How does the size of He- compare to that of a neon atom?

[wee00x]

I understand that He- is larger than He, but I don't know which is bigger between He- and Ne.. please help??

 

[wee00x]

Does anyone know the answer to this? How does the He- ion compare in size to that of the Ne atom?? How can you tell which one is bigger?

 

[voko]

First, compare the size of He- with that of Li. Second, compare Li with Ne. What is the general trend in atomic size within each row of the periodic table?

 

[wee00x]

Oh I see.. Well I understand the trends but I don't understand why it is that way. Do you have a explanation that can clear it up??

 

[voko]

Just to make sure we are on the same page, explain the trends the way you understand them.

 

[wee00x]

Well as you go down the periodic table, the size of the atoms will increase because of the addition of higher energy levels, increasing atomic radius. However, when you go across the periodic table to the right, the electrostatic force between electrons causes them to become more compact and smaller. However, once the energy level is completely filled with electrons, the atom (noble gas) is larger.

 

[voko]

I agree with everything except the last sentence. There is no reason for the noble gas atoms to break out of the general scheme. They are the smallest atoms within a period. However, between a noble gas atom of one period and the alkali metal of the next period, the radius rises sharply, because the next period starts a new electron shell.

 

[wee00x]

So would the He- be larger than the Ne atom, since it would be on the some energy level, but farther to the left on the periodic table?

 

[voko]

Would it be larger or smaller than neutral Li?

 

[wee00x]

neon would be smaller than lithium. Does this mean that He- would be larger than neon?

 

[voko]

I did not express myself clear. Would He- be larger or smaller than Li?

 

[wee00x]

I imagine that the helium ion would be smaller... because it has less protons than lithium does... but there is also the fact that since there are less protons than lithium, there is less electrostatic attraction, and so the electrons are not held in as close for the helium ion. I feel that this more likely explains why a helium ion is not likely to stay as an ion for very long. Am I correct?

 

[voko]

Yes, you are thinking correctly. It will be larger than Li EXACTLY for the same reason Li is larger than Be.

 

[wee00x]

Okay, thank you for helping me clear that up! So He- is larger than Ne because although they are on the same energy level, He- has less of an electrostatic attraction, being that it has less protons, and so the electrons are not bound as tightly to the nucleus as they would be with Ne.