Question

A meteorite is determined to be 4.5 billion years old. It now has 78 atoms of lead-206. Assuming no loss of daughter isotopes, how many atoms of uranium-238 did the meteorite have to begin with? The half-life of uranium-238 is 4.5 billion years.

a) 156
b) 312
c) 78
d) 238

Answer 1

Given the age of the meteorite is 4.5 billion years old with 78 atoms of lead-206 the answer should be B. 156. The half-life of uranium-238 in order to decay into lead-206 is 4.46 billion years, which means that the original number of atoms is 156.