Radioactive Emissions (OCR GCSE Physics A (Gateway)): Flashcards

After absorbing electromagnetic radiation, the electron moves to a higher energy level. It is then unstable and will eventually fall back down, emitting a wave of electromagnetic radiation as it does so.

All of the colours in the visible spectrum are produced by electrons moving down energy levels and emitting electromagnetic radiation.

The process in which an electron falls from a higher energy level to a lower one, releasing a wave of electromagnetic radiation as it does so.

Dark-coloured objects are good absorbers of electromagnetic radiation. They appear dark because they do not reflect the energy that hits them — it is absorbed rather than returned to the eye.

True.

Energy levels increase with distance from the nucleus. Lower energy levels are nearer to the nucleus and higher energy levels are further away.

The atomic number is the number of protons in the nucleus of an atom. It is unique to each element and determines which element the atom is. It also equals the number of electrons in a neutral atom.

The mass number is the total number of protons and neutrons in the nucleus of an atom. The number of neutrons equals the mass number minus the atomic number.

In nuclear notation, the mass number is written at the top and the atomic number is written at the bottom, next to the element symbol.

True.

A neutral atom has no overall charge, so the number of electrons (charge −1 each) must equal the number of protons (charge +1 each). This balance gives a total charge of zero.

Subtract the atomic number (number of protons) from the mass number (total protons and neutrons). The result is the number of neutrons in the nucleus.

Nuclear notation is a way of representing an atom using its element symbol with the mass number written above and the atomic number written below, showing the composition of the nucleus at a glance.

An electron carries a charge of −1, which is equal in size but opposite in sign to the +1 charge of a proton. Because they balance, a neutral atom with equal numbers of each has no overall charge.

False.

Electrons have negligible mass — roughly 1/2000 the mass of a proton or neutron. Almost all of the atom's mass comes from protons and neutrons in the nucleus.

Isotopes are atoms of the same element that have the same number of protons but a different number of neutrons. They have the same chemical properties but different masses.

False.

Isotopes of the same element always have the same number of protons (the same atomic number). They differ only in their number of neutrons.

Chemical properties depend on the number of electrons, which equals the number of protons. Because isotopes have the same number of protons, they have the same number of electrons and therefore behave identically in chemical reactions.

Isotopes are atoms of the same element with the same number of protons but a different number of neutrons.

Chlorine has two naturally occurring isotopes: chlorine-35 and chlorine-37. Chlorine-35 is about three times more abundant, so the average mass number across all chlorine atoms is approximately 35.5 rather than a whole number.

An unstable isotope is an isotope whose nucleus is unstable due to an imbalance of protons and neutrons. It will undergo radioactive decay to become more stable, emitting radiation in the process.

False.

Which element an atom belongs to is determined by its number of protons, not its neutrons. Adding a neutron creates a different isotope of the same element.

Radioactive decay is the process in which an unstable nucleus emits radiation (a high-energy particle or wave) to become more stable. It is a random process.

False.

Radioactive decay is a random process. It is not possible to predict exactly when any individual nucleus will decay, only the average rate at which a large number of nuclei will decay.

Activity is the rate at which unstable nuclei in a source decay.

During alpha decay, the mass number of the nucleus decreases by 4 and the atomic number decreases by 2.

During beta decay, a neutron turns into a proton and an electron. The electron is emitted, so the mass number stays the same but the atomic number increases by 1.

An alpha particle is a particle emitted during alpha decay, identical to a helium nucleus. It is made of two protons and two neutrons and carries a charge of +2.

During gamma decay, a gamma ray (electromagnetic wave) is emitted, carrying away energy. Unlike alpha and beta decay, the mass number and atomic number of the nucleus do not change — only its energy decreases.

True.

A beta particle is a fast-moving electron produced when a neutron in the nucleus changes into a proton. It has a charge of −1 and negligible mass.

The number of decays recorded each second by a detector is called the count rate.

The parent nucleus is the original unstable nucleus that undergoes radioactive decay.

True.

both the total mass number and the total atomic number must balance on each side of a nuclear equation.

The daughter nucleus is the new nucleus that remains after a parent nucleus has undergone radioactive decay.

In alpha decay the daughter nucleus has a mass number 4 less and an atomic number 2 less than the parent nucleus.

In alpha decay, an alpha particle is identical to a helium nucleus. The daughter nucleus has an atomic number that is 2 less than the parent.

In beta minus decay a neutron changes into a proton and an electron. The emitted electron is the beta particle and the atomic number increases by 1.

False.

during gamma decay neither the mass number nor the atomic number changes. The nucleus releases energy as a gamma ray but its structure stays the same.

In beta minus decay a neutron converts into a proton. Both particles have a mass number of 1, so the total mass number of the nucleus is unchanged.

Complete the table showing how mass number and atomic number change in each decay.

Ionisation is the process by which nuclear radiation knocks electrons out of atoms, giving them a non-zero charge.

True.

alpha particles carry a charge of +2, making them the most strongly ionising form of nuclear radiation. Because they ionise so readily, they have the shortest range in air.

Complete the table showing the penetrating power of each radiation type.

Alpha particles are strongly ionising, so they quickly lose energy by ionising air molecules. This means alpha only travels a few centimetres in air before stopping.

Penetrating power describes how far radiation can travel through a material before being stopped. Gamma rays have the greatest penetrating power of the three main types.

False.

gamma radiation is not absorbed by air, so it has an effectively infinite range in air. It does become less intense with distance but is never fully stopped.

Beta particles are stopped by a few millimetres of aluminium. Thinner aluminium will not stop them, so the thickness matters in exam questions about shielding.

Of alpha, beta and gamma radiation, alpha is the most ionising and gamma has the greatest penetrating power.

The half-life of a radioactive isotope is the time taken for the number of unstable nuclei in a sample to decrease by half.

True.

After one half-life, 50% remain. After two half-lives, half of that 50% has decayed, leaving 25% of the original nuclei undecayed.

Radioactive decay is a random process. It is impossible to know when any particular nucleus will decay, but we can measure the half-life to describe how quickly a large sample decays overall.

Carbon-14 has a half-life of 5700 years. After two half-lives, 25 % of the original carbon-14 remains.

The activity of a radioactive sample is the rate at which its nuclei decay. After one half-life, the activity falls to half its original value.

False.

Different isotopes have different half-lives, which can range from a fraction of a second to billions of years. For example, uranium-235 has a half-life of 704 million years.

After each half-life the activity of the sample halves again. The activity keeps decreasing by half each time, so after two half-lives it is 25% of the original and after three it is 12.5%.

The half-life of a radioactive isotope is the time for half the unstable nuclei to decay. It is constant for a particular isotope, regardless of the size of the sample.

The initial activity, A₀, is the activity of a radioactive sample at the start of a measurement. It is used as the reference value when calculating half-life from a graph.

True.

On an activity-time graph, the half-life is the time taken for the activity to fall from any value to exactly half that value. This interval is the same wherever you measure it on the curve.

Measure the initial activity A₀, then find the time at which the activity reaches ½ A₀. The time interval between these two points is the half-life. The same interval appears repeatedly as activity keeps halving.

A sample starts with 2 000 000 undecayed atoms. After one half-life 1 000 000 atoms remain. After two half-lives 500 000 atoms remain.

Divide the total time elapsed by the number of half-lives that have passed. For example, if 2 half-lives take 1 year, the half-life is 1 year ÷ 2 = 6 months.

False.

The half-life of an isotope is constant and does not depend on sample size. A larger sample has more atoms decaying but the proportion that decays in each half-life stays the same.

Complete the table for a sample with initial activity 800 Bq and a half-life of 3 hours.

No matter how large or small the sample, the same fraction of undecayed nuclei always decays in each half-life. The rate of decay slows as fewer atoms remain, but the half-life itself never changes.