The periodic table is a valuable predictive tool. Mendeleev very accurately predicted the existence and properties of five elements unknown in 1870. Three of these elements were discovered within a few years of his predictions. The predicted properties of the elements and their compounds were very close to the actual properties determined when the elements were discovered.

There are no longer empty spots in the periodic table that need to be filled, but the ability to make predictions about elements and compounds based on trends in the periodic table is still very useful. The patterns you will study in detail are those most useful to a chemistry student in predicting and explaining the basic behavior of atoms during chemical and physical changes.

The major trend that Mendeleev used in his organization of the table was the formula of the highest oxide of the element (ex. SO₂ and SO₃ - SO₃ is the highest oxide of sulfur). The ratio of moles of oxygen to moles of element increases in steps of .5 across the eight main groups of the table. This concept of valence served Mendeleev very well. This trend is dependent upon the number of valence electrons. There are limitations and exceptions to most periodic trends.

In a similar fashion, the formula of the highest fluoride of the elements increases in whole number steps (of the representative elements), from a 1:1 ratio with Na to 1:6 at SF₆.

The trends you will examine in detail are for atomic radius, ionization energy, electron affinity, and electronegativity.