Question bank — Alkaline earth metals (Group 2) — physical - chemical properties, anomaly of Be, diagonal Be-Al

False — $Z_{eff}$ on the valence electrons stays roughly constant (even rises slightly); IE falls because the principal quantum number $n$ rises, so $IE\propto Z_{eff}^2/n^2$ shrinks via the growing $n^2$.

False — a +2 ion needs two ionisations, so removing electrons costs more; Group 2 is less reactive than the corresponding Group 1 metal.

True — a tough, adherent oxide layer protects it, so unlike Mg (which reacts with steam) Be resists water at high temperature.

False — Ca (1.54 g/cm³) is actually the lowest in the group; its large atomic volume and inefficient packing beat Be (1.85), whose tiny size gives tight, dense packing despite low mass.

False — Be is highest (1560 K); Mg is anomalously the lowest (923 K) because of its particular packing and weaker metallic bonding.

False — ${\text{Be(OH)}}_2$ is amphoteric (reacts with both acids and bases); basicity rises down the group, so only the large cations give strongly basic hydroxides.

False — ${\text{BeCl}}_2$ is essentially covalent; Be's high charge density polarises the chloride so strongly that electrons are shared, not fully transferred.

True — forming ${\text{M}}^{2+}$ removes the entire outermost shell and leaves more protons per remaining electron, so ${\text{M}}^{2+}$ is always smaller than $\text{M}$.

False — more negative $E°$ means the metal is more readily oxidised, i.e. a stronger reducing agent (Ba at −2.91 V is the strongest here).

Error — Ba (and partly Sr) also forms the peroxide ${\text{BaO}}_2$; the large ${\text{Ba}}^{2+}$ stabilises the large ${\text{O}}_2^{2-}$ ion in the lattice.

Error — both are +2; the small, high-charge-density ${\text{Be}}^{2+}$ polarises and shatters ${\text{O}}_2^{2-}$ into ${\text{O}}^{2-}$, so it favours the simple oxide, not the peroxide.

Error — cause and label are swapped; radius increases because each element adds a shell and inner-shell shielding outweighs the added protons, giving $r\propto n^2/Z_{eff}$.

Error — IE₂ is always much larger, because the second electron is pulled from an already positive ${\text{M}}^{+}$ ion, which grips its electrons harder.

Error — ionisation is only one term; the huge hydration energy of the small, doubly-charged ${\text{Ca}}^{2+}$ (≈ −1577 kJ/mol) more than repays it, making the overall reaction exothermic (≈ −430 kJ/mol).

Error — fluorides are the least soluble (their high lattice energy resists dissolving); the chlorides/bromides/iodides are the more soluble ones.

Error — Be shows a diagonal relationship with aluminium (up-right neighbour), because Be²⁺ and Al³⁺ have similar charge density and polarising power, not because they share a group.

The atom grows down the group, so its nucleus holds shared bonding electrons more weakly, lowering its pull on them.

Be²⁺'s enormous charge density polarises the O–H bonds so the compound can release H⁺ or accept OH⁻, giving both acidic and basic behaviour; larger cations can't do this, so they are purely basic.

Both ${\text{Mg}}^{2+}$ and ${\text{O}}^{2-}$ are small, so $r_{+}+r_{-}$ is tiny and $LE\propto \frac{q_{+}{q}_{-}}{r_{+}+r_{-}}$ becomes large — small ions plus double charges make an exceptionally stable lattice.

It has the lowest ionisation energy (loses its two electrons most easily) and the most negative $E°$ (−2.91 V), so it gives electrons away most readily.

Be²⁺ is so small and dense that it distorts the chloride's electron cloud heavily (high polarising power, Fajans-type), pulling toward shared electrons; the larger, gentler Ca²⁺ leaves the ions intact.

Bigger atoms hold their valence electrons more loosely, so Ba releases them most easily and is the most metallic, while tiny Be clings hardest and is the least metallic.

Its very small size and high charge density push it toward covalent bonding and amphoteric behaviour, so it resembles Al (diagonal) more than Mg (its group).

Beryllium — its protective oxide film blocks the reaction even at elevated temperature, the degenerate end of the reactivity trend.

Density and melting point both wobble — e.g. Ca is the least dense and Mg has the lowest melting point — because crystal packing and bonding electron count compete with size and mass.

It becomes more stable, because the large cation no longer has enough charge density to polarise and break ${\text{O}}_2^{2-}$, so peroxides appear only at the large-cation end (Sr, Ba).

${\text{Ba(OH)}}_2$ is far more soluble — hydroxide solubility rises down the group as lattice energy falls faster than hydration energy for the larger cations.