Question

Phosphorus is immediately under nitrogen in the Periodic Table. Predict the molecular formula for phosphine, the compound formed by phosphorus and hydrogen. Predict the H-P-H bond angle in phosphine.

Short answer

Answer: The molecular formula for phosphine is PH_3, and the predicted H-P-H bond angle is approximately 107 degrees.

Step-by-step solution

Identify the Group Number of Phosphorus

Phosphorus is in Group 15 (or Group 5A, depending on the notation). It has five valence electrons, as elements in this group have the electron configuration ns^2np^3.

Determine the Molecular Formula

As phosphorus is immediately under nitrogen in the Periodic Table, we can use ammonia (NH_3) as a comparison. Nitrogen forms ammonia by bonding with three hydrogen atoms, completing its octet by sharing electrons and forming 3 covalent bonds. Similarly, phosphorus will also form bonds with three hydrogen atoms, creating the molecular formula PH_3.

Apply VSEPR Theory to the Phosphine Molecule

VSEPR (Valence Shell Electron Pair Repulsion) theory helps us determine the geometry of a molecule based on the repulsion between electron pairs. Phosphorus (the central atom) has three bonding pairs with hydrogen atoms and one lone pair of electrons. Hence, the electron pair arrangement around the phosphorus should be tetrahedral to minimize repulsion.

Predict the H-P-H Bond Angle

While the electron pair geometry around the phosphorus is tetrahedral (109.5 degrees), the presence of a lone pair of electrons will push the hydrogen atoms closer together, decreasing the bond angle between H-P-H. As a result, the molecular shape is trigonal pyramidal, with the H-P-H bond angle being slightly less than 109.5 degrees, typically around 107 degrees.