Classification of Elements and Periodicity in Properties
Test 17

Cr (Z=24) has an exceptional configuration (\(3d^5 4s^1\)) due to the stability of a half-filled 3d subshell, unlike Sc (\(3d^1 4s^2\)), Mn (\(3d^5 4s^2\)), or Fe (\(3d^6 4s^2\)).

Third ionization enthalpy peaks when removing an electron from a noble gas-like \(\ce{X^2+}\) ion. Sr (after losing two electrons, becomes \(\ce{Sr^2+}\) with \(1s^2 2s^2 2p^6 3s^2 3p^6 3d^{10} 4s^2 4p^6\)) has the highest value in period 5.

In period 5 d-block (Y to Cd), Tc (group 7) achieves a +7 oxidation state (e.g., in \(\ce{Tc2O7}\)) due to the availability of d-electrons, unlike Zr, Nb, or Mo.

Bond energy in \(\ce{XO2}\) decreases down group 14 due to weaker orbital overlap. \(\ce{CO2}\) (C=O) has the highest bond energy compared to \(\ce{SiO2}\), \(\ce{GeO2}\), or \(\ce{SnO2}\).

Electronegativity decreases down group 15 due to increasing size. Bi (period 6) has the lowest value among N, P, As, and Bi.

Acidity of group 15 oxides increases with oxidation state and decreases down the group. \(\ce{N2O5}\) (N in +5) is the most acidic, forming \(\ce{HNO3}\), unlike \(\ce{P2O5}\), \(\ce{As2O5}\), or \(\ce{Sb2O5}\).

Fourth ionization enthalpy peaks when removing an electron from a stable \(\ce{X^3+}\) ion. Si (after losing three electrons, becomes \(\ce{Si^3+}\) with \(1s^2 2s^2 2p^6\)) has the highest value.

d-block elements (groups 3–12) have electrons filling d-orbitals. Mn (group 7) is a d-block element, while Ga (p-block), Rb (s-block), and Kr (p-block) are not.

The fifth period (\(n=5\)) includes \(5s\) (2 electrons), \(4d\) (10 electrons), and \(5p\) (6 electrons), totaling 18 electrons.

Group 16 trioxides (\(\ce{XO3}\)) show a +6 oxidation state. S forms \(\ce{SO3}\), the most stable and common trioxide, unlike O, Se, or Te where \(\ce{XO3}\) is less prevalent.