Radioactive decay is a spontaneous, random process where an unstable nucleus changes into a more stable one by emitting particles or energy. The AP CED lists four types you should know: alpha (He-4 nucleus ejected), beta-minus (a neutron → proton + electron + antineutrino), beta-plus (a proton → neutron + positron + neutrino), and gamma (an excited nucleus emits a photon). In every decay nucleon number, charge, and lepton number are conserved. Decay is probabilistic: each nucleus has a fixed chance per unit time to decay (characterized by a decay constant or half-life), so you can predict rates for large samples but not when a single nucleus will decay. Neutrinos/antineutrinos carry away tiny energy and barely interact with matter. You don’t need to memorize which isotopes decay how or specific half-lives for the AP exam. For a focused review, check the Topic 15.8 study guide (https://library.fiveable.me/ap-physics-2-revised/unit-7/8-types-of-radioactive-decay/study-guide/yQriAHMZEcNvi6kE) and more unit resources (https://library.fiveable.me/ap-physics-2-revised/unit-15); practice questions are at (https://library.fiveable.me/practice/ap-physics-2-revised).

Alpha, beta, and gamma decays are different ways an unstable nucleus changes to get more stable—and AP expects you to know how each conserves nucleon number, lepton number, and charge (CED 15.8.A.2.i). - Alpha (α): the nucleus ejects a He-4 nucleus (2 protons + 2 neutrons). Atomic number ↓ by 2, mass (nucleon) number ↓ by 4. Symbol: α or He2+ (CED 15.8.A.1.i, 15.8.A.2.ii). - Beta-minus (β−): a neutron becomes a proton and emits an electron and an antineutrino. Atomic number ↑ by 1, nucleon number unchanged (CED 15.8.A.2.iii). - Beta-plus (β+): a proton becomes a neutron and emits a positron and a neutrino. Atomic number ↓ by 1, nucleon number unchanged (CED 15.8.A.2.iv). - Gamma (γ): an excited nucleus drops to a lower energy state and emits a photon. No change in nucleon or charge—often follows α or β decay (CED 15.8.A.2.v). For a quick AP-aligned review, see the Topic 15.8 study guide (https://library.fiveable.me/ap-physics-2-revised/unit-7/8-types-of-radioactive-decay/study-guide/yQriAHMZEcNvi6kE) and try practice problems (https://library.fiveable.me/practice/ap-physics-2-revised).

Some nuclei “randomly” decay because they’re intrinsically unstable—they have a combination of protons and neutrons (an isotope) that has higher energy than a more stable arrangement. Quantum mechanics makes decay a probabilistic process: each unstable nucleus has a constant probability per unit time (decay constant) of undergoing a particular decay mode (α, β−, β+, or γ). Which mode can happen is fixed by the isotope and by conservation laws (nucleon number, charge, lepton number). Stable nuclei have low enough nuclear binding energy and a favorable proton/neutron balance, so they have essentially zero probability of spontaneous decay. Unstable ones have measurable half-lives (seconds to billions of years) that tell you how likely decay is over time—but you can’t predict when any single nucleus will decay, only the statistics for many nuclei. For a concise AP-aligned review of these ideas, see the Topic 15.8 study guide on Fiveable (https://library.fiveable.me/ap-physics-2-revised/unit-7/8-types-of-radioactive-decay/study-guide/yQriAHMZEcNvi6kE). For more practice, check the unit overview (https://library.fiveable.me/ap-physics-2-revised/unit-15) and the AP Physics 2 practice bank (https://library.fiveable.me/practice/ap-physics-2-revised).

An alpha particle is just a helium nucleus: two protons and two neutrons bound together (He-4), written as α or He2+. It has charge +2 and is relatively heavy compared to single subatomic particles, so when a nucleus emits one (alpha decay) the parent nucleus loses 4 in its nucleon number and 2 in its atomic number (mass number −4, charge −2). Alpha particles interact strongly with matter, so they don’t penetrate far—a sheet of paper or your skin can stop them—but they carry a lot of kinetic energy. On the AP Physics 2 CED you only need He-4 as the alpha particle (symbol α or He2+) and to remember that nucleon number, charge, and lepton number are conserved in decays. For a short review, see the Topic 15.8 study guide (https://library.fiveable.me/ap-physics-2-revised/unit-7/8-types-of-radioactive-decay/study-guide/yQriAHMZEcNvi6kE); for more practice problems check the unit page (https://library.fiveable.me/ap-physics-2-revised/unit-15) and the practice bank (https://library.fiveable.me/practice/ap-physics-2-revised).

Beta-minus (β−) and beta-plus (β+) are opposite kinds of beta decay that change a nucleon and emit a lepton so conservation laws hold. - β− decay: a neutron turns into a proton inside the nucleus and the nucleus emits an electron (β−) plus an antineutrino. Example summary: n → p + e− + ν̄. Charge, nucleon number, and lepton number are conserved (neutron → proton keeps nucleon number; emitted electron adds one lepton and the antineutrino cancels lepton number sign). - β+ decay: a proton turns into a neutron and the nucleus emits a positron (β+, the antielectron) plus a neutrino. Example: p → n + e+ + ν. Again charge, nucleon number, and lepton number are conserved. Remember: AP Physics 2 expects you to state these processes and the conservation rules, but you don’t need to memorize which isotopes decay which way (see the Topic 15.8 study guide for a quick review: https://library.fiveable.me/ap-physics-2-revised/unit-7/8-types-of-radioactive-decay/study-guide/yQriAHMZEcNvi6kE). For more practice, check Unit 15 resources and the practice set (https://library.fiveable.me/ap-physics-2-revised/unit-15) and (https://library.fiveable.me/practice/ap-physics-2-revised).

A neutrino is a neutral, very low-mass subatomic particle produced in nuclear decays (like beta decay). Per the AP CED, neutrinos (v) and antineutrinos (v̄) have no electric charge and negligible mass, and they’re counted in lepton-number conservation for decays (e.g., a neutron → proton + electron + antineutrino). They hardly interact with “normal” matter because they don’t feel the electromagnetic force and interact only via the weak force (and gravity). The weak interaction is extremely short-range and much weaker than the electromagnetic or strong forces, so neutrinos mostly pass through atoms without affecting them—trillions pass through you every second with almost no collisions. For AP Physics 2 you only need to know their basic properties and that they rarely interact; you don’t need to explain the weak force details. Review Topic 15.8 in the Fiveable study guide (https://library.fiveable.me/ap-physics-2-revised/unit-7/8-types-of-radioactive-decay/study-guide/yQriAHMZEcNvi6kE) and practice more with the unit overview (https://library.fiveable.me/ap-physics-2-revised/unit-15) or Fiveable’s practice problems (https://library.fiveable.me/practice/ap-physics-2-revised).

You figure out the decay type from the nucleus’s imbalance, not by memorizing each isotope. Key rules (from the CED): conservation of nucleon number, charge, and lepton number always hold. Quick heuristics you’ll use on the exam-level problems: - Too many neutrons → beta-minus (n → p + e− + antineutrino). A neutron-rich nucleus converts a neutron to a proton. - Too many protons → beta-plus (p → n + e+ + neutrino)—positron emission (AP doesn’t require electron capture details). - Very heavy nuclei (large A, especially > ~200) → alpha emission (ejects a He-4 nucleus, lowers A by 4 and Z by 2). - After alpha/beta transitions the nucleus may be left excited → gamma emission (photon) with no change to A or Z. You don’t need to memorize which isotope does what for the AP exam (CED boundary), but you should be able to apply these conservation and imbalance ideas. For a focused review, see the Topic 15.8 study guide (https://library.fiveable.me/ap-physics-2-revised/unit-7/8-types-of-radioactive-decay/study-guide/yQriAHMZEcNvi6kE) and practice problems at Fiveable (https://library.fiveable.me/practice/ap-physics-2-revised).

Gamma decay is when an excited nucleus drops to a lower energy state by emitting a high-energy photon (a gamma, γ). It doesn’t change the number of protons or neutrons (nucleon number) or the charge—it just carries away excess energy. In the CED language: gamma decay occurs after an alpha or beta decay when the daughter nucleus is left in an excited state and relaxes by emitting a photon (15.8.A.2.v). Why it comes after alpha or beta: alpha or beta emission often leaves the remaining nucleus with extra internal energy. That nucleus then transitions to a lower-energy configuration and emits a gamma photon to conserve energy. Conservations required by AP (nucleon number, charge, lepton number) still hold because gamma emission changes only the nucleus’ energy state, not its particle counts. For a quick review, see the Topic 15.8 study guide (https://library.fiveable.me/ap-physics-2-revised/unit-7/8-types-of-radioactive-decay/study-guide/yQriAHMZEcNvi6kE) and more unit review (https://library.fiveable.me/ap-physics-2-revised/unit-15). Practice problems are also available (https://library.fiveable.me/practice/ap-physics-2-revised).

A positron is called an antielectron because it’s the electron’s antiparticle: it has the same mass as an electron but the opposite electric charge (+e instead of −e) and opposite lepton quantum numbers (CED 15.8.A.1.iv). That’s the key difference—same mass, opposite charge. In beta-plus (β+) decay a proton in the nucleus changes to a neutron and emits a positron and a neutrino (CED 15.8.A.2.iv), which conserves charge and lepton number. If a positron meets an electron they can annihilate, producing photons. You don’t need to learn the details of the weak force for AP Physics 2, but remember the CED point that positrons are just positively charged counterparts of electrons and appear in β+ decay. For a quick refresh see the Topic 15.8 study guide (https://library.fiveable.me/ap-physics-2-revised/unit-7/8-types-of-radioactive-decay/study-guide/yQriAHMZEcNvi6kE) and try practice problems (https://library.fiveable.me/practice/ap-physics-2-revised).

Conservation of charge and nucleon number means that during any nuclear decay the total electric charge and the total number of nucleons (protons + neutrons) stay the same before and after the decay. - Charge: add up the charges of the original nucleus and any emitted particles (like electrons e–, positrons e+, alpha particles He2+). The sum after the decay equals the original nucleus’s charge. For example, in β– decay a neutron → proton + electron + antineutrino: the nucleus gains one proton while an electron carries away –1 charge, so net charge is unchanged. - Nucleon number (mass number A): count protons + neutrons. In alpha decay the nucleus loses 2 protons and 2 neutrons (an alpha particle), so A decreases by 4 in both the emitted particle and the remaining nucleus combined—total A conserved. The CED explicitly states these conservation rules for alpha, β–, β+ and γ decays (see Topic 15.8). For a quick review, check the Fiveable study guide (https://library.fiveable.me/ap-physics-2-revised/unit-7/8-types-of-radioactive-decay/study-guide/yQriAHMZEcNvi6kE) and hit practice problems at (https://library.fiveable.me/practice/ap-physics-2-revised) to get used to applying these conservation checks on decay equations.

In beta-minus (β–) decay a neutron inside the nucleus turns into a proton while emitting an electron and an antineutrino. AP-level takeaway: this happens so charge, nucleon number, and lepton number are all conserved. Before: neutron (neutral). After: proton (+1 charge) + electron (−1 charge) + antineutrino (neutral). Nucleon number stays the same because neutrons + protons count doesn’t change; lepton number stays balanced because the emitted electron (lepton number +1) is paired with an antineutrino (lepton number −1). The CED statement you should know is 15.8.A.2.iii: “Beta-minus decay occurs when a neutron changes to a proton by emitting an electron and antineutrino.” If you want a quick review for Topic 15.8, see the Fiveable study guide (https://library.fiveable.me/ap-physics-2-revised/unit-7/8-types-of-radioactive-decay/study-guide/yQriAHMZEcNvi6kE). For extra practice, use the AP Physics 2 practice set (https://library.fiveable.me/practice/ap-physics-2-revised).

Short answer: neutrinos (ν) and antineutrinos (ν̄) are neutral, almost-massless particles that barely interact with matter. In AP Physics 2 you only need to know their roles in beta decay and that they’re distinct symbols. What matters for Topic 15.8: - Beta-minus: neutron → proton + electron + antineutrino (ν̄). - Beta-plus: proton → neutron + positron + neutrino (ν). - Conservation rules (on the exam): nucleon number, electric charge, and lepton number are conserved. Emitted electrons are leptons (+1), so an antineutrino (lepton number −1) is emitted in β− to keep total lepton number constant; in β+ a neutrino is emitted. - The CED intentionally keeps details minimal: you don’t need to explain the weak force or deep particle distinctions beyond symbols and these decay roles. For a quick review, see the Topic 15.8 study guide (https://library.fiveable.me/ap-physics-2-revised/unit-7/8-types-of-radioactive-decay/study-guide/yQriAHMZEcNvi6kE) and more unit review at (https://library.fiveable.me/ap-physics-2-revised/unit-15). Practice problems are at (https://library.fiveable.me/practice/ap-physics-2-revised).

Neutrinos barely interact because they only feel the weak and gravitational forces (CED 15.8.A.1.iii), so you detect them indirectly. Detectors use huge target masses (tons to cubic kilometers) so even the tiny interaction probability yields some events. When a neutrino occasionally interacts with a nucleus or electron it produces a charged particle (like an electron or positron) that moves faster than light in that medium and emits Cherenkov light, or it produces ionization/secondary particles. Arrays of photosensors or PMTs pick up the flashes (example technologies: water Cherenkov—Super-Kamiokande—or ice Cherenkov—IceCube) and timing/geometry let scientists infer the neutrino’s direction and energy. Long exposure times, shielding from other radiation, and careful background subtraction are essential. For AP study, remember the CED point that neutrinos have negligible mass and very little interaction with normal matter; see the Topic 15.8 study guide (https://library.fiveable.me/ap-physics-2-revised/unit-7/8-types-of-radioactive-decay/study-guide/yQriAHMZEcNvi6kE) and Unit 15 overview (https://library.fiveable.me/ap-physics-2-revised/unit-15). For practice questions, check Fiveable’s problem set (https://library.fiveable.me/practice/ap-physics-2-revised).

Because each isotope has a different number of protons and neutrons, its nucleus has a different balance of forces and energy. The type of decay that occurs is simply the easiest way for that specific nucleus to move toward a more stable, lower-energy state while conserving nucleon number, charge, and lepton number (CED 15.8.A.2.i). Practical patterns you should remember for AP-level reasoning: - Very heavy nuclei tend to lose an alpha (He-4) to reduce both protons and neutrons. - Neutron-rich isotopes favor beta-minus (n → p + e− + ν̄) to convert a neutron to a proton. - Proton-rich isotopes favor beta-plus (p → n + e+ + ν) when energetically allowed, or electron capture (not in AP2 scope). - Gamma decay happens when an excited daughter nucleus drops to a lower energy without changing composition. You don’t need to memorize which isotope decays which way for the exam—just understand these mechanisms and conservation rules. For a quick refresher, see the Topic 15.8 study guide (https://library.fiveable.me/ap-physics-2-revised/unit-7/8-types-of-radioactive-decay/study-guide/yQriAHMZEcNvi6kE) and practice problems (https://library.fiveable.me/practice/ap-physics-2-revised).

Alpha decay: 1. A heavy nucleus ejects an alpha particle (He-4 nucleus: 2 protons + 2 neutrons). 2. Nucleon number decreases by 4, charge (atomic number) decreases by 2. 3. Energy is shared as kinetic energy of the alpha and recoil of the daughter nucleus. 4. Nucleon number and charge are conserved overall (you can check by writing parent → daughter + α). Beta-minus (β–) decay: 1. A neutron inside the nucleus converts to a proton. 2. The nucleus emits an electron (β–) and an antineutrino (ν̄). 3. Charge increases by +1 (neutron → proton), nucleon number stays the same. 4. Lepton number is conserved: emitted electron (+1) balanced by emitted antineutrino (−1). Beta-plus (β+) decay: 1. A proton converts to a neutron inside the nucleus. 2. The nucleus emits a positron (β+) and a neutrino (ν). 3. Charge decreases by 1, nucleon number unchanged. 4. Lepton number conserved: positron (+1) with emitted neutrino (−1 for antileptons? AP scope: just know neutrinos/antineutrinos are emitted). Gamma (γ) decay: 1. After α or β decay the daughter nucleus may be in an excited state. 2. The nucleus drops to a lower energy level and emits a high-energy photon (γ). 3. Nucleon number and charge don’t change—only energy is released. You should be able to describe these step-by-step and check conservation of nucleon number, charge, and lepton number on the exam. For a short review, see the Fiveable study guide for Topic 15.8 (https://library.fiveable.me/ap-physics-2-revised/unit-7/8-types-of-radioactive-decay/study-guide/yQriAHMZEcNvi6kE).