Problem 12 - Chemistry Matter and Change

Question

A Describe what happens to unstable nuclei.

Step-by-Step Solution

Answer

Unstable nuclei undergo various decay processes to reach a more stable state, including alpha decay, beta decay, and gamma decay. In alpha decay, the nucleus emits an alpha particle, resulting in a change in both atomic mass and atomic number. Beta decay involves the conversion of a neutron into a proton or vice versa, accompanied by the emission of a beta-minus or beta-plus particle. Gamma decay involves the emission of high-energy photons called gamma rays, which reduce the energy of the excited nucleus to a more stable state without changing the atomic mass or atomic number.

Step 1: Understanding Unstable Nuclei

Unstable nuclei are atomic nuclei that have an excess of energy or mass, making them unstable. This often occurs when there is an imbalance between the number of protons and neutrons in the nucleus or when the nucleus is too large. As a result, these nuclei tend to lose energy and approach a more stable state through various decay processes.

Step 2: Alpha Decay

Alpha decay is a process in which an unstable nucleus emits an alpha particle, which consists of two protons and two neutrons. This results in a reduction of the atomic mass number by 4 and the atomic number by 2. The new atom formed after the alpha decay has a more stable nucleus. For example, when a uranium-238 nucleus undergoes alpha decay, it emits an alpha particle and becomes thorium-234.

Step 3: Beta Decay

Beta decay is a process in which an unstable nucleus converts a neutron into a proton or a proton into a neutron. There are two types of beta decay: a) Beta-minus decay: A neutron in the nucleus is converted into a proton, and an electron (called beta-minus particle) is emitted. The atomic number increases by 1, while the mass number remains the same. An example is the decay of carbon-14 to nitrogen-14. b) Beta-plus decay: A proton in the nucleus is converted into a neutron, and a positron (called beta-plus particle) is emitted. The atomic number decreases by 1, while the mass number remains unchanged. An example is the decay of sodium-22 to neon-22.

Step 4: Gamma Decay

Gamma decay is a process in which an unstable nucleus loses excess energy by emitting a high-energy photon called a gamma ray. Unlike alpha and beta decay, this process does not change the atomic number or mass number of the nucleus. Instead, it reduces the energy of the excited nucleus to a more stable state. For example, a cobalt-60 nucleus emits gamma radiation after undergoing beta decay to become nickel-60 in a more stable state.

Step 5: Summary

In conclusion, unstable nuclei undergo various decay processes to reach a more stable state. The three main decay processes are alpha decay, beta decay, and gamma decay. Alpha decay involves the emission of an alpha particle, resulting in a change in both atomic mass and atomic number. Beta decay involves the conversion of a neutron into a proton or vice versa and is accompanied by the emission of a beta-minus or beta-plus particle. Gamma decay involves the emission of high-energy photons called gamma rays, which reduce the energy of the excited nucleus to a more stable state without changing the atomic mass or atomic number.

Key Concepts

Unstable Nuclei

Unstable nuclei are like the ticking clocks of the atomic world. They are atoms that have an imbalance of neutrons and protons, leading to an excess of energy or mass. This can make them unstable.
There are key reasons why a nucleus becomes unstable:

• A mismatch between the number of protons and neutrons.
• An overall large size, making it hard for the nuclear strong force to hold everything together.

To find balance, these nuclei undergo radioactive decay processes. These processes allow them to lose energy and thus become more stable. It’s nature’s way of seeking equilibrium at the atomic level. Once stable, the atoms have less energy and are less likely to undergo further changes.

Alpha Decay

Alpha decay is a form of nuclear decay where an unstable nucleus releases an alpha particle.
An alpha particle is essentially composed of two protons and two neutrons - the same as a helium nucleus. During alpha decay:

• The atomic number of the nucleus decreases by 2.
• The atomic mass number decreases by 4.

This process results in the formation of a new, more stable atom. For instance, when uranium-238 undergoes alpha decay, it becomes thorium-234.
This type of decay is common in heavy elements because it helps them lose excess weight and protons, steering them to a more balanced state.

Beta Decay

Beta decay comes in two flavors: beta-minus and beta-plus decay. Both involve conversions within the nucleus to achieve stability.
- **Beta-minus decay**: A neutron turns into a proton. It results in the emission of an electron (beta-minus particle). This increases the atomic number by 1, but the atomic mass remains unchanged. An example is the decay of carbon-14 to nitrogen-14.
- **Beta-plus decay**: A proton is converted into a neutron and a positron (beta-plus particle) is emitted. The atomic number decreases by 1 without a change in mass number, like in the decay of sodium-22 to neon-22.
Beta decay affects the identity of the element by altering the number of protons, helping it to move toward a more stable state. Each time, an electron or positron is expelled, which carries away energy and helps in stabilizing the nucleus.

Gamma Decay

Gamma decay involves the emission of gamma rays, which are high-energy photons. This process allows nuclei that are left in an excited state after prior decay (like alpha or beta) to rid themselves of excess energy and settle down without altering their fundamental identity.

• Unlike alpha or beta decay, gamma decay does not change the atomic number or the mass number.
• It merely rids the nucleus of surplus energy.

So, if a cobalt-60 nucleus undergoes beta decay, it may emit gamma rays to become a stable nickel-60 nucleus. This is like the nucleus "sighing" with relief after reaching a comfortable energy state.
Gamma decay plays a crucial role in allowing nuclei to achieve their lowest energy form, ensuring the atom becomes as stable as possible.