(Polar molecules, Non-polar molecules, etc.)
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I do it based off of the charge. So I noticed that the charge on the atom was negative 3. If all atoms had single bonds then the charge would be -4 but if I had a double bond then that would bring the charge to -3 which is what I need. Also you can count how many electrons you have and if you don't have enough atoms to complete the octete, then you need to add another bond
Hi! Like Olivia mentioned above, I would also examine the charge of the molecule or ion. In this question, we want the formal charges of the atom to have a sum of -3, as the ion has a charge of -3. Thus, the structure that would give an overall formal charge of -3 would be if arsenic had one double bond with an oxygen and three single bonds with the rest. Additionally, we know that arsenic can have an extended octet due to its placement on the periodic table, so this structure is valid and favored. Hope this helps!
There are two ways to distinguish why the double bond is needed. You can either look at the charge of the compound or the number of valence electrons that As require. Oxygen has a -1 charge when it has a single bond, since it is surrounded by 7 electrons when it only needs 6 valence electrons. This means that with the compound's -3 charge, three of the oxygens have a single bond, meaning that one of the oxygens have a double bond. You can also notice that As requires 5 valence electrons, and when all of the oxygens have a single bond, As only has 4 electrons surrounding it. In order for As to have its needed 5 electrons, one of the oxygens needs to have the double bond.
You could count the number of formal chagrs of each atom and then you wil lfind that when it contains a double bond, the total formal charge is the minimum. It should not based on memorization but on the calculation of the formal charges.
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