Phosphine, denoted by the chemical formula PH3, is a highly toxic and flammable gas. Understanding its Lewis structure is fundamental to grasping its chemical properties and reactivity. The Lewis structure, also known as the electron dot structure, is a simplified representation of the molecule that shows how the electrons are arranged around the atoms.
To draw the Lewis structure of PH3, we start with the phosphorus (P) atom as the central atom, since it is less electronegative than hydrogen (H). Phosphorus is in group 15 of the periodic table and has 5 valence electrons. Hydrogen, being in group 1, has 1 valence electron.
Determine the Total Number of Valence Electrons:
- Phosphorus (P) has 5 valence electrons.
- Each hydrogen (H) has 1 valence electron.
- Since there are three hydrogen atoms, the total valence electrons contributed by hydrogen are 3*1 = 3 electrons.
- The total number of valence electrons for PH3 is 5 (from P) + 3 (from 3 H) = 8 valence electrons.
Central Atom:
- Phosphorus is chosen as the central atom because it is less electronegative than hydrogen.
Connect Atoms with Single Bonds:
- Connect the phosphorus atom to each of the three hydrogen atoms with single bonds. Each single bond represents 2 shared electrons.
- This step uses 6 electrons (2 electrons per bond * 3 bonds).
Satisfy the Octet Rule for Each Atom:
- After forming the single bonds, phosphorus has 6 electrons involved in bonding (3 single bonds * 2 electrons per bond) and needs 2 more electrons to achieve a stable octet (8 electrons).
- The remaining 2 valence electrons of phosphorus are placed as a lone pair on the phosphorus atom, satisfying its octet.
- Each hydrogen atom, having 2 electrons from the single bond, already has a stable configuration (duet), which is the goal for hydrogen atoms.
The final Lewis structure of PH3 shows a central phosphorus atom bonded to three hydrogen atoms with single bonds and having one lone pair of electrons. This arrangement indicates that phosphorus in PH3 does not achieve a stable noble gas configuration (octet) through bonding alone, which is a characteristic feature of this molecule.
Key Points about PH3’s Lewis Structure: - Central Atom: Phosphorus (P) - Bonding: Three single covalent bonds between phosphorus and hydrogen atoms. - Lone Pairs: One lone pair of electrons on the phosphorus atom. - Octet Rule: Phosphorus does not strictly follow the octet rule in PH3 due to its ability to expand its octet when reacting with certain substances, but in the context of PH3 itself, it has a stable configuration with an effective octet when considering the formal charges and lone pairs. - Electronegativity: Phosphorus is less electronegative than the elements in group 16 (like oxygen), which is why it does not follow the octet rule as strictly in its hydride as oxygen does in water (H2O).
Understanding the Lewis structure of PH3 is crucial for recognizing its reactivity and chemical properties, including its basicity and its role in various chemical reactions. The presence of a lone pair on phosphorus makes PH3 a Lewis base, capable of donating these electrons to form coordination compounds with metal ions or to act as a nucleophile in organic reactions.
