Suppose a radioactive material has 10 atoms initially. So after 1st half life its no. of atoms will become 5 but what will happen after the 2nd half life. How can it be 2.5 ???

as you will be aware, N=Noe^-λt. THIS DOES NOT IN ANY WAY SAY THAT AFTER TIME 't',   EXACTLY  Noe^-λt ATOMS WILL NOT HAVE DECAYED. IT MERELY STATES THAT AFTER TIME 't', the PROBABILISTIC EXPECTATION OF THE NUMBER OF ATOMS THAT HAVE NOT DECAYED IS Noe^-λt. It even applies to the case where only one undecayed atom is remaining.