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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: freswood on July 21, 2006, 07:04:37 PM

Title: hydronium dot diagram
Post by: freswood on July 21, 2006, 07:04:37 PM

I was wondering how you would draw this reaction using dot diagrams:

HCl + H₂O --> H₃O⁺ + Cl^-

I have a test on Monday so I'd really appreciate your help ASAP  :)

Title: Re: hydronium dot diagram
Post by: midwest9 on July 22, 2006, 12:26:18 AM

i think this is a dot diagram. lewis dot right?

Title: Re: hydronium dot diagram
Post by: freswood on July 22, 2006, 12:49:23 AM

Thanks so much! I understand why the Cl is negative, but why is the hydronium positive? I can see that it is missing an electron, but it has a full outer-shell nevertheless.

Title: Re: hydronium dot diagram
Post by: Yggdrasil on July 22, 2006, 02:49:57 AM

To see whether something is positive or negative, you aren't necessarily looking at whether the outer shell is full or not.  There are two ways to look at why the hydronium ion is positive:

1)  Number of protons v. number of electrons.  Each of the three hydrogens has one proton each and the oxygen has eight protons, giving a total of eleven protons.  Hyronium has only ten electrons (eight outer shell + two in oxygen's inner shell [1s orbital]).  Therefore, it has a net charge of +1.

2)  Formal charge (seehttp://en.wikipedia.org/wiki/Formal_charge#Formal_Charge for a more complete discussion).  Formal charge is defined as:

(number of valence electrons of the atom) - (1/2) * (number of bonding electrons) - (number of lone-pair electrons)

So since oxygen atoms have six valence electrons and in hydronium it has six bonding electrons and two lone-pair electrons, its formal charge is 6 - 3 - 2 = +1

Title: Re: hydronium dot diagram
Post by: freswood on July 22, 2006, 03:37:42 AM

Thanks so much for the clarification. I feel a lil' more prepared for the test.

Title: Re: hydronium dot diagram
Post by: sdekivit on July 22, 2006, 05:07:19 AM

the movement of electrons in the picture is wrong. From HCl a single proton is released where Cl takes the electron of the H-atom.

--> So therefore Cl- will be formed and the proton will bind to the free electronpai of oxygen.

Therefore oxygen donates 1 electron to the proton to form a bond and therefore oxygen has only 5 electron from itself and has therefore a postive charge.

Title: Re: hydronium dot diagram
Post by: Dan on July 22, 2006, 10:58:41 AM

Quote from: sdekivit on July 22, 2006, 05:07:19 AM

--> So therefore Cl- will be formed and the proton will bind to the free electronpai of oxygen.

Yeah, only two of the electrons on the H₃O⁺ should be red, the rest black.

Quote

Therefore oxygen donates 1 electron to the proton to form a bond and therefore oxygen has only 5 electron from itself and has therefore a postive charge.

I don't think so. A lone pair  of electrons is donated to the proton. So all atoms will have full outer shells.

Title: Re: hydronium dot diagram
Post by: sdekivit on July 22, 2006, 11:15:53 AM

yes, you're right of course.  ;D they share the complete pair. i said it incorrectly.

Title: Re: hydronium dot diagram
Post by: midwest9 on July 22, 2006, 07:03:48 PM

oops. yeah the guy above is completely right.