lithium (from greek "lithos" 'stone') is a soft, silver-white metal with symbol li. it is the third chemical element in the periodic table. this means that it has 3 protons in its nucleus and 3 electrons around it. its atomic number is 3. its mass number is 6.94. it has two common isotopes, 6li and 7li. 7li is more common. 92.5% of lithium is 7li. lithium is a soft silvery metal that is very reactive. it is used in lithium batteries and certain medicines.