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: Determine valence electrons

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

Step2: Analyze bonding and lone pairs

In \(PF_3\), P forms 3 single bonds (6 electrons used in bonding). Remaining electrons: \(26 - 6 = 20\). These are distributed as lone pairs: each F has 3 lone pairs (\(3\times6 = 18\) electrons) and P has 1 lone pair (2 electrons).

Step3: Check Lewis dot structure rules

• In Lewis dot structures, bonded atoms are shown with lines (single bond = 2 electrons) and lone pairs are shown as dots.
• First structure: Incorrect as it doesn't show proper bonding (should be lines for bonds).
• Second structure: Correct. It shows 3 single bonds (P - F) and all lone pairs (each F has 3 lone pairs and P has 1 lone pair).
• Third structure: Incorrect as it doesn't show lone pairs on F and P (violates octet rule for F as F needs 8 electrons).
• Fourth structure: Incorrect as P should have a lone pair (violates octet rule for P, P in \(PF_3\) has 8 electrons: 3 bonds + 1 lone pair).

Answer:

The second Lewis dot structure (the one with \(:\ddot{F}-P-\ddot{F}:\) and a \(:\ddot{F}:\) below P) correctly represents a molecule of \(PF_3\).