Rank the following elements... A) in order of decreasing EN: In, Rb, I. & B) order of decreasing number of valence electrons: Sb, S, Cs.?

A)
All of In, Rb and I are Period 5 elements.

In the same period, metals have lower EN than non-metals, and thus metals lose electron(s) more readily while non-metals gain electrons more readily. Among the three given elements, In and Rb are metals while I is a non-metal. Therefore, I has the highest EN.

A more reactive metal loses electron(s) more readily, and thus a more reactive metal has a lower EN. As Rb is a Group 1 metal (or Group IA) while In is Group 13 (or Group IIIA), Rb is more reactive than In. Therefore, Rb has a lower EN than In.

Therefore, EN : I > In > Rb


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B)
Sb is a Group 15 (or Group VA) element which has 5 valence electrons. For example, SbF₅ is formed by using all the 5 valence electrons of Sb.
S is a Group 16 (or Group VIA) element which has 6 valence electrons. For example, SF₆ is formed by using all the 5 valence electrons of S.
Cs is a Group 1 (or Group IA) element which has 1 valence electrons. For example, Cs releases its 1 valence electrons to form Cs⁺ ion.

Number of valence electrons : S > Sb > Cs