Atomic and nuclear physics: Class 10 Physics

Key points

An atom has a small, dense, positively charged nucleus at the centre with electrons moving around it. The nucleus contains protons (positive) and neutrons (no charge), together called nucleons. The atom is neutral because the number of electrons equals the number of protons. Almost all the mass of the atom is in the nucleus, which is about 100 000 times smaller in size than the atom. Rutherford's alpha particle scattering experiment led to this model: most alpha particles passed through a thin gold foil, a few were deflected strongly and very few bounced back.

Nuclear notation:

• Atomic number Z: the number of protons in the nucleus.
• Mass number A: the total number of protons and neutrons.
• Number of neutrons N = A − Z.
• A nuclide is written as ᴬ_Z X. For example, uranium 235 has Z = 92 and A = 235, so it has 143 neutrons.

Isotopes are atoms of the same element with the same atomic number but different mass numbers, that is, different numbers of neutrons. Hydrogen has three isotopes: protium (no neutron), deuterium (one neutron) and tritium (two neutrons). Isotopes have the same chemical properties but different masses and, in some cases, different nuclear stability.

Some nuclei are unstable. They change into more stable nuclei by emitting radiation. This spontaneous process is called radioactivity. It was discovered by Becquerel. It is not affected by temperature, pressure or chemical combination. Radioactivity is random: we cannot say when a given nucleus will decay, but a large sample decays in a predictable way. Radiation is detected by a Geiger-Müller counter, a cloud chamber or photographic film.

Three kinds of radiation:

• Alpha (α): a helium nucleus (2 protons and 2 neutrons), charge +2e. Strongly ionizing, short range of a few centimetres in air, stopped by paper or the skin.
• Beta (β): a fast electron from the nucleus, charge −e. Moderate ionization, travels metres in air, stopped by a few millimetres of aluminium.
• Gamma (γ): a high energy electromagnetic wave with no charge. Weakly ionizing, very penetrating, reduced by thick lead or concrete. Not deflected by electric or magnetic fields.

In alpha decay the nucleus loses 2 protons and 2 neutrons, so A falls by 4 and Z by 2: ²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He. In beta decay a neutron changes to a proton and an electron, so A stays the same and Z rises by 1: ¹⁴₆C → ¹⁴₇N + ⁰₋₁e. In gamma emission the nucleus loses only energy, so A and Z do not change. In every nuclear equation the total mass number and the total charge (atomic number) are the same on both sides.

The half-life T½ of a radioactive nuclide is the time in which half of the nuclei in a sample decay. The activity falls to half in each half-life. After n half-lives the fraction left is (½)ⁿ. Half-lives range from a fraction of a second to billions of years. A short half-life means a rapidly decaying, highly active source. The activity is measured in becquerel (Bq), where 1 Bq = 1 decay per second.

Einstein showed that mass and energy are equivalent: E = Δm c². The energy released is equal to the loss of mass times the square of the speed of light, c = 3 × 10⁸ m/s. A tiny mass converts to a huge energy. Mass defect is the difference between the mass of the separate nucleons and the mass of the nucleus. The energy equivalent of the mass defect is the binding energy that holds the nucleus together. The atomic mass unit is 1 u = 1.66 × 10⁻²⁷ kg, which is equivalent to about 931 MeV.

Nuclear fission is the splitting of a heavy nucleus, such as uranium 235, into two lighter nuclei when it absorbs a slow neutron. A few neutrons and a large amount of energy are released. These neutrons can split more nuclei, producing a chain reaction. In an uncontrolled chain reaction, as in an atomic bomb, the energy is released in an instant. In a nuclear reactor the chain reaction is controlled: control rods of boron or cadmium absorb extra neutrons, and a moderator such as heavy water or graphite slows neutrons. The heat produces steam that turns turbines to generate electricity. Pakistan generates part of its electricity from nuclear power plants, such as those at Karachi and Chashma.

Nuclear fusion is the joining of two light nuclei, for example hydrogen isotopes, to form a heavier nucleus with release of energy. It needs extremely high temperature (millions of degrees) and pressure to overcome the repulsion between positive nuclei. Fusion is the source of energy of the Sun and the stars. It gives more energy per unit mass than fission and produces little long lived waste, but it has not yet been made practical for power plants.

Radioisotopes have many uses. In medicine, cobalt 60 gamma rays treat cancer, and technetium 99m is used for imaging. In industry, radiation checks the thickness of paper and metal sheets and finds leaks in pipes. In agriculture, tracers study how plants take up fertilizer and radiation sterilizes insect pests. Carbon 14 dating finds the age of ancient wood and bones. Gamma rays sterilize surgical tools.

Radiation can ionize atoms in living cells, causing burns, cancer and genetic damage. Safety rules are: keep sources in lead-lined containers, handle them with tongs, limit the time of exposure, stay as far away as possible, use shielding, wear a film badge to record exposure, and dispose of waste safely.

Study tip: For nuclear equations, check the top numbers add up and the bottom numbers add up on both sides. For half-life, first find the number of half-lives n = t / T½ and then halve that many times. Learn the table comparing α, β and γ (nature, charge, ionizing power, penetration, stopping material), as it is often asked.

Definitions and flashcards

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Formula sheet

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Short questions with model answers

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Numericals with worked steps

Common mistakes

• Confusing atomic number (protons) with mass number (protons plus neutrons).
• Forgetting that the mass number and the charge must balance on both sides of a nuclear equation.
• Thinking half-life means the sample is gone after two half-lives. Half is left after one, a quarter after two.
• Mixing up the penetrating powers. Gamma is most penetrating, alpha least.
• Using mass in grams in E = Δm c². Convert to kilograms to get joules.

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