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F only has 7 valence electrons so for it to be able to stabilize and form an octet which is what the molecule ultimately needs to achieve stability it needs to covalently bond with H, sharing the pair formed between them which is why it cannot form a double or triple bond, the rationale for Fluorine bonding is similar to why oxygen can only form two bonds. Hope that helps.
In the example Sulfur Tetrafluoride, SF4 he showed us in lecture S acts as the central atom and the 4 fluorine atoms were each separately attached by a single bond. We know the SF4 has 34 electrons total so the Lewis structure should have 34 total. We know that the first thing we should ensure before thinking about adding a double bond or triple bond is to ensure each element meets the octet guideline. Keep in mind H, He, Li, and Be dont require the octet guideline. Once all the fluorine atoms reached 8 total electrons it checks the octet guideline and 2 electrons remain. Because fluorine is in period 2 it can only hold up to 8 electrons. If we were to add a double bond like the professor said Fluorine would have 10 electrons (4 by the double bond and 6 lone electrons) Period 3 elements and below can hold more than 8 since they can use the 3d orbital, so since Sulfur (S) is in period 3 it can hold the extra 2 electrons resulting Sulfur to have 10 electrons.
Fluorine is a halogen which means it is a group 17 element and all group 17 elements have 7 valence electrons so they only need 1 more valence electron to completely fill their outer valence shell and get their noble gas configuration so they're stable. This pattern follows for all elements in group 17. They all only form one covalent bond.
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