Sodium (Na) is an alkali metal, which belongs to Group 1 of the periodic table. Alkali metals share similar chemical properties, such as being highly reactive with water and forming strong bases (alkalies) when combined with water. Cesium (Cs), another alkali metal, is located in the same group as sodium, though it is further down the periodic table. Due to their similar chemical characteristics, cesium is the closest chemically similar element to sodium.
Therefore, sodium's closest chemically similar element is cesium (Cs), both being alkali metals in the same group.