Please explain atomic radii question.

[VDR]

#254 on Destroyer G-chem.
organize in decreasing order with respect to atomic radii.

Gives answer:

K+ > Ca++ > Al+3.

I am confused cause K+ and Ca++ seem to have the same atomic radii if i take one electron away from K and two from Ca.
What do you think.
Thanks,

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[PooyaH]

When you take an electron away from potassium, now it has 1 proton more than the electrons, this causes the remaining electrons to be held more tightly towards the nucleus. So in case of calcium, you have taken 2 electrons away, so now you have 2 protons more than the electrons which causes the remaining electrons to be held even more tightly to the nucleus, thus making the atomic radius smaller than of potassium

 

[VDR]

When you take an electron away from potassium, now it has 1 proton more than the electrons, this causes the remaining electrons to be held more tightly to towards the nucleus. So in case of calcium, you have taken 2 electrons away, so now you have 2 protons more than the electrons which causes the remaining electrons to be held even more tightly to the nucleus, thus making the atomic radius smaller than of potassium

Thank you. The way you explained it makes sense.

 

[kejs]

#254 on Destroyer G-chem.
organize in decreasing order with respect to atomic radii.

Gives answer:

K+ > Ca++ > Al+3.

I am confused cause K+ and Ca++ seem to have the same atomic radii if i take one electron away from K and two from Ca.
What do you think.
Thanks,

Since you are comparing their atomic radii, you would basically use the same trend rules as you would for comparing ionic radii. As you go down the period table (going down a column) the radius increases and as you go left to right on the period table (across a period) the radius decreases.

Also, I like to look at it this way, the more protons you have over electrons, the smaller it will be. And consequently, the more electrons you have over protons, the bigger it will be.

I'm usually bad at explaining things so hopefully what I said didn't confuse you.

 

[crax]

#254 on Destroyer G-chem.
organize in decreasing order with respect to atomic radii.

Gives answer:

K+ > Ca++ > Al+3.

I am confused cause K+ and Ca++ seem to have the same atomic radii if i take one electron away from K and two from Ca.
What do you think.
Thanks,

for more information, look into effective nuclear charge.