Questions: Main Group Chemistry Overview

Question 1

Thallium (Tl) most commonly forms Tl⁺ compounds rather than Tl³⁺, even though it is in Group 13 and has three valence electrons. Which concept explains this?

• A. The diagonal relationship between Tl and a lighter Group 12 element
• B. The inert pair effect — the 6s² electrons in Tl are stabilized by relativistic contraction and poor shielding, making them reluctant to participate in bonding
• C. Thallium's high electronegativity prevents it from losing more than one electron
• D. Crystal field stabilization energy favors the +1 oxidation state

Answer: (B)

The inert pair effect describes the increasing stability of the oxidation state two less than the group valence for heavier p-block elements. For Tl (Group 13), the 6s² electrons are stabilized by relativistic effects (the 6s orbital contracts and lowers in energy due to the high nuclear charge experienced by inner-shell electrons traveling at relativistic speeds) and by poor shielding from the filled 4f and 5d subshells. These stabilized 6s² electrons are reluctant to participate in bonding, making Tl⁺ (using only the 6p electron) more stable than Tl³⁺. The same effect explains Pb²⁺ over Pb⁴⁺ and Bi³⁺ over Bi⁵⁺.

Question 2

Lithium and magnesium show a diagonal relationship — they share similar properties despite being in different groups and periods.

• T. True
• F. False

Answer: True

Diagonal relationships arise because moving one step right and one step down in the periodic table produces opposing trends that approximately cancel: electronegativity and ionization energy decrease going down a group but increase going across a period. Li and Mg have similar charge densities (Li⁺ is small and singly charged; Mg²⁺ is slightly larger but doubly charged). Both form covalent organometallic compounds, both have carbonate and fluoride salts that decompose on heating (unlike their group congeners), and both form nitrides directly. Similar diagonal relationships exist between Be/Al and B/Si.

Question 3

Nitrogen forms stable N≡N triple bonds while phosphorus does not form analogous P≡P triple bonds under normal conditions. This is an example of the first-row anomaly.

• T. True
• F. False

Answer: True

First-row elements in each p-block group have uniquely small atomic radii, which allows effective lateral overlap of p-orbitals to form strong pi-bonds. N₂ has a bond energy of 945 kJ/mol — one of the strongest bonds in chemistry. Phosphorus, being larger, has poor p-orbital overlap for pi-bonding and instead forms single bonds to multiple neighbors (P₄ tetrahedra with P-P single bonds). This first-row anomaly extends across the p-block: carbon forms strong C=C and C≡C bonds while silicon prefers single bonds; nitrogen and oxygen form pi-bonds easily while their heavier congeners do not.

Question 4

Explain why boron chemistry is dominated by electron-deficient compounds and cluster structures, while aluminum — in the same group — forms conventional ionic and covalent compounds.

Model answer: Boron has three valence electrons but four valence orbitals (2s + three 2p), making it inherently electron-deficient — it cannot form enough conventional two-center, two-electron bonds to fill its octet. This forces boron into unconventional bonding: three-center two-electron bonds (as in diborane B₂H₆), electron-deficient bridge bonds, and polyhedral cluster compounds (boranes, carboranes). Aluminum, though also in Group 13 with three valence electrons, is much larger and more electropositive. It readily forms Al³⁺ in ionic compounds or uses its size to achieve higher coordination numbers (6 in Al₂O₃) through conventional bonding. Al can also form electron-deficient bridges (Al₂Cl₆ is a dimer with bridging chlorides), but this tendency is less dominant than in boron because aluminum's larger size and lower ionization energy favor ionic character.

This contrast between boron and aluminum is one of the clearest examples of the first-row anomaly combined with size effects. Boron's small size forces it into exotic bonding arrangements that are characteristic of its chemistry but unusual for the rest of the group.