NF3 and PF5 are stable molecules. Write the electron-dot formulas for these molecules. On the basis of structural and bonding considerations, account for the fact that NF3 and PF5 are stable molecules but NF5 does not exist
To write the electron-dot formulas for the molecules NF3 and PF5, we need to consider the valence electrons of the elements involved.
1. NF3:
Nitrogen (N) is in Group 15 of the periodic table, so it has 5 valence electrons. Fluorine (F) is in Group 17, so it has 7 valence electrons. Multiplying the number of fluorine atoms by the number of valence electrons per fluorine gives us a total of 3 × 7 = 21 valence electrons from fluorine atoms.
Total valence electrons in NF3 = 5 (from N) + 21 (from F) = 26 valence electrons.
To draw the electron-dot formula, we start by placing a single bond (one electron pair) between nitrogen and each fluorine atom. This accounts for 4 of the 26 valence electrons. We then distribute the remaining 22 valence electrons around the atoms, pairing them up to satisfy the octet rule (except for nitrogen, which will have a complete octet with 8 electrons in the valence shell). The electron-dot formula for NF3 is:
F
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F ─ N ─ F
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F
2. PF5:
Phosphorus (P) is in Group 15, so it has 5 valence electrons. Fluorine (F) is in Group 17, so it has 7 valence electrons. Multiplying the number of fluorine atoms by the number of valence electrons per fluorine gives us a total of 5 × 7 = 35 valence electrons from fluorine atoms.
Total valence electrons in PF5 = 5 (from P) + 35 (from F) = 40 valence electrons.
To draw the electron-dot formula, we start by placing a single bond (one electron pair) between phosphorus and each fluorine atom. This accounts for 5 of the 40 valence electrons. We then distribute the remaining 35 valence electrons around the atoms, pairing them up to satisfy the octet rule (except for phosphorus, which can exceed the octet rule due to the presence of empty d orbitals). The electron-dot formula for PF5 is:
F
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F ─ P ─ F
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F
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F
Now, let's address the stability and nonexistence of NF5.
NF5 does not exist because it violates the octet rule. Nitrogen already has 5 valence electrons, and in order to satisfy the octet rule, it would need to form 3 additional bonds with fluorine atoms. However, this would result in nitrogen having 10 total electrons around it, which is energetically unfavorable.
On the other hand, NF3 and PF5 are stable because they satisfy the octet rule for the central atom. In both molecules, the central atom (nitrogen in NF3 and phosphorus in PF5) has a complete octet of electrons around it, meaning it has 8 electrons in its valence shell. This provides stability to the molecules.