Question

question 3 of 10
a uranium-235 sample starts with 200 atoms, and 700 million years later, there are 100 atoms. what is the half-life of uranium-235?

a. 100 years
b. 700 million years
c. 235 years
d. 350 million years

Explanation:

Step1: Recall half - life definition

The half - life of a radioactive substance is the time it takes for half of the atoms of the substance to decay.

Step2: Analyze the given data

We start with 200 atoms of uranium - 235. After a certain time, we have 100 atoms left. Since \(200\times\frac{1}{2}=100\), this means that half of the original sample has decayed.

Step3: Determine the time taken for half - decay

The time taken for the number of atoms to go from 200 to 100 (i.e., for half of the sample to decay) is 700 million years. By the definition of half - life, this time period is the half - life of uranium - 235.

Answer:

B. 700 million years