In a reactive metal like sodium, the single electron on an electron shell furthest from the nucleus is more likely to be lost in a reaction with another element. This is because less energy is needed to pull them away. Other elements with similar properties like potassium that are lower down on the periodic table than sodium will be even more likely to lose their furthest electron in a reaction. This is because of the addition of more electron shells between the nucleus and the furthest electron, increasing distance.