Radioactivity, Radioisotopes and Types of Nuclear Emission – Radioactivity (Senior 6 Chemistry) | Ganzaa

Radioactivity

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Senior 6 · Chemistry

Radioactivity

5 lessons

Lesson 1 of 5

Radioactivity, Radioisotopes and Types of Nuclear Emission

25 min

A photograph taken in the dark

In February 1896 the French physicist Henri Becquerel laid crystals of a uranium salt on a photographic plate wrapped in thick black paper. When he developed the plate it was fogged in the shape of the crystals, even though no light had reached it. The uranium was giving out a penetrating radiation of its own, all the time, with no energy put in.

Two years later Marie and Pierre Curie showed that the strength of the radiation depended only on the amount of uranium, not on which compound it was in or whether it was hot or cold. Marie Curie called the property radioactivity and concluded that it came from inside the atom itself. The Curies went on to discover two new, far more radioactive elements, polonium and radium. That insight, a property of the nucleus that chemistry cannot touch, is the starting point of this unit. In Senior 6 Physics you studied the nucleus and its radiations as physics; here you look at them as chemistry: what changes, how to write the equations, and how radioisotopes are used.

Henri Becquerel's photographic plate of 1896, with his handwritten notes at the top. The two dark patches formed where crystals of a uranium salt lay on black paper over the plate, with no light reaching it. Image: Henri Becquerel, public domain, via Wikimedia Commons

Definition

Radioactivity: the spontaneous emission of radiation (particles or high-energy photons) by unstable atomic nuclei, which change as a result. Example: radium-226 emits alpha particles and becomes radon-222.

Nuclide: an atom with a particular number of protons $Z$ and neutrons $N$ , written ${}^{A}_{Z}\mathrm{X}$ , where the mass number $A = Z + N$ . Example: ${}^{14}_{6}\mathrm{C}$ has 6 protons and 8 neutrons.

Radioisotope (radionuclide): a radioactive isotope of an element. Example: carbon-12 and carbon-13 are stable, but carbon-14 is a radioisotope.

Nuclear reaction: a change in the composition of a nucleus, by decay or by bombardment with particles. Example: ${}^{14}_{6}\mathrm{C} \rightarrow {}^{14}_{7}\mathrm{N} + {}^{0}_{-1}\mathrm{e}$ .

Chemical reactions and nuclear reactions

	Chemical reaction	Nuclear reaction
Particles involved	outer electrons; nuclei unchanged	protons and neutrons in the nucleus
Elements	the same elements before and after	one element can become another
Isotopes of an element	react in the same way	behave quite differently (C-12 stable, C-14 radioactive)
Energy change	about $10^{1}$ to $10^{3}\,\mathrm{kJ\,mol^{-1}}$	about $10^{8}$ to $10^{10}\,\mathrm{kJ\,mol^{-1}}$
Mass change	far too small to measure	measurable ( $E = m c^{2}$ )
Effect of temperature, pressure, catalysts, chemical state	change the rate	almost no effect on the rate of decay

The types of nuclear emission

Emission	Symbol	What it is	Change in the nucleus	Effect on $A$ and $Z$
Alpha (α)	${}^{4}_{2}\mathrm{He}$ or ${}^{4}_{2}\alpha$	helium nucleus, charge +2	loses 2 protons and 2 neutrons	$A - 4$ , $Z - 2$
Beta-minus (β⁻)	${}^{0}_{-1}\mathrm{e}$ or ${}^{0}_{-1}\beta$	fast electron made in the nucleus	$\mathrm{n \rightarrow p + e^{-}}$ (+ antineutrino)	$A$ same, $Z + 1$
Positron (β⁺)	${}^{0}_{+1}\mathrm{e}$ or ${}^{0}_{+1}\beta$	positive electron (antimatter)	$\mathrm{p \rightarrow n + e^{+}}$ (+ neutrino)	$A$ same, $Z - 1$
Electron capture (EC)	${}^{0}_{-1}\mathrm{e}$ on the left	nucleus captures an inner-shell electron; X-rays follow	$\mathrm{p + e^{-} \rightarrow n}$	$A$ same, $Z - 1$
Gamma (γ)	${}^{0}_{0}\gamma$	high-energy photon	excited nucleus loses energy	no change

Alpha particles are slow and heavy and ionise strongly, so a sheet of paper or a few centimetres of air stops them. Beta particles go a few millimetres into aluminium; gamma rays are reduced only by thick lead or concrete. In an electric field (Figure 1.1) the charged emissions bend: β⁻ strongly towards the positive plate because it is so light, α a little towards the negative plate, and γ not at all.

Figure 1.1: Alpha, beta-minus and gamma radiation in an electric field (not to scale).

Worked example 1: which emission?

Problem. Identify the emission in each change: (a) ${}^{234}_{90}\mathrm{Th} \rightarrow {}^{234}_{91}\mathrm{Pa}$ ; (b) ${}^{226}_{88}\mathrm{Ra} \rightarrow {}^{222}_{86}\mathrm{Rn}$ ; (c) ${}^{11}_{6}\mathrm{C} \rightarrow {}^{11}_{5}\mathrm{B}$ , with no X-rays given off; (d) ${}^{99m}_{43}\mathrm{Tc} \rightarrow {}^{99}_{43}\mathrm{Tc}$ .

Solution.

(a) $A$ unchanged, $Z$ up by 1: a neutron became a proton: β⁻.

(b) $A$ down by 4, $Z$ down by 2: α.

(c) $A$ unchanged, $Z$ down by 1: a proton became a neutron. Both β⁺ emission and electron capture do this; the absence of the X-rays that follow capture points to positron emission.

(d) Neither $A$ nor $Z$ changes; "m" means metastable (excited): γ emission. This is why technetium-99m is used in medical imaging: it gives gamma rays but no particles.

Worked example 2: how big is a nuclear energy change?

Problem. Each alpha decay of radium-226 releases 4.87 MeV. Express this per mole and compare it with the enthalpy of combustion of methane, $-890\,\mathrm{kJ\,mol^{-1}}$ . ( $1\,\mathrm{eV} = 1.602 \times 10^{-19}\,\mathrm{J}$ , $N_\text{A} = 6.022 \times 10^{23}\,\mathrm{mol^{-1}}$ )

Solution.

E = 4.87 \times 10^{6} \times 1.602 \times 10^{-19}\,\mathrm{J} \times 6.022 \times 10^{23}\,\mathrm{mol^{-1}} = 4.70 \times 10^{11}\,\mathrm{J\,mol^{-1}} = 4.70 \times 10^{8}\,\mathrm{kJ\,mol^{-1}}

Ratio: $\frac{4.70 \times 10^{8}}{890} = 5.3 \times 10^{5}$ . One mole of radium decaying releases over half a million times more energy than one mole of methane burning. The difference comes from the strength of the forces involved: nuclear forces hold protons and neutrons together far more tightly than electrons hold atoms together.

Warning

Common mistakes

Thinking a β⁻ particle is an electron from the electron shells. It is made in the nucleus when a neutron changes into a proton.
Saying that heating or dissolving a radioactive compound speeds up its decay. The rate of decay is a property of the nucleus.
Forgetting that a nuclear change can change the element: after β⁻ emission the atom has one more proton, so it is the next element.
Mixing up β⁺ and electron capture. Both lower $Z$ by 1; a positron is emitted (right-hand side), a captured electron is a reactant (left-hand side).

Tip

For any emission, ask "what happened to one nucleon?": α, four nucleons leave together; β⁻, n → p; β⁺ and EC, p → n; γ, nothing, just energy. Then $Z$ follows: −2, +1, −1, 0.

The Organic Chemistry Tutor: Alpha Decay, Beta Decay, Gamma Decay, Electron Capture, Positron Production. Watch how each emission changes the mass number and atomic number, and how the equations are written.

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Practice

Define radioactivity and radioisotope.
Give four differences between chemical and nuclear reactions.
State the numbers of protons and neutrons in ${}^{131}_{53}\mathrm{I}$ and ${}^{32}_{15}\mathrm{P}$ .
Identify the emission: (a) ${}^{40}_{19}\mathrm{K} \rightarrow {}^{40}_{20}\mathrm{Ca}$ ; (b) ${}^{210}_{84}\mathrm{Po} \rightarrow {}^{206}_{82}\mathrm{Pb}$ ; (c) ${}^{7}_{4}\mathrm{Be} + {}^{0}_{-1}\mathrm{e} \rightarrow {}^{7}_{3}\mathrm{Li}$ .
Explain why β⁻ particles are deflected more than α particles in the same electric field, and in the opposite direction.
Two samples contain the same number of uranium atoms, one as uranium(VI) oxide and the other as uranyl nitrate. Explain why their activities are the same.
The fission of one uranium-235 nucleus releases about 200 MeV. Calculate the energy per mole in $\mathrm{kJ\,mol^{-1}}$ .

Show answersHide answers

Radioactivity: the spontaneous emission of radiation by unstable nuclei. Radioisotope: a radioactive isotope of an element, e.g. carbon-14.
Any four from the table, e.g. nuclei change in nuclear reactions; elements can change; energy changes are about a million times larger; isotopes behave differently; the rate is unaffected by temperature, pressure and catalysts.
Iodine-131: 53 protons, 78 neutrons. Phosphorus-32: 15 protons, 17 neutrons.
(a) β⁻ (Z up by 1); (b) α (A down 4, Z down 2); (c) electron capture.
A β⁻ particle has about $\frac{1}{7300}$ of the mass of an α particle (and half its charge), so the same force gives it a much larger deflection. Its charge is negative, so it bends towards the positive plate; α is positive and bends towards the negative plate.
Radioactivity comes from the uranium nuclei, which are not affected by the chemical bonding, so the same number of uranium atoms gives the same activity.
$200 \times 10^{6} \times 1.602 \times 10^{-19} \times 6.022 \times 10^{23} = 1.93 \times 10^{13}\,\mathrm{J\,mol^{-1}} = 1.93 \times 10^{10}\,\mathrm{kJ\,mol^{-1}}$ .

Sources and credits

Image: Becquerel plate.jpg by Henri Becquerel, via Wikimedia Commons, in the public domain.
Image: Marie Curie in her Paris Laboratory, 1912.jpg, photographer unattributed, via Wikimedia Commons, in the public domain.
Video: Alpha Decay, Beta Decay, Gamma Decay, Electron Capture, Positron Production, The Organic Chemistry Tutor, embedded from YouTube.
The text, tables, diagram, worked examples and Practice items were written and drawn by Ganzaa.

1A photograph taken in the dark

2Chemical reactions and nuclear reactions

3The types of nuclear emission

A photograph taken in the dark

Chemical reactions and nuclear reactions

The types of nuclear emission