# Homework Help: Which has the smallest radius?

1. Oct 14, 2008

### mrjoe2

1. The problem statement, all variables and given/known data
which has the smallest atomic radius: cl-, k+, Ar

2. Relevant equations
none

3. The attempt at a solution
i thought it was argon, but im not sure because it does have the most protons to bringin the electrons. please someone reply soon!!!

2. Oct 15, 2008

### Gannon

The radius of an atom is determined by its overall size and its electronegativity, but in this case the atoms are very similar in size. Chlorine is more electronegative and will pull its electrons in more, decreasing its radius. Potassium is very weakly electronegative, and would rather extend its electrons outward, increasing its radius. Argon is noble, and just sits in between.

3. Oct 15, 2008

### mrjoe2

are you sure about this answer? i dont doubt you or anything, im just making sure before i engrave this logic into my brain.

4. Oct 15, 2008

### mrjoe2

i think its argon or potassium because they have the more protons and thus greater attractions. cl- is out for sure. it cant be right sorry

5. Oct 15, 2008

### Staff: Mentor

Note all three have the same number of electrons - they differ only by the nucleus charge. The higher the nucles charge, the stronger the attraction.

6. Oct 15, 2008

### opuktun

Yes, I second to Borek answer.

The logic is simple. Positive (nucleus) attract negative (electrons).

So which is more in each atom? Positively charged species? Negatively charged species?

7. Oct 15, 2008

### Gannon

Ah, I didn't even notice that the charges were the same!

8. Oct 16, 2008

### Staff: Mentor

Charges are not the same, number of electrons is.

9. Oct 16, 2008

### Gannon

Of course; my mind has been a bit flighty the past few weeks. I guess I'll stop trying to help!

10. Oct 16, 2008

### Staff: Mentor

No need for that, just slow down when replying I am hitting the same wall now and again.