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Language English हिंदी

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The half life of a radioactive element which has only $\frac{1}{32}$ of its original mass left after a lapse of 60 days is

12 days

32 days

60 days

64 days

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Explanation

(a) $\frac{N_{0}}{32} = N_{0} {(\frac{1}{2})}^{60 / T} \Rightarrow 5 = \frac{60}{T} \Rightarrow T = 12 days$

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The half-life of ${Bi}^{210}$ is 5 days. What time is taken by (7/8)th part of the sample to decay

3.4 days

10 days

15 days

20 days

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Explanation

(c) By using $N = N_{0} {(\frac{1}{2})}^{t / T}$ where $N = (1 - \frac{7}{8}) N_{0} = \frac{1}{8} N_{0}$
So $\frac{1}{8} N_{0} = N_{0} {(\frac{1}{2})}^{t / T} \Rightarrow {(\frac{1}{2})}^{3} = {(\frac{1}{2})}^{t / 5} \Rightarrow t = 15 days$

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Radioactivity is

Irreversible process

Self disintegration process

Spontaneous process

All of the above

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Explanation

(d) All of the options apply for radioactivity.

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Half life of radioactive element depends upon

Amount of element present

Temperature

Pressure

Nature of element

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Explanation

(d) Half life of a substance doesn't depends upon Amount, temperature and pressure. It depends upon the nature of the substance.

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The decay constant of radium is $4.28 \times 10^{- 4}$ per year. Its half life will be

2000 years

1240 years

63 years

1620 years

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Explanation

(d) $T = \frac{0.6931 \times 1}{λ} = \frac{0.6931 \times 1}{4.28 \times 10^{- 4}} year = 1620 years$

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Which of the following is not a mode of radioactive decay

Positron emission

Electron capture

Fusion

Alpha decay

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Explanation

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Atomic mass number of an element thorium is 232 and its atomic number is 90. The end product of this radioactive element is an isotope of lead (atomic mass 208 and atomic number 82). The number of alpha and beta particles emitted is

$α$ =3, $β$ = 3

$α$ = 6, $β$ = 4

$α$ =6, $β$ =0

$α$ =4, $β$ =6

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Explanation

(b) $n_{α} = \frac{A - A'}{4} = \frac{232 - 208}{4} = 6$

and $n_{β} = (2 n_{α} - Z + Z') = (2 \times 6 - 90 + 82) = 4$

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A sample contains 16 gm of a radioactive material, the half life of which is two days. After 32 days, the amount of radioactive material left in the sample is

Less than 1 mg

$\frac{1}{4} gm$

$\frac{1}{2} gm$

1 gm

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Explanation

(a) Remaining amount

$= 16 \times {(\frac{1}{2})}^{32 / 2} = 16 \times {(\frac{1}{2})}^{16} = {(\frac{1}{2})}^{12} < 1 mg$

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Half life of a radioactive substance is T. The time taken for all the nuclei to disintegrate will be

$T^{2}$

Uncertain

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Explanation

(d) Uncertain, because it is infinite. No radioactive element can be disintegrated fully.

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The half life of polonium is 140 days. After how many days, 16 gm polonium will be reduced to 1 gm (or 15g will decay)

700 days

280 days

560 days

420 days

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Explanation

$\Rightarrow \frac{t}{140} = 4 \Rightarrow t = 560 days$

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Yes. You can attempt every NEET question on this page for free without logging in, and check the correct answer with a detailed explanation instantly.

No account is required to attempt questions and view answers. A free account adds bookmarks, personal notes, and progress tracking.

The bank mixes NEET previous year questions (PYQs) with practice questions, each tagged with its exam appearances where applicable.