Phosphorus pentachloride (\(\text{PCl}_5\)) represents a compound formed by one phosphorus atom and five chlorine atoms. Understanding the total number of valence electrons within this molecule is a foundational step in chemistry, providing the necessary information to determine how the atoms bond together and the resulting shape of the molecule. Valence electrons are the atomic components that participate in forming chemical bonds, which ultimately dictate the compound’s properties. By systematically accounting for the valence electrons contributed by each atom, we arrive at the total electron count available for sharing and pairing within the molecule.
What Are Valence Electrons?
Valence electrons are the electrons that occupy the outermost energy shell of an atom. These electrons interact with other atoms, making them responsible for an element’s chemical behavior and reactivity. The number of electrons in this outermost shell determines an atom’s tendency to gain, lose, or share electrons when forming compounds.
The primary function of these electrons is to facilitate the formation of chemical bonds, such as covalent bonds where electrons are shared between atoms. By engaging in bonding, atoms attempt to achieve a stable, full outer shell configuration, often containing eight electrons, a principle known as the octet rule.
Determining Valence Counts for Individual Atoms
The number of valence electrons for a main group element is determined by its position on the periodic table. For these elements, the number of valence electrons corresponds to the last digit of the group number (using the 1-18 numbering system). This relationship simplifies finding the electron count.
Phosphorus (P) is located in Group 15, indicating that a single phosphorus atom possesses five valence electrons. Chlorine (Cl) is found in Group 17, meaning one chlorine atom contributes seven valence electrons. These individual counts serve as the starting figures for calculating the total electron pool within the \(\text{PCl}_5\) molecule.
Calculating the Total Valence Electrons in \(\text{PCl}_5\)
To find the total number of valence electrons for the entire \(\text{PCl}_5\) molecule, the valence count of each individual atom must be summed. The formula specifies one phosphorus atom and five chlorine atoms. This structure requires multiplying the valence count of chlorine by five before adding the phosphorus contribution.
The calculation begins with the single phosphorus atom, which contributes five valence electrons. Next, the contribution from the five chlorine atoms is determined by multiplying the seven valence electrons of a single chlorine atom by five \((7 \times 5 = 35)\). Summing these two figures yields the total count.
The complete calculation is \(5 + 35\), resulting in a final total of 40 valence electrons for the \(\text{PCl}_5\) molecule. This number represents the absolute total of electrons that must be positioned around and between the atoms in the molecule’s structure.