Samples of four group 18 elements, antimony, arsenic, bismuth and phosphorus are in the gaseous phase. an atom in the ground state of which element requires the least amount of energy to remove its most loosely held electron.

The correct answer is Bismuth (Bi).  

An atom in the ground state of Bi needs the minimal amount of energy to withdraw its most loosely held electron. The Bi atoms exhibit 83 electrons, and the shell structure is 2.8.18.32.18.5.  

A ground state atom refers to an atom in which the whole energy of the electrons cannot be lowered by transferring one or more electrons to the distinct orbitals. Or it can be said that in the case of Bi, all the electrons are in the lowest possible levels of energy.  

periodic trends is an atom that gets larger as the number of electronic shells increase, the radius of atoms increases as you go down a certain group in the periodic table of elements. in general, the size of an atom will decrease as you move from left to right of a certain period.