Page 502 - Physics Coursebook 2015 (A level)

Cambridge International A Level Physics
Energy and the nucleus
Towards the end of the Second World War, nuclear weapons were dropped on the Japanese cities of Hiroshima and Nagasaki. Nuclear explosions release so much energy that their size is often given in ‘megatonnes’ – that is, their equivalent in millions
of tonnes of high explosive (Figure 31.1). A more peaceful use of nuclear materials is in nuclear power generation. Because materials such as uranium are such concentrated stores of energy, a nuclear power station requires only a small van-load of fuel each week, whereas a coal-fired power station may require a train-load every hour.
In this chapter, we will look at energy in the atomic nucleus and how this relates to nuclear stability;
we will also look at how we can write equations to represent radioactive decay.
Figure 31.1 Our understanding of nuclear physics has proved to be a mixed blessing. Nuclear weapons dominated global politics for much of the 20th century.
β+decay
ZA X
β−decay
α decay
490
■■ In α decay, the nucleon number decreases by 4 and the proton number decreases by 2.
■■ In β− decay, the nucleon number is unchanged and the proton number increases by 1.
■■ In β+ decay, the nucleon number is unchanged and the proton number decreases by 1.
■■ In γ emission there is no change in nucleon or proton number.
Balanced equations
When an unstable nucleus undergoes radioactive decay, the nucleus before the decay is often referred to as the parent nucleus and the new nucleus after the decay of the α-particle is known as the daughter nucleus.
Radioactive decay processes can be represented by balanced equations. As with all equations representing nuclear processes, both nucleon number A and proton number Z are conserved.
A +2 A A – 2
A – 4 Z – 2
Figure 31.2 Emission of α- and β-particles.
Z Z+ 2 Proton number Z
1 Study the decay equations given in Worked examples 1 and 2, and write balanced equations for the following:
a A nucleus of radon-220 (220Rn) decays by α 86
emission to form an isotope of polonium, Po.
b A nucleus of a sodium isotope (25Na) decays by 11
β− emission to form an isotope of magnesium, Mg.
2 Copy and complete this equation for the β− decay of a nucleus of argon:
41Ar→K+? 18
The emission of α- and β-particles can be shown on a graph of nucleon number plotted against proton number, as shown in Figure 31.2. The graph will appear different if neutron number is plotted against proton number.
QUESTIONS
Nucleon number A