnesium is in the same column with Calcium and when it combines with chlorine the formula is MgCl2
The ratio of Calcium and Magnesium to Chlorine in both compounds is 1:2
In fact all members of these two families will form compounds with this ratio. Therefore one general formula may represent many different compounds of elements from the same chemical families.
From the example above, a general formula for Alkaline earth metals (A) combining with Halogens (B) might be AB2.

periodic table
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Assignment #8
Chemical families and formulas

periodic table
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Assignment #9
Chemical Families