Which atom to add positive/negative charge?

Anh Trinh 1J
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Which atom to add positive/negative charge?

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
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Re: Which atom to add positive/negative charge?

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
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Re: Which atom to add positive/negative charge?

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 + $\frac{s}{2}$ )

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

Stephen Min 1I
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Re: Which atom to add positive/negative charge?

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.

Posts: 88
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Re: Which atom to add positive/negative charge?

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
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Re: Which atom to add positive/negative charge?

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.