I keep getting the wrong answer, please explain. Thanks Problem 5: An alpha part

Question

I keep getting the wrong answer, please explain. Thanks

Problem 5: An alpha particle (a helium nucleus, consisting of two protons and two neutrons) has a radius of approximately 1.6 x 1015 m. A certain heavy nucleus contains 79 protons in addition to all its neutrons and has a radius of approximately 5.8 x 1015 m. An alpha particle is shot directly from a large distance at such a resting heavy nucleus. Assuming that the heavy nucleus remains at rest during the ensuing process, what initial kinetic energy, in joules, must the alpha particle have in order to just reach the surface of the heavy nucleus.

Explanation / Answer

Using law of conservation of energy

initial kinetic energy = Final potential energy

0.5*m*v^2 = k*q1*q2/r

m is the mass of alpha particle = 6.64424*10^-27 kg

q1 = charge on alpha particle = 2*1.6*10^-19 = 3.2*10^-19 C

q2 = charge on heavy nucleus = 79*1.6*10^-19 = 1.264*10^-17 C

r = r1+r2 = (1.6+5.8)*10^-15 = 7.4*10^-15 m

then

0.5*m*v^2 = k*q1*q2/r

initial kinetic energy = (9*10^9*3.2*10^-19*1.264*10^-17)/(7.4*10^-15)

initial kinetic energy is Ki = 4.919*10^-12 J

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