[Nitin_Naudiyal]

Which among the following is the Biggest Ion?
1) Al³⁺
2) Ba²⁺
3) Mg²⁺
4) Na⁺

The Answer given is: Ba⁺²

When an atom loses an electron to form a cation, the shielding decreases and Hence the Ionic Radii Decreases.

However Ba has atomic no. 56. So, even after forming a cation it will still have more electrons compared to the other three elements.
So, how is Ba²⁺ the biggest ion in the above question.

Please help me with the question.

Thanking in Advance.

[Borek]

even after forming a cation it will still have more electrons compared to the other three elements.

And basically that's the reason why it is the largest one of all these listed.

[Big-Daddy]

As you go down the rows of the periodic table, the cationic radius increases. Within a row (a "period"), the cationic radius decreases as you go across because you have the same number of electrons but a greater number of protons in the nucleus as you go across the table. So the correct order is Ba2+ > Na+ > Mg2+ > Al3+ (from largest to smallest cation by radius).

[Nitin_Naudiyal]

Ba²⁺ still has more electrons as compared to other elements.
So, the attraction between electrons and nucleus will be more in case of Ba²⁺ as in case of other elements.
More the attraction, Smaller will be the Radius.
So, Shouldn't Ba²⁺ will have the smaller radius ??

[Borek]

Assuming your approach is right hydrogen would be the largest atom. It isn't.

[Nitin_Naudiyal]

Got the Answer.

Thank you for your Help

[Big-Daddy]

Ba²⁺ still has more electrons as compared to other elements.
So, the attraction between electrons and nucleus will be more in case of Ba²⁺ as in case of other elements.
More the attraction, Smaller will be the Radius.
So, Shouldn't Ba²⁺ will have the smaller radius ??

Just to be clear, when you say "more the attraction, smaller the radius", you were referring to the attraction between the electron clouds put together and the nucleus, but that's not the case: rather, "the more the attraction between the nucleus and outermost electron, the smaller the radius". Ba2+'s outermost electron occupies an orbital many energy levels higher than the other ones, so regardless of the extra protons in the nucleus the attraction of the outermost electron to the nucleus is smallest in Ba2+, so Ba2+ has the largest radius.

[Nitin_Naudiyal]

Yes.
Thank You for clearing my doubt.

I have another question.
Considering Na⁺, Mg²⁺ and Al³⁺, all three have same number of electrons but different number of Protons and hence The ionic radii decreases left to right.

Consider P^-3, S^-2, Cl^-1, They have same number of electrons but lesser number of protons to Hold on to the electrons.
So, ionic radii trend for these three elements would be
P^-3 > S^-2 > Cl^-1

i.e. Decreases from Left to Right

Am i correct?

[Big-Daddy]

Yes.
Thank You for clearing my doubt.

I have another question.
Considering Na⁺, Mg²⁺ and Al³⁺, all three have same number of electrons but different number of Protons and hence The ionic radii decreases left to right.

Consider P^-3, S^-2, Cl^-1, They have same number of electrons but lesser number of protons to Hold on to the electrons.
So, ionic radii trend for these three elements would be
P^-3 > S^-2 > Cl^-1

i.e. Decreases from Left to Right

Am i correct?

You are right. Atomic radius, cationic radius, anionic radius - all of these decrease as you go across (i.e. from left to right) a period of the Periodic table.

[Nitin_Naudiyal]

However, Since increase in the number of Electrons from
P^-3, S^-2, Cl^-1

goes on increasing, as the number has increased there will be more repulsion.

So, doesnt this affect the radii ?
Why doesnt the atom expand as a result of the repulsion?

[Borek]

However, Since increase in the number of Electrons from
P^-3, S^-2, Cl^-1

goes on increasing

No, all three have exactly the same number of electrons. It is Z than changes here.

Actually you correctly mentioned it a post before.

[Big-Daddy]

However, Since increase in the number of Electrons from
P^-3, S^-2, Cl^-1

goes on increasing, as the number has increased there will be more repulsion.

So, doesnt this affect the radii ?
Why doesnt the atom expand as a result of the repulsion?

Borek has answered your question. In fact you answered it yourself in your previous post.

Something interesting to note is that, if (as the case is for atomic radius) the number of electrons is increasing and so is number of protons, the overall effect is that the atomic radius decreases so long as the electrons are remaining within the same sub-shell; when they jump to a new sub-shell (e.g. from s to p in one period) then there is an increase in atomic radius.

[Nitin_Naudiyal]

Thank You All for Your Help.