What is the half-life of an isotope if 62.5 g of a 500. g sample remains after 18 years? PLS ASAP I NEED THIS

If a hydrogen atom is neutral, it must also have  one electron. an oxygen atom (o) has an atomic number of eight (8). this means a neutral oxygen atom has eight protons and eight electrons. the element actium (ac) has an atomic number of 89, so it has 89 protons and 89 electrons in a neutral atom