Sodium metal, consisting of sodium atoms, is only stable when immersed in an organic liquid, such as kerosene or diesel fuel, to prevent it from being exposed to water vapor in the air. When introduced to water, the sodium atom quickly loses the lone electron in its outer electron shell to become a sodium ion with a charge of +1. The sodium ion, in terms of mass, is not much different from the sodium atom, as the mass of the electron it lost was only 1/2000 of an atomic mass unit. But the reactivity is highly motivated, highly volatile, one of those reactions that will proceed uninterrupted until it has achieved its final destination. That is why sodium metal is in group one on the periodic table of elements, the elements in that group, such as hydrogen, all have one electron in their outer electron shell, and will quickly jettison that electron to a more electronegative element to become a group one ion.