A portion of a two-dimensional cleavage of a KBr(s) lattice is shown below:

Which circle represents K?

Provide a possible explanation for why the atomic radii of Nb and Ta are almost the same even if they are on different periods in the periodic table.

Calculate the atomic radius of nickel at 1453 °C if its distance with another nickel atom in a nickel metal is 0.124 nm.

Identify which of the following elements is expected to have the smallest atomic radius: K, Ca, Ga, Ge, As

Identify which pair is expected to be closest in size of atomic radius.
i.  O and Cl 

ii.  O and P

iii. O and S

What is the primary reason for the general increase in atomic radius as you descend a group among main-group elements?