To draw the Lewis structures for the given molecules, we must first determine the number of valence electrons for each atom involved.
A. SF₂ (Sulfur Difluoride)
- Sulfur (S) has 6 valence electrons.
- Fluorine (F) has 7 valence electrons, and there are 2 F atoms.
Total valence electrons = 6 + (2 × 7) = 20 valence electrons.
Now, we place the sulfur atom at the center, connecting it to the two fluorine atoms with single bonds. Each single bond consumes 2 electrons, so:
- Electrons used for bonds: 2 (for S-F) + 2 (for S-F) = 4 electrons.
Remaining electrons = 20 – 4 = 16 electrons. Each fluorine needs 6 more electrons (3 lone pairs) to complete their octet. Hence, we place the remaining 16 electrons as lone pairs on the fluorines:
.. ..
:F: - :S: - :F:
.. ..
Each F has 3 lone pairs, and S has no lone pairs. Thus, the Lewis structure for SF₂ is shown above.
B. H₂CCH₂ (Ethylene)
- Carbon (C) has 4 valence electrons and there are 2 C atoms.
- Hydrogen (H) has 1 valence electron, and there are 4 H atoms.
Total valence electrons = (2 × 4) + (4 × 1) = 8 valence electrons.
In ethylene, we place a double bond between the two carbon atoms to satisfy their valence needs. Each carbon atom is also bonded to two hydrogen atoms with single bonds:
H H
| |
H - C = C - H
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This Lewis structure shows that each C atom has 4 electrons (2 from the double bond and 2 from the single bonds), and each H atom has 2 electrons, completing the hydrogen’s valence requirement.
C. HNNH (Hydrazine)
- Nitrogen (N) has 5 valence electrons, and there are 2 N atoms.
- Hydrogen (H) has 1 valence electron, and there are 4 H atoms.
Total valence electrons = (2 × 5) + (4 × 1) = 14 valence electrons.
For hydrazine, we connect the two nitrogen atoms with a single bond and surround them with hydrogen atoms. Each nitrogen will have a lone pair to complete their octets:
H H
\/
H - N - N - H
/\
H H
Each NH group has 3 bonds (1 to N and 2 to H) fulfilling the valence for hydrogen and nitrogen. Thus, the Lewis structure for HNNH is illustrated above.