Question

which among is the following has largest radius
a) Al
b)Al3+
c)O
d)O2-

Answer 1

I think Al
option b) Al
Al is in third period so it has 3 shells where as o has 2 shells
now comparing Al+3 with Al
al+3 has effective​nuclear charge so protons will attract electrons so it's size will decrease than Al
so Al has largest radius

Answer 2

Correct answer - a) Al
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How many shells does Al have?
Let's see..
Number of electrons= 13
So electronic configuration = 2,8,3
So it has 3 shells

But what about Al³⁺?
Here the number of electrons = 10 (as it donates electrons to attain stability)
So electronic configuration = 2,8
So it has 2 shells

More the number of filled shells more the radius
If we compare we find that Al has greater radius

I gave this as an explanation
Just remember this point---

When an atom of element forms a cation (lose electrons ), their radius is smaller than the initial parent atom.And when an atom forms anion ( gains electrons), their radius is greater than the initial parent atom

Now see it.
Al³⁺ has lost electrons but Al is the parent atom.So Al cation radius is smaller than Al

O and O⁻ doesn't count, as O has lesser occupied shells than Al atom

Hope This Helps You!