Question

What happens to the mass number and the atomic number of an atom when its nucleus: (a) Ejects a beta particle? (b) Ejects a positron? (c) Undergoes electron capture? (d) Ejects an alpha particle?

Short answer

(a) When an atom ejects a beta particle, the atomic number (Z) increases by 1 while the mass number (A) remains unchanged. (b) When an atom ejects a positron, the atomic number (Z) decreases by 1 while the mass number (A) remains unchanged. (c) In electron capture, the atomic number (Z) decreases by 1 while the mass number (A) remains unchanged. (d) When an atom ejects an alpha particle, the atomic number (Z) decreases by 2 and the mass number (A) decreases by 4.

Step-by-step solution

(a) Ejection of a beta particle

When an atom ejects a beta particle, a neutron in the nucleus converts into a proton, and a beta particle (an electron) is emitted. The atomic number (Z) increases by 1, as there is an additional proton, while the mass number (A) remains the same because the total number of protons and neutrons does not change.

(b) Ejection of a positron

When an atom ejects a positron, a proton in the nucleus converts into a neutron, and a positron is emitted. The atomic number (Z) decreases by 1, as there is one less proton, while the mass number (A) remains the same because the total number of protons and neutrons does not change.

(c) Electron capture

In electron capture, an electron from the innermost shell is captured by the nucleus and combines with a proton to form a neutron. This process reduces the number of protons by 1 and increases the number of neutrons by 1. The atomic number (Z) decreases by 1, as there is one less proton, while the mass number (A) remains the same because the total number of protons and neutrons does not change.

(d) Ejection of an alpha particle

When an atom ejects an alpha particle, it loses two protons and two neutrons from its nucleus. The alpha particle is composed of these two protons and two neutrons. The atomic number (Z) decreases by 2, as there are two fewer protons, and the mass number (A) decreases by 4, as there are two fewer protons and two fewer neutrons.

Key concepts

Beta Particle Ejection

Beta particle ejection is one of the nuclear processes that atoms can undergo. In this event, a neutron within an atom's nucleus is transformed into a proton, and a beta particle—which is essentially an electron—is emitted. This transformation leads to an increase in the atomic number by 1 because the atomic number corresponds to the total count of protons within the nucleus. Despite the change in atomic makeup, the mass number remains unchanged. The mass number represents the total number of protons and neutrons, and since one neutron is replaced by one proton, the overall count does not vary.

In practical scenarios, beta particle ejection often occurs in radioactive isotopes, which are forms of elements that have unstable nuclei. The ejection of the beta particle helps to stabilize the nucleus, moving it toward a more stable state. As students explore these concepts, it's beneficial to remember that the emitted beta particle is a high-energy, high-speed electron, which is why it's considered a form of ionizing radiation, capable of penetrating materials and potentially causing chemical changes.

Positron Ejection

Positron ejection, or positron emission, is another type of nuclear reaction that can lead to the transformation of an atomic nucleus. During this process, a proton within the nucleus is converted into a neutron, and a positron—the antiparticle of an electron—is emitted. As opposed to beta particle ejection, the emission of a positron decreases the atomic number by 1, given that there is one less proton in the nucleus. The mass number remains the same because the sum of the protons and neutrons does not change as a result of this single proton-neutron exchange.

Positron emission is a common avenue for certain unstable isotopes to attain stability. It is important for students to comprehend that the emitted positron is a form of antimatter, which will typically annihilate with an electron, producing gamma radiation. This phenomenon is utilized in medical imaging techniques such as Positron Emission Tomography (PET) scans, providing practical applications in diverse fields like oncology and neurology.

Electron Capture

Electron capture is a process through which an inner shell electron is pulled into the nucleus and merges with a proton, forming a neutron. As a result, the atomic number is reduced by 1 because the nucleus has one less proton after the capture. Interestingly, the mass number doesn't change. This may seem counterintuitive at first, but it's because the newly created neutron takes the place of the proton, maintaining the overall count of nucleons.

It's fundamental for students to recognize electron capture as one of the two processes where electron involvement leads to a decrease in the atomic number. In the natural world and in various technologies, electron capture plays a vital role. For instance, many isotopes that undergo this process will emit characteristic X-rays, which are useful for analytical techniques in material science and chemistry.

Alpha Particle Ejection

Alpha particle ejection occurs when an atom emits an alpha particle, which consists of two protons and two neutrons bound together. This event causes the atom to lose a chunk of its mass and results in a decrease in both the atomic number and the mass number. Specifically, the atomic number drops by 2 due to the loss of the two protons, and the mass number, which accounts for the total number of protons and neutrons, falls by 4.

The ejection of alpha particles is a common decay mode for very heavy elements, such as uranium and radium. These emitted alpha particles are relatively heavy and carry a double positive charge, but they are not very penetrating due to their size. While they pose little risk when outside the body, they can be quite damaging if inhaled or ingested. In historical studies of atomic structure, alpha particles were famously used in Rutherford’s gold foil experiment, leading to the discovery of the nucleus. Their understanding is crucial for grasping the basics of nuclear physics and its implications in both natural and engineered nuclear reactions.