How do unstable nuclei decay, and how is the time for decay measured?
Nuclear chemistry and radioactivity: describe alpha, beta and gamma decay, balance nuclear equations, distinguish fission from fusion, and use half-life.
A focused Virginia SOL Chemistry answer on nuclear processes under CH.2: alpha, beta and gamma decay, balancing nuclear equations, the difference between fission and fusion, and using half-life to find how much of a sample remains.
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What this topic is asking
Standard CH.2 closes with nuclear chemistry. Virginia expects you to describe the three common types of radioactive decay, to balance nuclear equations by conserving mass number and atomic number, to distinguish fission from fusion, and to use half-life to find how much of a sample remains after a given time. Nuclear changes involve the nucleus, not the electrons, so they change the identity of the element.
Types of radioactive decay
Alpha particles are the largest and least penetrating (stopped by paper or skin); beta particles penetrate more (stopped by thin metal or wood); gamma rays are the most penetrating (requiring thick lead or concrete). All three are forms of ionizing radiation.
Balancing nuclear equations
This conservation is the key tool. To find an unknown product, make the mass numbers balance and the atomic numbers balance; the atomic number then identifies the element from the periodic table. Unlike a chemical equation, a nuclear equation changes the element because the number of protons changes.
Fission and fusion
Fission splits a heavy, unstable nucleus (such as uranium-235) into two smaller nuclei plus neutrons, releasing large amounts of energy; it powers nuclear reactors and is triggered when a nucleus absorbs a neutron. Fusion combines light nuclei (such as hydrogen isotopes) into a heavier nucleus, releasing even more energy per gram; it is the process that powers the Sun and stars. Both convert a small amount of mass into a large amount of energy, which is why nuclear changes release far more energy than chemical reactions.
Half-life
Half-life is independent of temperature, pressure or how much sample you start with, so it is a reliable clock used in radioactive dating. The amount remaining is found by counting how many half-lives have elapsed and halving the sample that many times.
Try this
Q1. Carbon-14 (atomic number ) undergoes beta decay. State the atomic number of the element formed. [1 point]
- Cue. (nitrogen); beta decay raises the atomic number by one while the mass number stays at .
Q2. Identify whether combining two hydrogen nuclei to form helium is fission or fusion. [1 point]
- Cue. Fusion; light nuclei combine to form a heavier nucleus.
Exam-style practice questions
Practice questions written in the style of VDOE exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SOL (multiple choice)1 marksA radioactive isotope has a half-life of days. What fraction of an original sample remains after days? (A) (B) (C) (D) Show worked answer →
The answer is (C) .
days is three half-lives (). After each half-life, half of the remaining sample decays. Starting at one whole: after one half-life remains, after two remains, after three remains. So of the original sample is left.
The trap is dividing by and using that as a fraction; instead count the number of half-lives and halve the amount that many times.
SOL (tech-enhanced, fill in blank)2 marksAn atom of uranium with mass number and atomic number undergoes alpha decay. (a) State the mass number and atomic number of the new element formed. (b) Identify the type of particle emitted.Show worked answer →
A 2-point nuclear-equation item.
(a) New nuclide (1 point): an alpha particle has mass number and atomic number , so the mass number drops by to and the atomic number drops by to (thorium).
(b) Particle (1 point): an alpha particle (a helium nucleus, mass number , charge ).
Markers reward conserving mass number and atomic number across the equation: the totals on each side must be equal. Alpha decay reduces both the mass number (by 4) and the atomic number (by 2).
Related dot points
- Structure of the atom: describe protons, neutrons and electrons, atomic number and mass number, and the historical development of the atomic model from Dalton to the modern view.
A focused Virginia SOL Chemistry answer on standard CH.2: the subatomic particles, atomic number and mass number, how they define an element and its ions, and the development of the atomic model from Dalton, Thomson and Rutherford to Bohr and the modern model.
- Isotopes and average atomic mass: define isotopes, write nuclide notation, and calculate the weighted average atomic mass of an element from its isotopes.
A focused Virginia SOL Chemistry answer on isotopes under CH.2: what isotopes are, how to read nuclide notation, and how to calculate the weighted average atomic mass of an element from the masses and natural abundances of its isotopes.
- Electron configuration and energy levels: describe how electrons occupy energy levels, write electron configurations, identify valence electrons, and relate ground and excited states to spectra.
A focused Virginia SOL Chemistry answer on electron arrangement under CH.2: energy levels and sublevels, writing electron configurations, counting valence electrons, and the difference between ground state and excited state and how it produces line spectra.
- The periodic table and periodic trends: describe the organization of the periodic table and the trends in atomic radius, ionization energy, electronegativity and reactivity across periods and down groups.
A focused Virginia SOL Chemistry answer on the periodic table under CH.2: how it is organized into groups, periods, metals, nonmetals and metalloids, and the trends in atomic radius, ionization energy, electronegativity and reactivity and why each runs the way it does.
- Endothermic and exothermic reactions: distinguish endothermic and exothermic processes by the direction of energy flow and the sign of the enthalpy change.
A focused Virginia SOL Chemistry answer on reaction energy under CH.6: the difference between endothermic and exothermic reactions, the direction of energy flow, the sign of the enthalpy change, and how temperature change signals each type.
Sources & how we know this
- 2018 Science Standards of Learning - Chemistry — Virginia Department of Education (2018)
- Chemistry Curriculum Framework — Virginia Department of Education (2018)