Question

phosphorus trifluoride (pf₃) is a covalent compound. the phosphorus atom shares a pair of electrons with each fluorine atom, forming three covalent bonds. which lewis dot structures correctly represent a molecule of pf₃?

Explanation:

Step1: Count valence electrons

Phosphorus has 5 valence electrons, and each fluorine has 7 valence electrons. So total valence electrons: \(5 + 3\times7=26\).

Step2: Check bonding and lone pairs

In \(PF_3\), phosphorus forms three single bonds (each bond has 2 electrons). So \(3\times2 = 6\) electrons are used in bonding. Remaining electrons: \(26 - 6=20\). These 20 electrons are distributed as lone pairs. Each fluorine gets 3 lone pairs (\(3\times6 = 18\) electrons) and phosphorus gets 1 lone pair (2 electrons).

Step3: Analyze Lewis dot structures

• First structure: Incorrect as it doesn't show lone pairs on fluorine and phosphorus.
• Second structure: Incorrect as it doesn't use line - bond notation (which is standard for Lewis dot structures of covalent compounds with bonds).
• Third structure: Correct. It shows three \(P - F\) bonds (each bond represented by a line, 2 electrons per bond) and lone pairs on fluorine (6 electrons around each \(F\) as three lone pairs) and a lone pair on phosphorus.
• Fourth structure: Incorrect as phosphorus should have a lone pair (it has 5 valence electrons, 3 used in bonding, so 2 left as a lone pair), which is not shown here.

Answer:

The third Lewis dot structure ( \(:\ddot{F}- \dot{P}-:\ddot{F}:\) with a \(F\) below and lone pairs on \(F\) and a lone pair on \(P\)) correctly represents a molecule of \(PF_3\).