The periodic table is a way of organizing known elements into groups with similar properties. Mendelev was a Russian scientist who was first to recognise that elements can be grouped together according to atomic mass.
The modern periodic table has elements arranged in order of electronic configuration. The table is divided into groups(columns) and periods(rows). Elements in the same group display similar chemical properties and trends immerge as we go along a period and down a group.

All elements in a particular group share similar chemical properties. All elements in group 1 are very reactive metals, with the exception of hydrogen gas. These metals react readily with other compounds and elements to give away some of their outer shell (valence) electrons. These metals react with water to produce hydrogen gas. Click on the blue writing below to see the reactions.

Sodium reacts slowly with water to produce hydrogen gas. Hydrogen gas builds up and ignites.

Potassium reacts faster than sodium to produce hydrogen gas.

Cesium reacts more violently with water

Elements in group 2 also react with water to form hydrogen gas but are not as reactive as group 1 elements.

Magnesium

Calcium

Notice how reactivity of metals increases as we go down a group. Compare the reactivity of sodium and potassium with water. Now compare the reactivity of calcium and magnesium with water. Is there a pattern?