Question

1. which element pb -207 or u-235 is more stable? how do you know? how long would it take for u-235 to undergo 3 half lives? 27. what is a half - life?

Explanation:

Step1: Define stability in terms of radioactivity

Stable elements do not decay spontaneously. Pb - 207 is a stable isotope, while U - 235 is a radioactive isotope that undergoes radioactive decay.

Step2: Determine half - life concept

The half - life of a radioactive substance is the time it takes for half of the atoms of the substance to decay. The half - life of U - 235 is about 704 million years.

Step3: Calculate time for 3 half - lives of U - 235

If the half - life of U - 235 ($t_{1/2}$) is 704 million years, the time ($t$) for 3 half - lives is $t = 3\times t_{1/2}$.
$t=3\times704$ million years = 2112 million years = 2.112 billion years.

Answer:

Pb - 207 is more stable because it is non - radioactive while U - 235 is radioactive. It would take 2.112 billion years for U - 235 to undergo 3 half - lives. A half - life is the time it takes for half of the atoms of a radioactive substance to decay.