11 -A large collection of nuclei are undergoing alpha decay. The rate of decay at any instant is proportional to:

A. the number of un-decayed nuclei present at that instant

B. the time since the decays started

C. the time remaining before all have decayed

D. the half-life of the decay

E. the average time between decays

12 .Of the three common types of radiation (alpha, beta, gamma) from radioactive sources, electric charge is carried by:

A. only beta and gamma

B.only beta

C. only alpha and gamma

D. only alpha

E. only alpha and beta

13 .A radioactive atom X emits a β− particle. The resulting atom

A. must be very reactive chemically

B. has an atomic number that is one more than that of X

C. has a mass number that is one less than that of X

D. must be radioactive

E. is the same chemical element as X

14 . A nucleus with mass number A and atomic number Z undergoes β− decay. The mass number and atomic number, respectively, of the daughter nucleus are :

A. A, Z − 1

B. A − 1, Z

C. A +1, Z − 1

D. A, Z +1

E. A, Z − 1

15.in the line spectrum of sodium , the number of lines obtained is :

A.3

B.2

C.4

 D.5