Sodium (Na) is an alkali metal and is extremely reactive, reacting vigorously with oxygen, water, and acids. Copper (Cu) is moderately reactive; it reacts with oxygen to form oxides and with certain acids (like nitric acid or hot sulfuric acid) but is relatively stable in air and water compared to alkali metals. Iron (Fe) is also reactive; it rusts in moist air (reacts with oxygen and water) and reacts with acids. Gold (Au) is a noble metal, known for its low reactivity. It resists reaction with oxygen at normal temperatures, water, and most acids. It can only be dissolved by highly corrosive mixtures like aqua regia (a mixture of nitric acid and hydrochloric acid).