Copy each passage into your book and fill in the numbered gaps using the word bank. Then check your answers. ← All topics
1. The structure of the atom (C+S)
An atom has a radius of about 1 × 10−10 m. At its centre is a tiny, positively charged (1)______ made of protons and (2)______; it contains almost all of the atom’s (3)______. Negatively charged (4)______ orbit the nucleus at different energy levels. An electron moves to a higher energy level (further from the nucleus) if it (5)______ electromagnetic radiation, and moves to a lower level if it (6)______ it.
Word bank: absorbs · electrons · emits · mass · neutrons · nucleus
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1 nucleus · 2 neutrons · 3 mass · 4 electrons · 5 absorbs · 6 emits
2. Atomic number, mass number and isotopes (C+S)
Atoms have no overall charge because the number of electrons equals the number of (1)______. The atomic number is the number of protons; the (2)______ number is the total number of protons and neutrons. Atoms of the same element always have the same number of protons. (3)______ are atoms of the same element with different numbers of (4)______. An atom that loses one or more outer electrons becomes a positive (5)______.
Word bank: ion · isotopes · mass · neutrons · protons
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1 protons · 2 mass · 3 Isotopes · 4 neutrons · 5 ion
3. How the model of the atom changed (C+S)
Atoms were once thought to be tiny spheres that could not be divided. The discovery of the (1)______ led to the plum pudding model: a ball of positive charge with electrons embedded in it. In the alpha (2)______ experiment, most alpha particles passed straight through gold foil, but a few were deflected or bounced back. This showed that the mass is concentrated in a tiny, positively charged (3)______ and that most of the atom is empty (4)______. Niels Bohr suggested electrons orbit at specific distances, and James (5)______ later showed that neutrons exist. Models change when new (6)______ can’t be explained by the old model.
Word bank: Chadwick · electron · evidence · nucleus · scattering · space
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1 electron · 2 scattering · 3 nucleus · 4 space · 5 Chadwick · 6 evidence
4. Types of nuclear radiation (C+S)
Radioactive decay is a (1)______ process in which an unstable nucleus gives out radiation to become more stable. An alpha particle is two protons and two neutrons – the same as a (2)______ nucleus. It is the most (3)______, travels only a few centimetres in air and is stopped by paper. A beta particle is a high-speed (4)______ from the nucleus; it is stopped by a few millimetres of (5)______. Gamma rays are (6)______ radiation; they are the least ionising and are only reduced by thick lead or concrete.
Word bank: aluminium · electromagnetic · electron · helium · ionising · random
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1 random · 2 helium · 3 ionising · 4 electron · 5 aluminium · 6 electromagnetic
5. Nuclear equations (C+S)
In alpha decay, the mass number decreases by (1)______ and the atomic number decreases by (2)______. In beta decay, a neutron turns into a (3)______, so the atomic number increases by 1 and the mass number stays the (4)______. Emitting a gamma ray does not change the mass or the (5)______ of the nucleus. In a balanced equation, the mass numbers and atomic numbers on each side must add up to the same totals.
Word bank: 2 · 4 · charge · proton · same
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1 4 · 2 2 · 3 proton · 4 same · 5 charge
6. Activity and half-life (C+S)
Activity is the rate at which a source of unstable nuclei decays; it is measured in (1)______ (Bq). Count-rate is the number of decays recorded each second by a detector such as a (2)______ tube. Half-life is the time it takes for the number of unstable nuclei in a sample to (3)______, or for the count-rate to fall to half its initial value. After two half-lives, (4)______ of the original nuclei remain. (HT) After three half-lives, the net decline is (5)______ of the original.
Word bank: ¼ · ⅞ · becquerels · Geiger-Muller · halve
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1 becquerels · 2 Geiger-Muller · 3 halve · 4 ¼ · 5 ⅞ (⅛ remains)
7. Contamination and irradiation (C+S)
(1)______ is the unwanted presence of materials containing radioactive atoms on or in other materials. (2)______ is the process of exposing an object to nuclear radiation – the object does not become (3)______. Ionising radiation can damage (4)______ and cause cancer. Results of studies on the effects of radiation should be published and shared with other scientists so that they can be checked by (5)______ review.
Word bank: cells · contamination · irradiation · peer · radioactive
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1 Contamination · 2 Irradiation · 3 radioactive · 4 cells · 5 peer
8. Background radiation and uses (S only)
Background radiation comes from natural sources, such as (1)______ and cosmic rays from space, and from man-made sources, such as fallout from nuclear weapons testing and nuclear accidents. The (2)______ a person receives is measured in sieverts. Radioactive isotopes are used as medical tracers and to treat cancer (radiotherapy). A tracer must have a (3)______ half-life and emit (4)______ rays so they can be detected outside the body.
Word bank: dose · gamma · rocks · short
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1 rocks · 2 dose · 3 short · 4 gamma
9. Fission and fusion (S only)
Nuclear fission is the (1)______ of a large, unstable nucleus such as uranium. It usually happens when the nucleus absorbs a (2)______. It splits into two smaller nuclei and releases two or three neutrons, gamma rays and energy. The released neutrons can cause further fissions – a (3)______ reaction. In a nuclear reactor, (4)______ rods absorb some neutrons to keep the reaction steady. Nuclear (5)______ is the joining of two light nuclei to form a heavier nucleus; some of the mass is converted into energy.
Word bank: chain · control · fusion · neutron · splitting
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1 splitting · 2 neutron · 3 chain · 4 control · 5 fusion
