An alkaline earth element is radioactive. It and its daughter elements decay by emitting a total of three alpha particles in succession. In what group of the periodic table is the element resulting from the emission of the third alpha particle?
A)4A (14)
B)5A (15)
C)6A (16)
D)7A (17)
E)8A (18)

Question

Quizplus · Accepted Answer

The atomic number of an element decreases by 2 for each alpha particle emitted. After emitting three alpha particles, the atomic number of the element will decrease by 6. Alkaline earth elements have an atomic number of 2, so the element resulting from the emission of the third alpha particle will have an atomic number of -4. This is not possible, so the element must have started with an atomic number of 8. The only alkaline earth element with an atomic number of 8 is radium, which is in Group 2A (4A).

An Alkaline Earth Element Is Radioactive