What is the total number of electrons needed to completely fill all of the orbitals in an atoms second principal energy level

Different isotopes indicate that an element will have different numbers of

2. how is heavy water different from normal water

How many liters of hydrogen gas will be produced at stp from the reaction of 7.179×10^23 atoms of magnesium with 54.219g of phosphoric acid (h3po4) the equation is 3mg + 2h3(> mg(po4)2+3h2