Atomic structure

Explanation

• An atom has a tiny, positively charged nucleus containing protons and neutrons, with negatively charged electrons arranged at different energy levels around it.
• Compare scales using a ratio: an atom has radius about $1\times10^{-10}\,\mathrm{m}$, while its nucleus has less than $1/10\,000$ of the atom's radius.
• Most atomic mass is concentrated in the nucleus; absorbing electromagnetic radiation can move an electron to a higher energy level, while emitting it can move the electron lower.
• A common error is to draw the nucleus as most of the atom: it contains most of the mass but occupies only a very small central region.

Explanation

• Atomic number is the number of protons; mass number is the total number of protons and neutrons, so neutron number is mass number minus atomic number.
• For a neutral atom, electron number equals proton number; for a positive ion, subtract the positive charge from the proton number to find its electrons.
• Isotopes are atoms of the same element with the same proton number but different neutron numbers, so their atomic numbers match but their mass numbers differ.
• A common error is to change the nucleus when an ion forms: losing outer electrons changes the charge, not the atomic number or mass number.

Explanation

• Atoms were first treated as indivisible spheres; discovery of the electron led to the plum pudding model, with electrons embedded in a ball of positive charge.
• Use scattering evidence in order: most alpha particles passed through, so atoms are mostly empty space; a few were strongly deflected, so charge and most mass occupy a tiny nucleus.
• Bohr proposed electrons at specific distances, later evidence identified protons in the nucleus, and Chadwick's work provided evidence for neutrons.
• A common error is to say Rutherford expected every alpha particle to rebound: the key comparison is between the observed pattern and the plum pudding model's prediction of only small deflections.

Explanation

• An unstable nucleus decays at random; activity is its decay rate in becquerels ($\mathrm{Bq}$), while count rate is the number of decays recorded each second by a detector such as a Geiger-Muller tube.
• Identify radiation by composition and properties: alpha is two protons plus two neutrons, beta is a fast electron from the nucleus, gamma is electromagnetic radiation, and a neutron may also be emitted.
• Alpha has the shortest range and greatest ionising power, beta is intermediate, and gamma is the most penetrating and least ionising of the three.
• A common error is to call beta an orbital electron or gamma a charged particle: beta forms when a neutron changes into a proton, while gamma has no charge or mass.

Explanation

• Balance a nuclear equation by making the total mass number and total atomic number equal on both sides.
• For alpha emission use ${}^{4}_{2}\alpha$; for beta-minus emission use ${}^{0}_{-1}\beta$, so the daughter's atomic number is one greater while its mass number is unchanged.
• For example, after one alpha emission a parent labelled $A,Z$ becomes $A-4,Z-2$; gamma emission changes neither number.
• A common error is to decrease atomic number in beta-minus decay: the emitted beta has atomic number $-1$, so the daughter must increase by $1$ to balance.

Explanation

• Half-life is the time for the number of undecayed nuclei, activity or net count rate to fall to half its initial value; individual nuclear decays remain unpredictable.
• Subtract background count rate before finding successive halvings, then divide the elapsed time by the number of half-lives.
• A fall from $960$ to $120$ is three halvings because $960\rightarrow480\rightarrow240\rightarrow120$; Higher tier can express the remaining-to-original ratio as $1:8$.
• A common error is to subtract the same amount in every half-life or to halve a gross detector reading without first removing background.

Explanation

• Contamination is the unwanted presence of material containing radioactive atoms; irradiation is exposure to nuclear radiation without transfer of radioactive material.
• Compare hazards by asking whether the source can remain on or enter the body, which radiation it emits, and how exposure can be shortened, shielded or kept at a distance.
• An alpha contaminant outside the body may be stopped by skin, but the same material inside the body can be especially hazardous because alpha is strongly ionising at short range.
• A common error is to say an irradiated object must become radioactive; irradiation stops when exposure ends, whereas contaminating atoms continue to decay until removed or decayed.

Explanation

• Background radiation is always present and includes natural sources such as radioactive rocks and cosmic rays, plus man-made fallout from weapons tests and nuclear accidents.
• When comparing measurements, allow for location, altitude, surrounding rock and occupation, and subtract a local background count rate when isolating a source's count rate.
• For dose, $1000\,\mathrm{mSv}=1\,\mathrm{Sv}$; a worker's total dose can be estimated by adding the contributions from different exposures over the stated time.
• A common error is to assume a detector should read zero after a test source is removed: background radiation continues to produce counts.

Explanation

• Radioactive isotopes span a very wide range of half-lives, so the duration and rate of a hazard depend on the isotope present.
• Compare hazards using both activity and persistence: a short half-life means rapid decay and a quickly falling hazard, while a long half-life can leave material radioactive for much longer.
• For equal numbers of unstable nuclei, a shorter-half-life isotope undergoes decays more rapidly at first; a longer-half-life isotope generally creates the longer waste-management problem.
• A common error is to call either short or long half-life always safer: risk also depends on quantity, radiation type, route into the body and exposure time.

Explanation

• A medical tracer for exploring an organ should be detectable outside the body, so a penetrating radiation such as gamma is useful, and its half-life should limit the time the patient remains radioactive.
• For controlling or destroying unwanted tissue, direct radiation at the target or place a suitable source close to it while limiting dose to healthy cells.
• Evaluate a use by comparing diagnostic or treatment benefit with absorbed dose, radiation type, half-life, exposure time and the consequences of not carrying out the procedure.
• A common error is to discuss only usefulness: every evaluation needs a linked risk, such as ionisation damaging healthy cells, and a way the exposure is controlled.

Explanation

• Fission is the splitting of a large, unstable nucleus; it usually begins when the nucleus absorbs a neutron.
• A fission event produces two smaller nuclei of roughly equal size, two or three neutrons and gamma rays, with released energy appearing as kinetic energy of the products.
• Emitted neutrons can trigger further fissions: limiting how many continue gives a controlled reactor chain reaction, while continued multiplication gives an uncontrolled release.
• A common error is to describe fission as two small nuclei joining; that is fusion, whereas fission starts with one large nucleus splitting.

Explanation

• Fusion is the joining of two light nuclei to form a heavier nucleus.
• Identify fusion from the pattern of two small nuclear reactants becoming one larger nuclear product, rather than from the presence of radiation alone.
• The combined mass of the nuclear product can be slightly less than that of the starting nuclei; the mass difference is converted into energy carried by radiation.
• A common error is to call any energy-releasing nuclear process fusion: fission splits one large nucleus, while fusion joins two light nuclei.