Draw a Lewis structure for SiF5-. Do not include overall ion charges or formal charges in your drawing. If the species contains oxygen, do not draw double bonds to oxygen unless they are needed in order for the central atom to obey the octet rule.
The Correct Answer and Explanation is:
Correct Lewis Structure for SiF₅⁻ (without charges shown):
F
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F - Si - F
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F
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F
Here, silicon (Si) is in the center bonded to five fluorine (F) atoms arranged around it. Each fluorine has three lone pairs (not shown explicitly here), and the silicon is bonded to all five fluorines with single bonds. The entire structure carries an extra electron (represented by the negative charge on the ion), which affects electron count but is not shown here as requested.
Detailed Explanation
1. Identify the central atom:
Silicon (Si) is less electronegative than fluorine (F), so Si is the central atom, and the five fluorine atoms surround it.
2. Count valence electrons:
- Silicon (Si) has 4 valence electrons.
- Each fluorine (F) has 7 valence electrons; with 5 fluorines, that’s 5 × 7 = 35 electrons.
- Since the ion has a -1 charge, it means one extra electron is added.
- Total valence electrons = 4 (Si) + 35 (F) + 1 (extra) = 40 electrons.
3. Form bonds:
Each Si-F bond is a single bond, representing 2 electrons. With 5 bonds, that accounts for 10 electrons.
4. Distribute remaining electrons to complete octets:
- After bonding, 40 – 10 = 30 electrons remain.
- These electrons are placed as lone pairs around the fluorine atoms to complete their octets (each fluorine needs 8 electrons total including bonding electrons).
- Each fluorine already shares 2 electrons in bonding, so it needs 6 more as lone pairs (3 lone pairs per fluorine).
- 5 fluorines × 6 electrons = 30 electrons, which matches perfectly.
5. Check the central atom (silicon):
Silicon has 5 single bonds, so it is surrounded by 10 electrons (more than an octet). This is allowed because silicon is in period 3 of the periodic table and can have an expanded octet by using d-orbitals.
6. No double bonds are needed:
Since fluorine only forms single bonds and silicon can expand its octet, there is no need for double bonds to oxygen or any other atom here (fluorine is the only surrounding atom and does not form double bonds).
7. Negative charge:
The extra electron that creates the -1 charge is accounted for in the total electron count and is reflected in the lone pairs around fluorines or in the overall electron distribution, but charges are not shown in this drawing.
In summary, the Lewis structure of SiF₅⁻ consists of a central silicon atom with five single bonds to fluorines, each fluorine having three lone pairs to complete their octets, and silicon having an expanded octet with 10 electrons around it. This fits all the bonding rules and electron counts for the ion.
