Born Theory - Finding the value of Z (solvation of ions)

Wek

1. Homework Statement

What's the energy for transfer from
ε=1 to water with r=2.5Å
ε=1 to water with r=5Å.

2. Homework Equations

ΔG = (1/2)*(Z^2/r)*[(1/e1)-(1/e2)]

3. The Attempt at a Solution

ΔG = (1/2)*(Z^2/2.5)*[(1/80)-(1/1)]

However, I am stuck on the Z variable. Do I assume Z is 1 for a monovalent ion? I am not given any other information. The previous question (not related I think) does not say anything about the ion either.

Also, from an article I was reading, the was a calculation of the energy needed to transport a K+ ion into a membrane using the same equation. I tried to solve for the Z value but it was not 1 even though K+ is a monovalent ion.

EDIT: I had the Z symbol confused. I think it's the atomic number but I still cannot get the numbers in the article (my number is a bit higher).

EDIT 2: It seems like the article rounded up 0.4875 to 0.5. If I use 0.5 I can get the same numbers they did but I don't understand why they would round up the number.

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"Born Theory - Finding the value of Z (solvation of ions)"

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