radioactivity - topic 7

Describe an atom

Atom has a tiny positive nucleus of protons and neutrons containing almost all mass, with electrons orbiting at relatively huge distances.

Why the nucleus is positively charged

Because protons are +1 and neutrons are neutral.

Where most mass of atom is

In the nucleus.

Typical radius of atom

1×10⁻¹⁰ m.

Typical radius of nucleus

1×10⁻¹⁴ m.

Structure of isotopes

Same protons, different neutrons.

Atomic number definition

Number of protons.

Mass number definition

Number of protons + neutrons.

Isotope symbol example

¹³₆C means mass number 13, atomic number 6.

Why isotopes have same chemistry

They have the same electron arrangement.

Nuclear charge definition

Determined by number of protons.

Relative masses of subatomic particles

Proton 1, neutron 1, electron ~0, positron ~0.

Relative charges of subatomic particles

Proton +1, neutron 0, electron –1, positron +1.

What makes an atom neutral

Protons = electrons.

Electron orbit rules

Electrons occupy fixed energy levels

When electrons move up a shell

When they absorb electromagnetic energy.

When electrons move down a shell

When they emit electromagnetic energy.

How positive ions form

By losing electrons.

How negative ions form

By gaining electrons.

What unstable nuclei emit

Alpha, beta-minus, beta-plus, gamma, and neutrons.

Definition of ionising radiation

Radiation that removes electrons from atoms.

Which radiations are ionising

Alpha (strong), beta (moderate), gamma (weak).

Definition of background radiation

Low-level radiation present around us all the time.

Sources of background radiation

Radon gas, cosmic rays, rocks, medical uses, nuclear industry.

Why radon matters

It is a radioactive gas released from rocks.

Detectors of radiation

Photographic film and Geiger–Müller tube.

What alpha radiation is

Helium nucleus (2p + 2n).

What beta-minus radiation is

Electron emitted when neutron → proton.

What beta-plus radiation is

Positron emitted when proton → neutron.

What gamma radiation is

High-energy electromagnetic wave.

Penetration of alpha

Stopped by paper or skin.

Penetration of beta

Stopped by a few mm of aluminium.

Penetration of gamma

Reduced by thick lead or concrete.

Ionisation of alpha

Very high.

Ionisation of beta

Medium.

Ionisation of gamma

Low.

Range of alpha in air

A few cm.

Range of beta in air

A few metres.

How the atomic model changed

Plum pudding → Rutherford nuclear model → Bohr shells.

Rutherford scattering showed

Atoms are mostly empty space with a small positive nucleus.

β⁻ decay definition

Neutron → proton + electron.

β⁺ decay definition

Proton → neutron + positron.

Effect of alpha decay

Mass number –4, atomic number –2.

Effect of beta-minus decay

Atomic number +1.

Effect of beta-plus decay

Atomic number –1.

Effect of gamma emission

No change to proton or mass number.

Effect of neutron emission

Mass number –1.

Why gamma is emitted

To lose excess nuclear energy.

Balancing nuclear equations

Mass and atomic numbers must match on both sides.

Definition of activity

Rate of decay per second.

Unit of activity

Becquerel (Bq).

Why activity decreases

Fewer unstable nuclei remain.

Definition of half-life

Time taken for activity or number of nuclei to halve.

Why half-life is constant

Decay is random but statistically predictable.

Why individual nuclei can’t be predicted

Decay events are random.

Uses of half-life

Carbon dating, medical tracers, nuclear waste assessment.

Shape of decay graphs

Exponential decrease.

Meaning of short half-life

High activity

Meaning of long half-life

Low activity

Effect of neutron radiation

Causes nuclei to become unstable or form new isotopes.

Danger of alpha inside body

Very dangerous due to high ionisation.

Danger of alpha outside body

Generally safe

Difference between exposure and contamination

Exposure = radiation near you

Why gamma useful in medicine

High penetration

What a positron is

Antimatter electron with +1 charge.

What annihilation is

Positron meets electron → energy as gamma.

Why radiation damages cells

Ionisation breaks DNA and cell molecules.

What determines isotope suitability for medical use

Short half-life, gamma emission, low ionisation.