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, Li and Li. Li is more common. 92.5% of lithium is Li. Lithium is a soft silvery metal that is very reactive. It is used in lithium batteries and certain medicines.