The general properties of the alkali metals in the modern periodic table
The alkali metals are located on the left side of the modern periodic table in the group (1) or (1 A). They are the first group of s-block, The alkali metals are mono-valent elements as they have only one electron in their outermost energy levels.
The alkali metals are all metals which have one electron in the outer shell, they are very reactive, they are the most reactive metals where they are located in group 1, the alkali metals are good conductors of the heat and the electricity.
Most of the alkali metals have low density. Lithium (Li). Sodium (Na) and potassium (K) elements float on the water surface as their densities are smaller than the water density. Rubidium (Rb) and cesium (Cs) elements sink in the water as their densities are greater than the water density.
The alkali metals react to lose the electrons, These metals are very reactive with reactivity increasing down the group, They are active elements, so, they are kept under the surface of kerosene or paraffin oil.
Lithium is not kept under the surface of kerosene as it floats on its surface and burns at once so, it is kept in paraffin oil, Sodium and potassium are kept under the surface of kerosene to prevent them from the reaction with the moist air as they are active metals.
If you would like to know why Cesium is the most active metal, Look at the modern periodic table, You will know that the chemical activity of the alkali metals increases as the atomic size increases, therefore, Cesium (Cs) is considered the most active metal (in the periodic table) as it has the largest atomic size.