Question

Which of the following has the largest atomic radius?

a. S^2-

b. Ca²⁺

c. Cl^1-

d. Ar

e. K¹⁺

Answer 1

Please do rate

A) S2-

For this you need to know two basic concepts and the position of the elements in the periodic table

1)A cation is smaller than the parent atom and an anion will be bigger than the parent atom

2)The atomic radii tend to decrease across a period from left to right. Theatomic radius usually increases while going down a group due to the addition of a new energy level

so K is to the left of Ca so is bigger than Ca

K+(lost on electron) will also hence be bigger than Ca2+(which has lost 2 electrons)

S is on the left of Cl and Ar in the same period and hence S is bigger than Ar and Cl. S2- which is even bigger than S is also hence bigger than Ar and Cl-

so now we only have to determine which is bigger K+ or S-

K: 1s2 2s2 2p6 3s2 3p6 4s1

K+:1s2 2s2 2p6 3s2 3p6

Now take a look at a neutral Sulphur atom

S: 1s2 2s2 2p6 3s2 3p4

Its outermost electrons are alredy on the third energy level. When the S2- anion is formed, twp electron are being added to that same energy level, so that you have

Cl−:1s2 2s2 2p6 3s2 3p6

Now turn to the second concept, effective nuclear charge.

Both ions have their outermost electrons located on the same energy level, but they do not have the same ionic radius.

That happens because the number of protons each species has in its nucleus varies.

More specifically, potassium, which has an atomic number equal to 19, has 19 protons in its nucleus. Sulphur, on the other hand, only has 16.

This is important because in the case of the potassium atom, the outermost electrons will be attracted by the nucleus more.

In other words, K+ has bigger effective nuclear charge than S2−, which translates to a bigger net positive charge felt by the outermost electrons.

This will compress the energy levels a bit and make the ionic radius smaller for the potassium cation.

Therefore, the Sulphur anion will have the larger atomic radius.