Which atom to add positive/negative charge?

Moderators: Chem_Mod, Chem_Admin

Anh Trinh 1J
Posts: 147
Joined: Wed Sep 30, 2020 9:55 pm
Been upvoted: 1 time

Which atom to add positive/negative charge?

Postby Anh Trinh 1J » Sat Nov 07, 2020 2:25 pm

When I'm drawing the Lewis structures in Sapling homework, I have trouble determining which atom in the molecule to add the positive/negative charge. How do I assign formal charges given a polyatomic ion? Do I use the number of electrons in the periodic table to find which atom has the charge?

Joseph Hsing 2C
Posts: 91
Joined: Wed Sep 30, 2020 9:42 pm

Re: Which atom to add positive/negative charge?

Postby Joseph Hsing 2C » Sat Nov 07, 2020 2:39 pm

Nonmetals will typically gain electrons and metals will typically lose electrons. If an ion gains e- it will have a negative charge and if it loses e- it will have a positive charge. Hope this helps!

Mackenzie Fernandez 3G
Posts: 104
Joined: Wed Sep 30, 2020 10:07 pm
Been upvoted: 1 time

Re: Which atom to add positive/negative charge?

Postby Mackenzie Fernandez 3G » Sat Nov 07, 2020 2:43 pm

Anh Trinh 1B wrote:When I'm drawing the Lewis structures in Sapling homework, I have trouble determining which atom in the molecule to add the positive/negative charge. How do I assign formal charges given a polyatomic ion? Do I use the number of electrons in the periodic table to find which atom has the charge?


hi!

So for FC, Dr. Lavelle goes over some pretty good examples in his lectures. He gives the following procedure for calculating FC:

1) determine # of valence e- for each bound atom
--> count # bonding/shared e- (s)
--> each atom contributes 1 e- (s/2)
--> count # lone pairs (L)

2) compare s & L with # of valence e- of free atoms (V)

and you get the equation: FC = V - (L + )

He goes over a couple of problems in the beginning of his lecture on 11/04.
Hope this helps!

Stephen Min 1I
Posts: 90
Joined: Wed Sep 30, 2020 10:01 pm

Re: Which atom to add positive/negative charge?

Postby Stephen Min 1I » Sat Nov 07, 2020 3:36 pm

To find the charge of the atom, you want to subtract the number of valence electrons as depicted in your structure from the the number of valence electrons that the element has according to the periodic table.

Jaden Haskins 2E
Posts: 89
Joined: Wed Sep 30, 2020 9:38 pm

Re: Which atom to add positive/negative charge?

Postby Jaden Haskins 2E » Sat Nov 07, 2020 4:09 pm

I believe that you also can determine the charge of an atom based on what column it resides in the periodic table. F is negative because it has seven valence electrons, meaning that it only wants to gain one to reach the octet. This makes is F-. Calcium has two valence electrons, so it wants to lose them both to become Ca 2+, reaching its full octet.

Gillian Gabrielsen2F
Posts: 95
Joined: Wed Sep 30, 2020 10:03 pm
Been upvoted: 2 times

Re: Which atom to add positive/negative charge?

Postby Gillian Gabrielsen2F » Sun Nov 08, 2020 9:20 am

Jaden Haskins 2F wrote:I believe that you also can determine the charge of an atom based on what column it resides in the periodic table. F is negative because it has seven valence electrons, meaning that it only wants to gain one to reach the octet. This makes is F-. Calcium has two valence electrons, so it wants to lose them both to become Ca 2+, reaching its full octet.

This charge would be the charge of the element when it's an ion, not the formal charge. For example, since Cl has 7 valence electrons, it needs to gain an electron to have a full valence shell. The additional electron makes it's charge -1 and forms the ion Cl-. So to form an ionic bond, it needs to be paired with an equal number of + charge (ex, Na+ and Cl- for NaCl). This is different then formal charge. Formal charge, Cl shares electrons with another element, rather than actually "stealing" some like Cl- does. So in a covalent structure, Cl doesn't need to have a formal charge of -1. In PCl5, each Cl has a formal charge of 0 (7 valence - (6 lone electrons + 1 bond)). You have to use the number of valence e-s an element has to find the formal charge, but the charge of an ion is different than the same element's formal charge in a covalent structure. Formal Charge of atoms aims to be 0, individual ions won't have a charge of 0.

Keep in mind that after you've found the formal charge of all the atoms, when you add up all the formal charges you should get the charge of the compound as a whole. This is a good way to make sure you have the correct numbers.

Jaden Haskins 2E
Posts: 89
Joined: Wed Sep 30, 2020 9:38 pm

Re: Which atom to add positive/negative charge?

Postby Jaden Haskins 2E » Tue Nov 10, 2020 11:14 pm

I apologize, I did not read the question closely enough! Thank you Gillian for the clarification and great explanation of formal charge.


Return to “Lewis Structures”

Who is online

Users browsing this forum: No registered users and 1 guest