Order the following in terms of decreasing atomic radius:

Ba, Cs, Hf, Bi, Ta

Cs > Ba > Bi > Hf > Ta

Explanation:

The atomic radius of a chemical element defined as the distance between nucleus and electron from the center.

The atomic number and atomic radius of given elements are:

Barium (Ba):

Atomic number - (56)

Atomic radius - 268 pm

Caesium (Cs)

Atomic number - 55

Atomic radius - 343 pm

Hafnium (Hf):

Atomic number - 72

Atomic radius - 225 pm

Bismuth (Bi):

Atomic number - 83

Atomic radius - 230

Tantalum (Ta):

Atomic number - 73

Atomic radius - 220 pm

According to the given atomic radius the decreasing order of atomic radius is Cs > Ba > Bi > Hf > Ta.

Caesium has highest atomic radius among all even though lowest atomic number because the force of attraction between the nucleus and the outermost electron is low and outermost electron are loosely held.

Barium (Ba) has lower atomic number than Bismuth (Bi) but still atomic radius of Barium is high because moving across the periodic table atomic radius decreases as force of attraction between nucleus and outer electrons is high.

Bismuth (Bi) has higher atomic number that Hafnium (Hf), atomic radius of Bi is more than Hf.

Tantalum (Ta) have higher atomic number than Hafnium (Hf) but still have lower atomic radius as as force of attraction between nucleus and outer electrons is high.

Hence, the decreasing atomic radius is Cs > Ba > Bi > Hf > Ta.