Valence describes an atom’s combining capacity with other atoms. It indicates the number of chemical bonds an atom typically forms when creating molecules or compounds. This property helps explain how different elements interact and build diverse substances.
Understanding Valence
Valence relates directly to an atom’s outermost electrons, known as valence electrons, which are involved in chemical bonding. Atoms generally strive for a stable electron configuration, often by achieving eight electrons in their outermost shell, a principle known as the octet rule. This stability can be met by gaining, losing, or sharing electrons. The number of electrons an atom needs to gain, lose, or share to reach this stable state determines its valence.
The Valences of Phosphorus
Phosphorus, a nonmetal element, typically exhibits valences of 3 and 5. A neutral phosphorus atom has 15 electrons, with an electron configuration of 1s² 2s² 2p⁶ 3s² 3p³. This means phosphorus has five valence electrons in its outermost shell: two in the 3s orbital and three in the 3p orbital.
Phosphorus can achieve a valence of 3 by sharing its three unpaired electrons in the 3p orbitals. This allows it to form three covalent bonds to complete its octet. It can also exhibit a valence of 5, which is possible due to empty 3d orbitals in its electron structure. This allows phosphorus to “expand” its octet, meaning it can accommodate more than eight electrons in its outermost shell. By promoting an electron from a filled 3s orbital to an empty 3d orbital, phosphorus can have five unpaired electrons available for bonding.
How Phosphorus Forms Compounds
The different valences of phosphorus are evident in the variety of compounds it forms. For instance, in phosphine (PH₃), phosphorus displays a valence of 3. The phosphorus atom forms three single bonds with three hydrogen atoms, and it retains one lone pair of electrons. This configuration satisfies its combining capacity of three.
When phosphorus forms compounds like phosphorus pentachloride (PCl₅), it exhibits a valence of 5. In PCl₅, the phosphorus atom forms five single bonds with five chlorine atoms. Another common example is the phosphate ion (PO₄³⁻), where phosphorus is bonded to four oxygen atoms. In these compounds, the phosphorus atom utilizes its expanded octet to form additional bonds.