CHEMISTRY · Single correct · A Level · Chemistry · Periodicity and periodic trends · Periodicity and periodic trends
Problem
The graph below is described verbally: From Z=19 to Z=36, the first-ionization energy curve rises overall but shows two sharp local maxima at Z=24 and Z=29 and two local minima at Z=25 and Z=30. A second curve on the same axes represents atomic radius and exhibits minima at Z=24 and Z=29.
Which conclusion is most strongly supported by simultaneous inspection of the two curves?
(a) Because the 3d electrons added between Z=19 and Z=30 shield the nucleus very poorly, the effective nuclear charge is greatest at Z=24 and Z=29, producing simultaneous maxima in ionization energy and minima in radius.
(b) Half-filled and fully filled 3d subshells at Z=24 and Z=29 create unusually stable electron configurations, resulting in higher ionization energies and smaller atomic radii at those atomic numbers.
(c) Ionization energy peaks at Z=24 and Z=29 because those elements have just completed the 4p subshell, giving them noble-gas-like stability and correspondingly small radii.
(d) The unusually high ionization energies at Z=24 and Z=29 occur because the first electron removed comes from the lower-energy, more tightly bound 4s orbital, whereas neighboring elements lose an electron from the more diffuse 3d orbital.