Question

39 during a radioactive process, the number of active nuclei disintegrating per unit time in the undecayed sample.
a undecayed sample.
b. activity.
c. half - life.
d. decay constant.

40 the number of electrons in uranium nucleus $\ce{_{92}^{235}u}$ is
a. 164.
b. 143.
c. 327.
d. 92

Explanation:

39)

• Option A: "undecayed sample" refers to the remaining undecayed nuclei, not the number disintegrating per unit time. Eliminate A.
• Option B: Activity (or decay rate) is defined as the number of active nuclei disintegrating per unit time. This matches the description.
• Option C: Half - life is the time taken for half of the radioactive nuclei in a sample to decay, not the number disintegrating per unit time. Eliminate C.
• Option D: Decay constant is a proportionality constant in the decay law \(N = N_0e^{-\lambda t}\), related to the probability of decay per unit time, not the number of nuclei disintegrating per unit time. Eliminate D.

Step 1: Recall the composition of an atom

For a neutral atom, the number of electrons (\(e^-\)) is equal to the number of protons (\(p^+\)). The atomic number (\(Z\)) of an element is equal to the number of protons. For uranium - 235 (\(_{92}^{235}\text{U}\)), the atomic number \(Z = 92\).

Step 2: Determine the number of electrons

Since a neutral uranium nucleus (atom) has an equal number of electrons and protons, and the number of protons is equal to the atomic number (\(Z = 92\)), the number of electrons is also 92.

Answer:

B. activity

40)