Flashcards · 8463 · Topic 4

Atomic structure flashcards

27 cards on Topic 4, subtopics 4.4.1 to 4.4.4. Pick a subtopic or study them all.

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Size of an atom and of its nucleus: Atom radius: about 1 × 10⁻¹⁰ m. Nucleus radius: less than 1/10 000 of the radius of the atom.
Atomic number vs mass number: Atomic number: protons only. Mass number: protons + neutrons.
Isotope vs ion: Isotopes of an element have different numbers of neutrons. An ion has a different number of electrons from protons (so it is charged).
How many protons, neutrons and electrons are in an atom of sodium-23 (atomic number 11)?: 11 protons, 23 − 11 = 12 neutrons, 11 electrons.
What happens to an electron when an atom absorbs or emits electromagnetic radiation?: Absorbs: the electron moves to a higher energy level (further from the nucleus). Emits: it moves to a lower energy level (closer to the nucleus).
Plum pudding model vs nuclear model: Plum pudding: a ball of positive charge with electrons embedded in it. Nuclear: mass and positive charge concentrated in a tiny nucleus, with electrons outside.
Alpha scattering: what each observation showed: Most went straight through: the atom is mostly empty space. Some were deflected: the centre is charged. Very few bounced back: the centre is tiny and holds most of the mass.
Who gave evidence for the neutron, and when, relative to the nuclear model?: James Chadwick, about 20 years after the nucleus became an accepted idea.

Alpha, beta and gamma: what each is: Alpha: 2 protons + 2 neutrons (a helium nucleus). Beta: a high-speed electron ejected from the nucleus as a neutron turns into a proton. Gamma: electromagnetic radiation from the nucleus.
Penetration and ionising power of alpha, beta and gamma: Alpha: stopped by paper, a few cm in air, most ionising. Beta: stopped by thin aluminium. Gamma: reduced by thick lead or concrete, least ionising.
Activity: The number of decays per second of a source, in becquerel (Bq).
Count-rate: The number of decays recorded each second by a detector (for example a Geiger-Muller tube).
Effect of alpha and of beta decay on mass number and atomic number: Alpha: mass number − 4, atomic number − 2. Beta: mass number unchanged, atomic number + 1. Gamma: no change to either.
Half-life: The time it takes for the number of nuclei of the isotope in a sample to halve, or for the count-rate (or activity) to fall to half its initial level.
A source has activity 800 Bq and half-life 2 days. What is its activity after 6 days?: 6 ÷ 2 = 3 half-lives: 800 → 400 → 200 → 100 Bq
Net decline as a ratio after n half-livesHigher: 1 : 2ⁿ. For example, after 4 half-lives it is 1 : 16.
Contamination vs irradiation: Contamination: radioactive atoms get on or into an object, so the hazard continues. Irradiation: the object is exposed to radiation from outside; it does not become radioactive.

Sources of background radiation: Natural: rocks and cosmic rays from space. Man-made: fallout from nuclear weapons testing and nuclear accidents.
Unit of radiation dose: The sievert (Sv). 1000 millisieverts (mSv) = 1 Sv.
Hazard of a source with a short half-life vs a long half-life: Short: very active at first but the activity falls quickly. Long: less intense, but stays hazardous for a long time.
What kind of source is chosen as a medical tracer, and why?: A gamma emitter, so the radiation can leave the body and reach the detector, with a half-life of hours: long enough to do the job, short enough to limit the dose.
Why is a beta source used to monitor the thickness of thin card?: Alpha would be stopped by the card; gamma would pass through almost unchanged, so small changes in thickness would not show.

Nuclear fission: A large unstable nucleus (such as uranium or plutonium) absorbs a neutron and splits into two smaller nuclei of roughly equal size, releasing two or three neutrons, gamma rays and energy.
Chain reaction: The neutrons released by one fission go on to cause further fissions. It is controlled in a nuclear reactor and uncontrolled in a nuclear weapon.
Nuclear fusion: Two light nuclei join to form a heavier nucleus. Some of the mass may be converted into the energy of radiation.
What usually has to happen before a uranium-235 nucleus undergoes fission?: It must absorb a neutron. Spontaneous fission is rare.
Fission vs fusion: Fission: a heavy nucleus splits. Fusion: light nuclei join.