Question

By how many units does the mass number of a nucleus change when the nucleus produces an alpha particle? By how many units does the mass number of a nucleus change when the nucleus produces a beta particle? Is each change an increase or decrease in mass number?

Short answer

The mass number of a nucleus decreases by 4 units when it produces an alpha particle and does not change when it produces a beta particle. For alpha decay, the change is a decrease in mass number, while for beta decay, there is no change in the mass number.

Step-by-step solution

Identify the particles involved in alpha and beta decay

An alpha particle (α) consists of 2 protons and 2 neutrons, which means it has a mass number of 4 units. A beta particle (β) is an electron and has a very small mass, so it does not significantly affect the mass number of the nucleus.

Determine the mass number change for alpha decay

When a nucleus produces an alpha particle, it loses 2 protons and 2 neutrons. The mass number decreases by 4 units (the mass of an alpha particle). It is a decrease in the mass number.

Determine the mass number change for beta decay

When a nucleus produces a beta particle, it is due to a neutron converting into a proton within the nucleus. Since the mass of a neutron is roughly equal to the mass of a proton, and the mass of a beta particle (electron) is negligible, the mass number does not change during beta decay. Therefore, there is no change in the mass number.

Summarize the results and answer the questions

The mass number of a nucleus changes by 4 units when the nucleus produces an alpha particle (a decrease in mass number). When the nucleus produces a beta particle, there is no change in the mass number.

Key concepts

Alpha Decay

In the fascinating world of nuclear physics, alpha decay represents a type of radioactive decay where an unstable atomic nucleus emits an alpha particle, significantly altering its identity. An alpha particle, symbolized as $\alpha$, is essentially a helium nucleus composed of two protons and two neutrons, bound together into a particle identical to a helium-4 nucleus.

When an alpha particle is emitted, the original atom loses a chunk of its mass and takes a step down in the periodic table. The mass number of the atomic nucleus, which is the total number of protons and neutrons, decreases by 4 during this process. So, when confronted with the question of how alpha decay affects an atom’s mass number, the answer is a decrease by 4 units. For instance, if uranium-238 undergoes alpha decay, it transforms into thorium-234, shedding its alpha particle and, correspondingly, reducing its mass number by 4.

Understanding the Mass Number Change

This decrease in mass number is not just a numerical change but has implications for the chemical and physical properties of the element. It transforms into a different element altogether, moving two places back in the periodic table due to the loss of two protons encompassed in the alpha particle.

Beta Decay

Beta decay is another genre of radioactive decay, quite a contrast to its alpha counterpart. It involves a more subtle transformation within the nucleus of an atom. There are two types of beta decay: beta-minus (${\beta }^{-}$) and beta-plus (${\beta }^{+}$), but for this explanation, we'll focus on beta-minus decay which is commonly referred to when mentioning beta decay.

This process occurs when a neutron in the nucleus is transformed into a proton and emits a beta particle, which is a fast-moving electron. Despite this internal reshuffle, the mass number of the atom remains untouched. Why? Because the mass number accounts for protons and neutrons, but not electrons. The beta particle has an insignificant mass in comparison to nucleons (protons and neutrons), and thus, the mass number–this sum of protons and neutrons in the nucleus–stays constant.

No Change in Mass Number

The proton count increases by one, which means the atomic number, not the mass number, increases during beta decay. Consequently, the element transmutes to its neighboring element in the periodic table with an unchanged mass number. For example, carbon-14 transforms into nitrogen-14 while maintaining its mass number after emitting a beta particle during decay.

Mass Number Change

The mass number is a critical factor in nuclear chemistry, effectively determining the specific isotope of an element. It represents the sum of protons and neutrons in the atomic nucleus. Isotopes of an element have the same number of protons but differ in their number of neutrons, hence showing variations in their mass numbers.

When addressing nuclear decay, the mass number can change significantly in the case of alpha decay, or it can remain constant as observed in beta decay. It is essential to differentiate between these types of decay because they do not only alter the identity of an element but also affect the stability and radioactivity of isotopes.

Why Mass Number Is Key

These changes in mass number are paramount in applications such as radiometric dating, medical imaging, and nuclear power generation, where the precise identity and properties of isotopes are utilized. A clear understanding of how alpha and beta decays influence the mass number helps students piece together the complex puzzle of nuclear reactions and their far-reaching implications.