The periodic table is a useful tool because it arranges all the elements in an organized and informative manner.
The periodic table arranges the elements into families and periods (vertical and horizontal rows). The elements in each family have similar properties. As you go across a row, the properties vary gradually from one element to the next. The table tells you what elements may have similar chemical and physical properties.
The periodic table describes the atomic structure of all known elements. For instance, by looking at the periodic table, you can find out the atomic mass and the number of electrons the element has. Each element has its own separate set of such data. No two elements are the same.
This is perhaps the most useful feature of the Periodic Table. It is an excellent reference tool. In one place, you can find many properties of an element.
The video below provides a quick review of how to determine the number of valence electrons for some commonly used elements in science classes.
Because elements in each family have the same number of valence electrons, they tend to have several similar characteristics.
For example, elements in group one, which therefore only have one valence electron, are soft, highly reactive metals, while those in group 18, with full outer shells, all exist in their pure form as a colorless gas and are highly nonreactive.
There are other ways to use the periodic table to predict properties of an element. As you move down the periodic table vertically, the ionization energy and electron affinity both decrease.
As you move from left to right the electron affinity and ionization energy both increase. Since elements of the same family have similar characteristics, you can often predict the properties of one element if you know the properties of another element in its family.