The modern periodic table has changed since Mendeleev's original table, yet both the first tables and the modern table are important for the same reason: The periodic table organizes elements according to similar properties so you can tell the characteristics of an element just by looking at its position on the table.
Before all the naturally occurring elements were discovered, the periodic table was used to predict the chemical and physical propertiesof elements in the gaps on the table.
Today, the table can be used to predict properties of elements yet to be discovered, although these new elements are all highly radioactive and break down into more familiar elements almost instantly.
The table is useful for modern students and scientists because it helps predict the types of chemical reactions that are likely for an element. Rather than memorize facts and figures for each element, a quick glance at the table reveals a lot about the reactivity of an element, whether it is likely to conduct electricity, whether it is hard or soft, and many other characteristics.
For example, the elements in the first column (the alkali metals) are all metals that usually carry a 1+ charge in reactions, react vigorously with water, and combine readily with nonmetals.
Elements in the same row as each other (periods) share highest unexcited electron energy level.
Another useful feature of the periodic table is that most table provide all the information you need to balance chemical reactions at a glance.
The table tells an elements atomic number and usually its atomic weight. The usual charge on an element is indicated by an element's group.
Trends or Periodicity
The periodic table is organized according to trends in element properties:
Moving Left to Right Across the Row
Atomic Radius Decreases
Ionization Energy Increases
Electron Affinity Generally Increases (except Noble Gases)
Moving Top to Bottom Down a Column
Atomic Radius Increases
Ionization Energy Decreases
Electron Affinity Usually Decreases Moving Down a Group
To summarize, the periodic table is important because it is organized to provide a lot of information about elements and how they relate to each other in one easy-to-use reference: