Atoms are the smallest entities which resemble the properties of an element and cannot be broken down into smaller parts. Molecules are formed when two or more atoms are combined. This formation can happen in a variety of ways. When two oxygen atoms join O2 is formed, and when three oxygen atoms join O3 or ozone is formed. Both of these have different properties. Different atoms can also combine to form molecules. Hydrogen combines with sulfur to form H2S and hydrogen and oxygen also combine to form H2O. Dmitri Mendeleev’s Periodic table is based on atomic weight, whereas, the modern periodic table keeps the atomic number as the base. An atomic table arranges elements from left to right in a row with the atomic number and properties gradually changing. The elements on the next row will actually exhibit similar properties as the ones above or below it. This provides an example of the periodicity nature of the elements. The columns of different elements in a period have the same or very similar properties. The properties of the elements change gradually as the columns move down through the periodic table. Atoms in similar families, such as H2S and H2O, exhibit both similar properties and molecular structure. This shows periodicity in both the molecules and the individual atoms.
Alkali metals contain a single electron in their outer shell rendering them highly reactive. This allows them to be ready to donate this single electron to form molecules with substances like water. Halogens are also extremely reactive containing seven electrons in their outer shell. Similar to alkali metals, halogens need only one electron in their outer shell to become stable. The reactivity of an element increases the when the number of outer shell electrons is closer to one or seven. Elements that contain eight electrons in their outer shell exhibit little or no