Question

which element will form the ion with the largest ionic radius? periodic table image showing elements h, he, li, be, b, c, n, o, f, ne, na, mg, al, si, p, s, cl, ar, k, ca, ga, ge, as, se, br, kr, rb, sr, in, sn, sb, te, i, xe, cs, ba, tl, pb, bi, po, at, rn, fr, ra, nh, fl, mc, lv, ts, og with k, rb, cs highlighted in yellow. options: k, cs, rb

Explanation:

Step1: Recall ionic radius trend

For group 1 metals (alkali metals like K, Rb, Cs), when they form ions (\(M^+\)), the ionic radius increases down the group. This is because as we go down a group, the number of electron shells increases, so the distance from the nucleus to the outermost electron (in the ion) increases.

Step2: Locate elements in group

K (potassium), Rb (rubidium), and Cs (cesium) are all in Group 1 of the periodic table. Cs is the lowest in the group among these three (K is above Rb, Rb is above Cs? No, wait: K is period 4, Rb period 5, Cs period 6. So down the group: K < Rb < Cs in terms of period number (and atomic size, ionic size). So Cs is the lowest, so it has the most electron shells when forming \(Cs^+\) ion.

Step3: Determine largest ionic radius

Since ionic radius increases down the group for group 1 metal ions, Cs (cesium) will form an ion with the largest ionic radius among K, Rb, and Cs.

Answer:

Cs (the element is Cesium, and among the options, the correct one is the middle option labeled "Cs")