The metallic character of elements generally increases as you move down a group in the periodic table and decreases as you move across a period from left to right.
So, the order of increasing metallic character for Rb, Cs, K, and Na would be:
Na < K < Rb < Cs
Step-by-step explanation:
Na (sodium) is the least metallic of the four elements because it is located at the beginning of period 3, which means it has the smallest atomic radius and the highest effective nuclear charge.
K (potassium) is more metallic than Na because it is located below Na in group 1, which means it has a larger atomic radius and a lower effective nuclear charge.
Rb (rubidium) is more metallic than K because it is located below K in group 1, which means it has an even larger atomic radius and an even lower effective nuclear charge.
Cs (cesium) is the most metallic of the four elements because it is located at the bottom of group 1, which means it has the largest atomic radius and the lowest effective nuclear charge