ch23

Question 1

unified atomic mass units

Answer 1

One unified atomic mass unit (1u) is defined as being equal to one-twelfth of the mass of a carbon-12 atom. 1u is equal to 1.66 × 10−27kg

Question 2

mass defect

Answer 2

difference between mass of nucleus and total mass of nucleons where nucleons are separated to infinity

Question 3

binding energy

Answer 3

energy required to separate nucleons to infinity

Question 4

Binding energy per nucleon

Answer 4

the total energy needed to completely separate all the nucleons in a nucleus divided by the number of nucleons in
the nucleus

Question 5

nuclear fusion

Answer 5

two nuclei combine to form a single nucleus. it is initiated by very high temperature

Question 6

nuclear fission

Answer 6

a single large nucleus divides to form smaller nuclei. it is initiated by neutrons bombardment

Question 7

thermonuclear reactions

Answer 7

Reactions requiring conditions of extremely high temperature and pressure, similar to those found at the centre of the Sun

Question 8

Radioactive decay

Answer 8

when unstable nuclei emit particles and/or electromagnetic radiation to become stable. it is a random process in that it cannot be predicted which nucleus will decay next. it is a spontaneous process because it is not affected by any external factors, such as temperature or pressure

Question 9

decay curve

Answer 9

A graph, such as that seen in radioactive decay, that shows an exponential decrease – the value decreases by the same fraction over equal time intervals

Question 10

decay constant λ

Answer 10

probability of decay per unit time

Question 11

becquerels

Answer 11

Unit used to measure the activity of a radioactive source, where 1 becquerel is 1 decay per second.

Question 12

activity

Answer 12

the number of nuclear decays occurring per unit time in aradioactive source

Question 13

The half-life

Answer 13

time taken for the number of undecayed nuclei to be reduced to half its original number

Question 14

activity equation

Answer 14

-dN/dt== λN

Question 15

−λN

Answer 15

dN/dt

Question 16

equation of N

Answer 16

N0e^−λt where N0 = the initial number of undecayed nuclei (when t = 0)

Question 17

exponential decay

Answer 17

An exponential decrease – the value decreases by the same fraction over equal time intervals.

Question 18

equation of A

Answer 18

A0 e^−λt where A0= is the initial activity

Question 19

decay constant equation

Answer 19

0.693/t(1/2)

Question 20

Isotopes of hydrogen have binding energies per nucleon of less than 3MeV.
Suggest why a nucleus of helium‑4 does not spontaneously break down to become nuclei of
hydrogen.

Answer 20

binding energy per nucleon is much greater M1
so would require a large amount of energy to separate the nucleons in helium

Question 21

Suggest an explanation for any difference between the actual activity of the sample
shown in Fig. 12.1 and the curve you have drawn for the activity of isotope X.

Answer 21

(activity of sample is greater than activity of X so) there must be an additional source of activity C1
the decay product (of isotope X) is radioactive

Question 22

The decay constant λF of nuclide F is very much larger than the decay constant λE of
nuclide E.
By reference to the half-life of nuclide F, explain why the number of nuclei of nuclide F in the
sample is always small.

Answer 22

half-life of F is much shorter than half-life of E B1
nuclei of F decay (almost) as soon as they are produced