Sapling #20

(Polar molecules, Non-polar molecules, etc.)

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Ayesha Aslam-Mir 3C
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Joined: Wed Sep 30, 2020 9:43 pm

Sapling #20

Postby Ayesha Aslam-Mir 3C » Sun Nov 29, 2020 5:42 pm

How do we know that one of the bonds with the As and O atoms in the As)4(3-) molecule has to be a double bond? How can I recognize this trend in further mleculres? Is it just bc the As is the central atom?

Olivia Smith 2E
Posts: 87
Joined: Wed Sep 30, 2020 9:43 pm

Re: Sapling #20

Postby Olivia Smith 2E » Sun Nov 29, 2020 5:44 pm

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

Megan Lu 3D
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Joined: Wed Sep 30, 2020 9:49 pm

Re: Sapling #20

Postby Megan Lu 3D » Sun Nov 29, 2020 6:03 pm

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!

Savannah Torella 1L
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Joined: Wed Sep 30, 2020 9:40 pm

Re: Sapling #20

Postby Savannah Torella 1L » Sun Nov 29, 2020 6:08 pm

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.

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Re: Sapling #20

Postby Yuehan_Wu_3K » Sun Nov 29, 2020 6:49 pm

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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Joined: Wed Sep 30, 2020 9:37 pm

Re: Sapling #20

Postby FionaHunter21 » Sun Nov 29, 2020 7:03 pm

Always check to see if there are unnecessary formal charges on atoms, and if there are any that can be zero by adding or removing a bond in place of a lone pair!

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