WJEC Chemistry for AS Level Student Book: 2nd Edition (Draft)

 1.2 Basic ideas about atoms

Ionisation involves a transfer of energy from the radiation passing through the matter to the matter itself. As the alpha particle is the most strongly ionising of the radiations, this transfer happens most rapidly and so they are the least penetrating. Conversely, since gamma rays are the least ionising they are the most penetrating of the radiations. When alpha, beta and gamma radiations pass through an electric field, gamma rays are undeflected, while alpha particles are deflected towards the negatively charged plate and beta particles towards the positive plate.

Exam tip Do not just state that β particles are electrons. You must make it clear that they come from the nucleus.

Charged plates

+

β− particles

γ− rays

α , β and γ radiation

α− particles

–

Study point

▲ The effect of electric field on radiation

In equations: 4 2 He

Magnetic fields have a similar effect on alpha, beta and gamma radiation. When a charged particle cuts through a magnetic field it experiences a force referred to as the motor effect. Alpha particles are deflected by a magnetic field, confirming that they must carry a charge. The direction of deflection (which can be determined by Fleming’s left-hand rule) demonstrates that they must be positively charged. Beta particles are deflected by a magnetic field in an opposite direction to alpha particles confirming they must hold a charge opposite to alpha particles. Gamma rays are unaffected by a magnetic field. This shows gamma rays are uncharged as they do not experience a force when passing through the lines of a magnetic field.

2+ is acceptable for 4

2 α .

0 −1 e is acceptable for

0 −1 β .

Study point Electron capture can be regarded as an equivalent to positron emission, since capture of an electron results in the same transmutation as emission of a positron.

01.02.03 AS Chemistry Eduqas Barking Dog Art Effect on mass number and atomic number α and β particle emissions result in the formation of a new nucleus with a new atomic number therefore the product is a different element. Whe an element emits an α particle its mass number decreases by 4 and its atomic number decreases by 2. 4 2 α The product is two places to the left in the periodic table. When an element emits a β particle its mass number is unchanged and its atomic number increases by 1. 19 DRAFT 238 92 U → 234 90 Th + 14 0 −1 β The product is one place to the right in the periodic table. 6 C → 14 7 N + A process of inverse beta decay can also occur. This is known as electron capture . In the process of electron capture, one of the orbital electrons is captured by a proton in the nucleus, forming a neutron (and emitting an electron neutrino, ν e .) Stretch & challenge Because positron emission decreases proton number relative to neutron number, positron decay happens typically in large ‘proton- rich’ radionuclides. Electron capture is an alternative decay mode for radioactive isotopes with insufficient energy to decay by positron emission. It therefore occurs much more often in smaller atoms than positron emission. Electron capture always competes with positron emission, however it occurs as the only type of beta decay in proton-rich nuclei when there is not enough decay energy to support positron emission.