Understanding lewis structure answer

Jan 2020
2
0
Norway
The following picture is a posted solution to an earlier exam,

The question was to draw Ca (s) + Br2 (g) = CaBr2 as a lewis structure, the suggested answer was like this:
Skjermbilde 2020-01-03 kl. 13.04.26.png

I understand that this forms an ionic bond given that Ca (metal) donated electrons to Br (non metal) and becomes attached. But I don't understand where the "extra" electrons come from in the product? I am talking about the four "net extra" electrons around the Ca in the product, for clarity.

Best
 
Sep 2018
68
16
England
I agree it is a bit misleading:
The diagram is only showing the electrons of the outer "shell" of the comonents.
The left part of the diagram is showing the 2 electrons of outer 4th "shell" of the calcium atom and 7 electrons from the outer "shell" of the bromine atom.
The right part of the diagram is showing the 8 electrons of the 3rd "shell" of the calcium ion and 8 electrons from the outer "shell" of the bromine ion.
It might have been clearer if the right side of the diagram showed \(\displaystyle Br^-\) and \(\displaystyle Ca^{+ +}\) to emphasize the ionic nature of the components being shown.

Note that I have put "shell" in quotes, this is to indicate that the electron shell model is a simplified model of what is actually occurring with electrons in atoms.
 
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Jan 2020
2
0
Norway
I agree it is a bit misleading:
The diagram is only showing the electrons of the outer "shell" of the comonents.
The left part of the diagram is showing the 2 electrons of outer 4th "shell" of the calcium atom and 7 electrons from the outer "shell" of the bromine atom.
The right part of the diagram is showing the 8 electrons of the 3rd "shell" of the calcium ion and 8 electrons from the outer "shell" of the bromine ion.
It might have been clearer if the right side of the diagram showed \(\displaystyle Br^-\) and \(\displaystyle Ca^{+ +}\) to emphasize the ionic nature of the components being shown.

Note that I have put "shell" in quotes, this is to indicate that the electron shell model is a simplified model of what is actually occurring with electrons in atoms.

Thanks for the answer, it sort of makes sense, but still not quite. I have a follow-up question for clarity.

This is how I look at it now:

- Every element have electron "layers", each layer can hold 2n^2 electrons (2, 8, 18 etc).
- The octet rule applies to bonding, and doesn't care about these "real" layers. 8 is ideal as a "shell" in bonding - even though it isn't a "real" electron shell compatible with that model (in which they can hold more than 8, M shell etc)

If the calcium gives away the last two electrons in it's 4th shell (by the octet rule model), then its another 8 electrons beneath that shell, because it is (2, 8, 8, 2 - again, by the octet rule).

Now:
Even if this is correct, the calcium atom, which "begun" at its next available octet shell only appears to have 4 electrons (not the expected 8), because the other 4 belongs to the bromine elements. It now looks like som kind of covalent bond where calcium has 4 electrons and bromine 8, then they complete each other - if you know what I mean.

Thanks.
 
Sep 2018
68
16
England
The key is (I think) the move from atoms on one side to ions on the other.
The mutual interaction of electrons in a "full shell" makes this arrangement more stable than a partially filled shell.
The outer two electrons of the calcium atom are rattling around in an almost empty shell,
this makes them relatively loosely attached to the atom.
The Bromine atoms have a space to fill in the outer shell, to reach the more stable arrangement.
The Bromine atoms pinch the loose outer electrons from the Calcium atoms, leaving Calcium ions with just the highly stable inner shells.
and Bromine ions, also with a highly stable arrangement of electrons in a full outer shell.
CaBr2.png
Here's a (slightly silly) analogy:
Imagine octopuses on a roundabout.
Each octopus can hold the arm of just one other octopus.
An arrangement of eight octopuses is obviously going to be the most stable situation,
with every octopus holding one arm of every other octopus.