Radioactive decay Revision Notes

Radioactive decay is a nuclear process where an unstable atomic nucleus changes into a different state to achieve greater stability. This transformation involves the emission of radiation and particles from the nucleus.

The process is spontaneous, meaning it occurs without any external cause or trigger. It is also random, so it is impossible to predict exactly when a particular nucleus will decay.

The instability arises because the nucleus has an imbalance in the forces or an excess of energy, causing it to seek a more stable configuration through decay.

• Decay is a spontaneous nuclear transformation.
• It occurs randomly and unpredictably for individual nuclei.
• Instability in the nucleus drives the decay process.

Three main types of radiation can be emitted during radioactive decay: alpha (α), beta (β), and gamma (γ) radiation. Each type has distinct properties and origins within the nucleus.

Alpha radiation consists of helium nuclei (2 protons and 2 neutrons) emitted from the nucleus.

Beta radiation involves the emission of electrons (beta-minus) or positrons (beta-plus) resulting from changes in the neutron-to-proton ratio within the nucleus, but this objective focuses on the general emission concept without detailed mechanisms or equations as per scope limitations here. Gamma radiation is electromagnetic radiation of very high frequency emitted when the nucleus transitions from a higher to a lower energy state after alpha or beta decay or other nuclear processes.

• Alpha (α) radiation: helium nuclei emitted.
• Beta (β) radiation: electrons or positrons emitted.
• Gamma (γ) radiation: high-energy electromagnetic waves emitted.

Radioactive decay is inherently spontaneous and random. This means that no external factors can cause or predict the exact moment when a particular unstable nucleus will decay.

The probability of decay for a large number of nuclei can be described statistically, but individual decay events occur unpredictably.

This randomness is a fundamental property of radioactive materials and underpins the concept of half-life, which is the average time for half the nuclei in a sample to decay (covered in other objectives).

• Decay timing is unpredictable for individual nuclei.
• Decay probability can be statistically described for many nuclei.
• Spontaneity means no external trigger is needed.

The nucleus of an atom is held together by the strong nuclear force, which balances the repulsive electromagnetic force between protons.

When the balance of forces or the neutron-to-proton ratio is not optimal, the nucleus becomes unstable.

To reach a more stable state, the nucleus undergoes transformations that emit radiation and particles, reducing excess energy or adjusting the nuclear composition.

• Strong nuclear force holds nucleus together.
• Instability arises from force imbalance or neutron-proton ratio.
• Decay transforms nucleus to a more stable state.

To draw a simple diagram representing radioactive decay:

1. Draw a circle labelled 'Unstable nucleus'. Inside, indicate 'Excess energy or imbalance'.

2. From this nucleus, draw arrows pointing outward labelled 'α', 'β', and 'γ' to represent the three types of radiation emitted during decay. Use distinct symbols: α for alpha particle, β for beta particle, and γ for gamma ray (wave symbol).

• Circle representing unstable nucleus with imbalance.
• Arrows showing emission of α, β, and γ radiation.
• Labels identifying each radiation type.

Radioactive decay is a spontaneous process where unstable atomic nuclei emit radiation to become more stable. Two common types of decay are alpha (α) decay and beta (β) decay. Both involve changes in the nucleus that result in the formation of a different element, a process known as transmutation.

• Radioactive decay changes the identity of the element by altering the number of protons in the nucleus.
• Alpha and beta decay involve emission of particles that carry away protons or electrons, changing the nucleus composition.

In alpha decay, the nucleus emits an alpha particle, which consists of two protons and two neutrons (identical to a helium-4 nucleus). This emission reduces the atomic number (number of protons) by 2 and the mass number (total protons and neutrons) by 4.

Because the atomic number decreases, the element changes to one with an atomic number two less than the original. For example, uranium-238 (atomic number 92) undergoes alpha decay to form thorium-234 (atomic number 90).

• Alpha particle = 2 protons + 2 neutrons (mass number 4, charge +2).
• Atomic number decreases by 2; mass number decreases by 4.
• Element changes to one with atomic number two less than original.

Beta decay involves the conversion of a neutron into a proton within the nucleus. This process emits a beta particle, which is an electron (β⁻), and an antineutrino (not required at this level).

The atomic number increases by 1 because a neutron becomes a proton, but the mass number remains unchanged since the total number of nucleons stays the same.

For example, carbon-14 (atomic number 6) undergoes beta decay to form nitrogen-14 (atomic number 7).

• Beta particle = electron emitted from nucleus.
• Neutron converts to proton, increasing atomic number by 1.
• Mass number remains constant.
• Element changes to one with atomic number one greater than original.

The key difference between alpha and beta decay is how the atomic and mass numbers change, which determines the new element formed.

Alpha decay decreases atomic number by 2 and mass number by 4, while beta decay increases atomic number by 1 with no change in mass number.

• Alpha decay: atomic number −2, mass number −4.
• Beta decay: atomic number +1, mass number unchanged.
• Both processes result in a different element due to change in proton number.

To draw a nuclear decay diagram: start with the original nucleus symbol showing its atomic number (Z) and mass number (A).

For alpha decay, draw an arrow pointing to the emission of an alpha particle (⁴₂He) and the daughter nucleus with atomic number Z−2 and mass number A−4.

For beta decay, draw an arrow pointing to the emission of a beta particle (⁰₋₁e) and the daughter nucleus with atomic number Z+1 and mass number A unchanged.

• Use nuclide notation: A (mass number) over Z (atomic number) before and after decay.
• Alpha particle notation: ⁴₂He.
• Beta particle notation: ⁰₋₁e (electron).
• Show changes in atomic and mass numbers clearly.

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons in their nuclei. The number of protons defines the element, while the number of neutrons affects the isotope's mass and nuclear stability.

For example, carbon has isotopes such as carbon-12 (6 protons, 6 neutrons) and carbon-14 (6 protons, 8 neutrons). Both are carbon because they have 6 protons, but their neutron numbers differ.

• Isotopes have identical proton numbers (atomic number) but different neutron numbers.
• Neutron number affects nuclear stability and mass number (protons + neutrons).

The nucleus is held together by the strong nuclear force, which acts between protons and neutrons. Stability depends on the balance between the strong nuclear force and the electrostatic repulsion between protons.

When the neutron-to-proton ratio is too high or the nucleus is very heavy, the nucleus becomes unstable. This instability causes the nucleus to undergo radioactive decay, emitting radiation to reach a more stable state.

• Strong nuclear force binds nucleons (protons and neutrons) together.
• Excess neutrons or very heavy nuclei disrupt stability.
• Unstable nuclei emit radiation spontaneously to become stable.

Radioactivity in isotopes arises mainly from two causes:

1. Excess Neutrons: Too many neutrons compared to protons can cause instability because the balance of nuclear forces is disturbed. This often leads to beta decay, where a neutron converts into a proton, emitting a beta particle (electron) and an antineutrino.

2. Excessive Nuclear Mass: Very heavy nuclei (such as uranium or thorium isotopes) have so many protons that electrostatic repulsion overcomes the strong nuclear force, causing instability. These nuclei often undergo alpha decay, emitting an alpha particle (2 protons and 2 neutrons) to reduce mass and increase stability.

• Excess neutrons cause neutron-to-proton imbalance, leading to beta decay.
• Heavy nuclei have strong proton repulsion, leading to alpha decay.
• Radioactive decay reduces instability by changing nucleon numbers.

Radioactive isotopes have unstable nuclei due to neutron excess or large nuclear mass.

They spontaneously emit radiation (alpha, beta, or gamma) to reach a more stable nuclear configuration.

The type of decay depends on the nature of the instability: neutron excess often leads to beta decay; heavy nuclei often emit alpha particles.

• Radioactivity is a nuclear property, not chemical.
• Isotopes with balanced neutron-proton ratios tend to be stable.
• Radioactive decay changes the nucleus, often transforming the element.

Atomic nuclei are composed of protons and neutrons. Stability depends on the balance between these particles. Unstable nuclei have excess neutrons or protons, causing them to undergo radioactive decay to reach a more stable state.

Radioactive decay occurs spontaneously and randomly, emitting radiation in the form of alpha (α) particles, beta (β) particles, or gamma (γ) rays. Each type of decay changes the nucleus differently, affecting its composition and energy state.

• Nuclei with too many neutrons or protons are unstable.
• Radioactive decay reduces instability by changing the nucleus.
• Alpha, beta, and gamma emissions are the main decay types.

Alpha decay involves the emission of an alpha particle, which consists of 2 protons and 2 neutrons (a helium-4 nucleus). This emission reduces the atomic number by 2 and the mass number by 4, transforming the original element into a different element with a smaller nucleus.

Because alpha particles remove both protons and neutrons, alpha decay decreases the size of the nucleus and helps reduce excess nucleons, contributing to increased nuclear stability.

Example decay equation: ^{238}_{92}U 14}_{90}Th + ^{4}_{2}He (alpha particle)

• Alpha particle = 2 protons + 2 neutrons.
• Atomic number decreases by 2; mass number decreases by 4.
• Nucleus becomes smaller and more stable.

Beta decay occurs when a neutron inside the nucleus transforms into a proton, emitting a beta particle (an electron) and an antineutrino. This process increases the atomic number by 1 while the mass number remains unchanged because a neutron is replaced by a proton.

This conversion reduces the number of excess neutrons, improving the proton-to-neutron ratio and thus increasing nuclear stability.

The emitted beta particle is a high-speed electron ejected from the nucleus, not from the electron cloud around the atom, and the antineutrino carries away some energy and momentum to conserve these quantities in the decay process.

• Neutron 1210 proton + electron (beta particle) + antineutrino.
• Atomic number increases by 1; mass number unchanged.
• Reduces excess neutrons, increasing stability.
• Beta particle is a high-speed electron emitted from the nucleus.

Gamma emission involves the release of gamma rays, which are high-energy electromagnetic waves. Unlike alpha and beta decay, gamma emission does not change the number of protons or neutrons in the nucleus.

Gamma rays are emitted when the nucleus transitions from an excited energy state to a lower energy state after alpha or beta decay. This emission helps the nucleus lose excess energy and reach a more stable configuration.

Because no particles are lost or gained, gamma emission only affects the energy state, not the identity or mass of the nucleus.

• Gamma rays are electromagnetic radiation with no mass or charge.
• No change in proton or neutron number.
• Nucleus loses excess energy to become more stable.
• Often follows alpha or beta decay.

Alpha decay reduces both the atomic number and mass number, removing nucleons and decreasing the nucleus size.

Beta decay converts a neutron into a proton, increasing the atomic number by one and reducing neutron excess without changing mass number.

Gamma emission releases energy, allowing the nucleus to reach a lower energy, more stable state without changing particle numbers.

• Alpha decay: atomic number -2, mass number -4.
• Beta decay: atomic number +1, mass number unchanged.
• Gamma emission: energy loss only, no particle change.
• All processes increase nuclear stability by balancing nucleon numbers and energy.

Nuclide notation represents a nucleus by its chemical symbol, preceded by its mass number (A) as a superscript and atomic number (Z) as a subscript. For example, \(^{A}_{Z}X\) denotes an element X with mass number A (total protons and neutrons) and atomic number Z (number of protons).

Radioactive decay involves an unstable nucleus transforming into a more stable nucleus, often emitting particles or radiation. This process changes the identity or energy state of the nucleus, which is represented using decay equations in nuclide notation.

• Mass number (A) = number of protons + neutrons.
• Atomic number (Z) = number of protons, defines the element.
• Decay equations show initial nucleus → emitted particle(s) + resulting nucleus.

An alpha particle consists of 2 protons and 2 neutrons, identical to a helium-4 nucleus, symbolised as \(^{4}_{2}He\) or \(\alpha\).

When a nucleus emits an alpha particle, its mass number decreases by 4 and atomic number decreases by 2, forming a new element.

The general alpha decay equation is: \[^{A}_{Z}X \rightarrow ^{A-4}_{Z-2}Y + ^{4}_{2}He\] where X is the parent nucleus and Y is the daughter nucleus.

• Alpha particle: \(^{4}_{2}He\), charge +2, mass 4.
• Mass number decreases by 4.
• Atomic number decreases by 2.
• Example: \(^{238}_{92}U \rightarrow ^{234}_{90}Th + ^{4}_{2}He\).

Beta decay involves the emission of a beta particle, which is an electron (\(\beta^-\)) emitted when a neutron in the nucleus converts into a proton and an electron.

The mass number remains unchanged because a neutron changes into a proton, so total nucleons stay the same.

The atomic number increases by 1, changing the element to the next higher proton number in the periodic table.

• Beta particle: electron, symbol \(^{0}_{-1}e\) or \(\beta^-\).
• Mass number (A) unchanged.
• Atomic number (Z) increases by 1.
• General equation: \[^{A}_{Z}X \rightarrow ^{A}_{Z+1}Y + ^{0}_{-1}e\]
• Example: \(^{14}_{6}C \rightarrow ^{14}_{7}N + ^{0}_{-1}e\).

Gamma radiation is high-energy electromagnetic radiation emitted from an excited nucleus returning to its ground state.

Gamma emission does not change the mass number or atomic number because no particles are lost or gained; only energy is emitted.

Gamma rays often accompany alpha or beta decay to remove excess energy from the daughter nucleus.

• Gamma ray symbol: \(\gamma\).
• No change in mass number or atomic number.
• Represents energy release only.
• Example: \(^{60}_{27}Co^* \rightarrow ^{60}_{27}Co + \gamma\), where \(^*\) denotes an excited state.

To write a decay equation, identify the parent nuclide and the type of emission.

Apply the changes in atomic and mass numbers according to the emission type.

Ensure conservation of nucleon number and charge: total mass and atomic numbers on both sides must balance, including emitted particles or radiation (gamma has zero mass and charge).

• Check mass number: decreases by 4 for alpha, unchanged for beta and gamma.
• Check atomic number: decreases by 2 for alpha, increases by 1 for beta, unchanged for gamma.
• Include emitted particle or radiation on the right side.
• Example: \(^{210}_{84}Po \rightarrow ^{206}_{82}Pb + ^{4}_{2}He\) (alpha decay).