Problem 61 - Introductory Chemistry Atoms First

Question

What happens to an atom's atomic number and mass number when it undergoes alpha emission?

Step-by-Step Solution

Answer

When an atom undergoes alpha emission, its atomic number decreases by 2 (\(Z - 2\)) and its mass number decreases by 4 (\(A - 4\)).

Step 1: Introduction to Alpha Emission

Alpha emission is a type of radioactive decay in which an unstable atomic nucleus loses two protons and two neutrons in the form of an alpha particle. The alpha particle is essentially a helium-4 nucleus, which consists of 2 protons and 2 neutrons.

Step 2: Impact of Alpha Emission on Atomic Number

The atomic number (Z) of an element is the number of protons in its nucleus. Since an alpha particle contains 2 protons, when an atom undergoes alpha emission, it loses 2 protons in the process. Thus, the atomic number of the resulting atom decreases by 2. In other words, if the original atomic number was \(Z\), the atomic number of the new atom after alpha emission will be \(Z - 2\).

Step 3: Impact of Alpha Emission on Mass Number

The mass number (A) of an atom is the total number of protons and neutrons in its nucleus. An alpha particle contains a total of 4 nucleons (2 protons and 2 neutrons). Therefore, when an atom undergoes alpha emission and releases an alpha particle, it loses a total of 4 nucleons. Consequently, the mass number of the resulting atom decreases by 4. If the original mass number was \(A\), the mass number of the new atom after alpha emission will be \(A - 4\). In summary, when an atom undergoes alpha emission, its atomic number decreases by 2 and its mass number decreases by 4.

Key Concepts

Radioactive Decay

Radioactive decay is a natural process that transforms unstable atomic nuclei into more stable ones. It occurs because some nuclei have too much energy or the wrong neutron-to-proton ratio. This results in the emission of particles or energy. There are different types of radioactive decay, such as alpha decay, beta decay, and gamma decay.

• **Alpha decay** involves the release of an alpha particle, which we will discuss further in this article.
• **Beta decay** results in the conversion of a neutron into a proton or vice versa.
• **Gamma decay** involves the release of energy without changing the number of protons or neutrons.

Each type helps atoms reach a stable energy state. Radioactive decay keeps occurring until a stable isotope is formed. During this time, the atom can change its identity completely.

Atomic Number

The atomic number is crucial in identifying the nature of an element. This number signifies how many protons reside in an atom's nucleus. Protons carry a positive charge and determine the element's chemical properties. When an atom undergoes alpha emission, two protons are lost as they exit in the form of an alpha particle. As a result, the element transforms into a different element. For example, if an atom has an original atomic number of 92, it will become an element with an atomic number of 90 after alpha emission, as the atomic number decreases by 2.

Mass Number

A mass number represents the total number of protons and neutrons in an atom's nucleus. It's a key factor in understanding an atom's overall mass. Unlike the atomic number, which changes the element, the mass number gives insight into the isotope of that element. During alpha decay, an alpha particle, which consists of 2 protons and 2 neutrons, is released. This results in a decrease of the mass number by 4. So, if an atom initially has a mass number of 238, it will have a mass number of 234 after shedding an alpha particle.

Alpha Particle

An alpha particle is a small, dense component released during alpha decay. It consists of 2 protons and 2 neutrons, making it identical to a helium-4 nucleus. Its release allows the original atom to transform into a more stable form. Some key properties of alpha particles include:

• They have a double positive charge due to the 2 protons.
• They are less penetrating than other forms of radiation like beta particles or gamma rays.
• However, they can still cause significant ionization along their path.

While alpha particles can be stopped by something as thin as paper, they remain crucial in understanding the process of radioactive decay and the transformation of atoms.