In the last activity, you looked at how elements are organized based on their atomic number and atomic mass. In this next activity, you will look more closely at how the periodic table is organized.

Since Dmitri Mendeleev's idea of organizing elements into a chart or table, many more elements have been discovered. They have all fit into a pattern of organization similar to that developed by Mendeleev. There were 63 known elements in Mendeleev's day; now there are over 100.

As you just learned, the elements are arranged by atomic number. This number tells us how many protons are in the nucleus of one atom of the element. This is a slight change from Mendeleev's system, which arranged the elements by atomic mass (the mass of all the protons and neutrons in an atom).

The individual squares you see on the periodic table include the symbol for each element along with information about the element's chemical properties. Some periodic tables include more information than others, but they all follow this basic format. In an upcoming lesson, you will learn how to use the periodic table to describe and understand chemical formulas.