What did rutherford discover about the structure of the atom

He discovered atom is composed from a central part - a nucleus, positively charged, surrounded by electrons - very small negative charged particles. 

The answer is 4f======since electrostatic force is given by    we let f be the electrostatic force when  r = 1 m    if we let  denote the electrostatic force when the distance is decreased so that r = 0.5m, then    we established earlier that f = kq₁q₂. therefore    the force will now be 4f.