Arrange in order of decreasing atomic radii: Kr, K, Ge, Ca

K > Ca > Ge > Kr

Explanation:

The elements given are all in the 4th period on the periodic table.

       Kr, k, Ge, Ca;

Arranging their order from left to right;

         K, Ca, Ge, Kr

Atomic radius is taken as half the of the inter-nuclear distance (d) between two covalently bonded atoms of non-metallic elements or half the distance between two nuclei in the solid state of metals.

Across the periodic table atomic radii decreases progressively from left to right. This is due to progressive increase in the nuclear charge (number of protons in the nucleus) without an attendant increase in the number of electronic shells.

From the left to right; decreasing radii;

         K > Ca > Ge > Kr