Question

A hydrogen ion can bond to a phosphine molecule, \(\mathrm{PH}_{3}\) forming \(\mathrm{PH}_{4}{ }^{+}\). Draw the electron dot formula for \(\mathbf{P H}_{4}{ }^{+}\) and label a coordinate covalent bond.

Short answer

In \( \text{PH}_4^+ \), the coordinate covalent bond is marked by an arrow pointing from P to H.

Step-by-step solution

Determine Valence Electrons for PH3

Phosphorus (P) has 5 valence electrons, and each hydrogen (H) has 1 valence electron. In phosphine, \( \text{PH}_3 \), there are a total of \( 5 + 3 \times 1 = 8 \) valence electrons.

Add Hydrogen Ion to Form PH4+

Adding a hydrogen ion \( \text{H}^+ \) will utilize an electron pair from \( \text{PH}_3 \), forming \( \text{PH}_4^+ \). This doesn't add any electrons to the total, but it results in a positive charge for the new molecule, giving us 8 valence electrons to arrange.

Draw Electron Dot Formula for PH4+

Place phosphorus (P) in the center and use one pair of electrons to bond each of the four hydrogen atoms to the phosphorus atom. This should use all 8 valence electrons, with each \( \text{H} \) bonded by a pair of electrons shared with \( \text{P} \).

Identify and Label the Coordinate Covalent Bond

In \( \text{PH}_4^+ \), one of the hydrogen atoms forms a bond using a pair of electrons that were originally from phosphine and not from the \( \text{H}^+ \). This is the coordinate covalent bond, and you can indicate it by drawing an arrow pointing towards the hydrogen ion \( \text{H}^+ \) from the phosphorus.

Key concepts

Electron Dot Formula

The electron dot formula is a simple way to illustrate how atoms share electrons in a molecule. It uses dots around the element symbols in a compound to represent valence electrons - the outermost electrons involved in bonding. In the case of \(\text{PH}_4^+\), we first identify the total number of valence electrons in the molecule.
\(\text{P}\) has five valence electrons, and each \(\text{H}\) has one, totaling eight electrons for \(\text{PH}_3\). Upon adding an \(\text{H}^+\), the electron count remains eight, though the structure gains a positive charge.
In the electron dot formula for \(\text{PH}_4^+\), \(\text{P}\) is central, sharing electrons with four \(\text{H}\) atoms. By illustrating each bond as a shared pair of dots, you'll visualize how electrons are distributed, ensuring all atoms reach stable configurations.

Phosphine

Phosphine, represented by the chemical formula \(\text{PH}_3\), is a compound where phosphorus is bonded to three hydrogen atoms. It's a colorless, gaseous molecule characterized by a pyramidal shape. Let's explore some important notes about phosphine:

• The three hydrogen atoms form bonds with phosphorus using a pair of shared electrons for each bond.
• Phosphine exhibits a lone pair of electrons on phosphorus, not involved in bonding with hydrogens.
• This lone pair plays a crucial role when phosphine forms \(\text{PH}_4^+\) as it participates in coordinate covalent bonding with an added hydrogen ion.

Understanding phosphine and its structure helps make sense of how molecules like \(\text{PH}_3\) evolve into acids or positive ions under certain conditions.

Coordinate Covalent Bond

A coordinate covalent bond is a unique type of covalent bond where one atom provides both electrons for the shared pair in the bond. This differs from typical covalent bonds where each atom supplies one electron. In \(\text{PH}_4^+\), this plays an interesting and educational role:

• As phosphine reacts with an \(\text{H}^+\), the lone electron pair from phosphorus forms a bond with the hydrogen ion.
• This bond is specifically called "coordinate" because it originates from phosphorus entirely, distinguishing it from the typical electron-sharing bonds found in covalent bonds.
• To label this in structural diagrams, an arrow can be drawn from the donor atom (phosphorus) pointing towards the acceptor (\(\text{H}^+\)). This arrow visually signifies the single-sided flow of electrons.

Developing a clear understanding of coordinate covalent bonds enhances knowledge of how molecules can form with unusual configurations and contributions.