Determining the Number of Neutrons in Ions

To determine the quantity of subatomic particles within an atom or ion, one must understand the standard notation and the roles of the three primary particles: protons, neutrons, and electrons.
Atomic Number ( $Z$ ): - The atomic number represents the total number of protons found in the nucleus of an atom. - It defines the identity of the chemical element. - In the provided example, the atomic number ( $Z$ ) is specified as $7$ .
Mass Number ( $A$ ): - The mass number, also known as the nucleon number, is the sum of the protons and neutrons within the nucleus. - It is expressed by the formula: $A = Z + N$ , where $N$ represents the number of neutrons. - In the provided example, the mass number ( $A$ ) is specified as $15$ .
The Neutron ( $N$ ): - A neutron is a subatomic particle with no net electric charge and a mass slightly larger than that of a proton. - Neutrons and protons together constitute the nucleus of an atom.

Isotopic Notation: - Chemical species are often written in the format ${_{Z}^{A}X^{C}}$ , where: - $X$ is the chemical symbol. - $A$ is the mass number (top left). - $Z$ is the atomic number (bottom left). - $C$ is the net charge of the species (top right).
Specific Case Analysis: $_{7}^{15}X^{2-}$ : - The transcript identifies a species referred to as "x two minus fifteen seven." - This corresponds to an ion with an atomic number ( $Z$ ) of $7$ and a mass number ( $A$ ) of $15$ . - The "two minus" ( $2-$ ) indicates that this is an anion, which is a negatively charged ion.
The Role of Ionic Charge: - The charge of an ion ( $C$ ) is determined by the difference between the number of protons and the number of electrons: $C = \text{protons} - \text{electrons}$ . - A charge of $-2$ signifies that the ion has two more electrons than it has protons. - Crucial Distinction: The ionic charge only affects the electron count; it has no impact on the number of neutrons in the nucleus.

To find the number of neutrons ( $N$ ), one must subtract the atomic number ( $Z$ ) from the mass number ( $A$ ).
Step-by-Step Procedure: 1. Identify the Mass Number ( $A$ ): In this scenario, $A = 15$ . 2. Identify the Atomic Number ( $Z$ ): In this scenario, $Z = 7$ . 3. Apply the subtraction formula: $N = A - Z$ .
Mathematical Execution: - $N = 15 - 7$ - $N = 8$
Conclusion: - The number of neutrons in the species $_{7}^{15}X^{2-}$ is exactly $8$ .