Read the following passage and then answer questions (a) — (e) on the basis of your understanding of the passage and the related studied concepts.
Henry Becquerel, in 1896, discovered the radioactivity purely by accident when he observed that pieces of uraniumpotassiumsulphate emit something which are able to penetrate black paper as well as a silver foil. Subsequent experiments showed that radioactivity was a nuclear phenomenon in which a unstable nucleus undergoes a decay. Three types of radioactive decay occur in nature, namely (i) `alpha`-decay in which a helium nucleus, `(" "_(2)^(4)He)` is emitted, (ii) `beta`-decay in which electrons (or positrons) are emitted, and (iii) `gamma`-decay in which high energy photons are emitted.
The decay rate of a radioactive sample is a measurable quantity and is known as 'activity'. A common way to characteristic decay rate of a substance is 'half-life period' `(T_(1/2))` which is the time during which half of the radioactive material decays and one half remains intact. Another related measure is the average or mean life `tau`. It is related to `T_(1/2)` as per relation `T_(1/2)=0.693tau`.
(a) What is radioactivity?
(b) Give two examples of radioactive elements?
(c) Out of three types of radioactive radiations which are deflected by an electric and a magnetic field and why?
(d) A radioactive sample has a half-life period of 5 hours. What fraction of original sample remains undecayed after 20 hours.
(e) Complete the following nuclear reaction :
`" "_(72)^(180)A overset(alpha)(to)A_(1)overset(beta^(-))(to)A_(2)overset(alpha)(to)A_(3)overset(gamma)(to)A_(4)`

Explanation

(a) The phenomena of spontaneous emission of some particles, without any outside stimulation, by an unstable nuclide is called radioactivity.
(b) Uranium and radium.
(c) Alpha and beta particles because these are charged particles.
(d) As `T_(1/2)=5h and t=20h=4T_(1/2)`
`therefore` Fraction of radioactive sample left intact (undecayed)=`[1/2]^(4)=1/16 or 6.25%`
(e) Completed reaction is :
`" "_(72)^(180)A overset(alpha)(to)" "_(70)^(176)A_(1)overset(beta^(-))(to)" "_(71)^(176)A_(2)overset(alpha)(to)" "_(69)^(172)A_(3)overset(gamma)(to)" "_(69)^(172)A_(4)`