2.2.7 · D5Periodic Trends
Question bank — Diagonal relationship — Li - Mg, Be - Al, B - Si
Every trap here targets one idea from the parent note: the driver is polarising power via ionic potential , working through whichever input ends up matched — never an exact cancellation.
True or false — justify
Diagonal partners are similar because they share the same number of valence electrons.
False — Li has 1 valence electron, Mg has 2; the similarity comes from matched size / polarising power, a cross-trend effect, not a group (valence-count) effect.
Moving down-and-right makes cancel exactly, so the pair has identical ionic potential.
False — the right-move raises and the down-move lowers it, but only partly; for Li→Mg, actually roughly doubles (). The trends oppose; they do not annihilate.
For the Li/Mg pair it is the ionic potential itself that is matched.
False — for Li/Mg the radii are nearly equal ( vs pm) while differs by ~2×; the matched input here is size, which controls the lattice-vs-hydration energy balance.
For the Be/Al pair it is the ionic potential itself that is closely matched.
True — values ( vs ) are both high and close, so both cations strongly polarise anions and give covalent bonding; here (not radius) is the matched input.
All magnesium salts are insoluble, just like lithium salts.
False — only the carbonate, phosphate and fluoride of Mg are sparingly soluble (large lattice energy); MgCl₂, Mg(NO₃)₂ and MgSO₄ dissolve readily, as do LiCl and LiNO₃.
Lithium is an alkali metal, so Li₂CO₃ dissolves easily like Na₂CO₃.
False — Li⁺ is anomalously small and high-, so Li₂CO₃ is sparingly soluble and decomposes on heating, matching MgCO₃ and breaking the alkali-metal pattern.
A real B³⁺ cation exists and its ionic potential is what links B to Si.
False — B³⁺ is never actually formed; B and Si chemistry is covalent, so the link is a qualitative analogy of covalent radius and electronegativity, not an ionic argument.
Because Be is in group 2 it must behave like a typical metal such as Ca.
False — Be²⁺ is tiny with very high , so BeO is amphoteric and BeCl₂ is covalent/polymeric, matching Al; Ca (large, low ) is a straightforwardly basic, ionic metal.
Sodium also forms a nitride with N₂ under mild conditions, like Li and Mg.
False — only Li (→ Li₃N) and Mg (→ Mg₃N₂) form nitrides directly; Na does not, which is one of the sharpest signs Li tracks Mg rather than its own group.
Spot the error
"Li₂CO₃ is insoluble because Li⁺ is a large ion with weak lattice energy."
Wrong on both counts — Li⁺ is small and high-, giving a large lattice energy relative to hydration energy, which is precisely why the carbonate stays insoluble.
"BeCl₂ is ionic because Be is a metal in group 2."
Wrong — Be²⁺'s very high polarising power (Fajans) distorts Cl⁻ so strongly that the bonding is covalent; BeCl₂ forms polymeric chains and fumes/hydrolyses, unlike an ionic salt like CaCl₂.
"B resembles Si because both would form the same +4 ion."
Wrong — neither forms a simple +4 (or +3) cation; the resemblance is covalent, based on similar small covalent radius (~84 vs ~111 pm) and electronegativity (~2.0 vs ~1.9).
"Diagonal similarity proves φ is conserved along the down-right diagonal."
Wrong — is not conserved; the diagonal step leaves one input of (radius for Li/Mg, or itself for Be/Al) nearly matched, which is a weaker statement than φ-conservation.
"Al₂O₃ is basic like Na₂O, so Be/Al can't be an amphoteric pair."
Wrong — Al₂O₃ (and BeO) are amphoteric: they dissolve in acid () and in base (), which is exactly the shared property.
"Since B and C are group neighbours, boron hydrides behave like stable alkanes."
Wrong — boranes are volatile, flammable and hydrolysed by water, matching silanes (SiₙH₂ₙ₊₂) rather than the inert C–H alkanes; B tracks Si diagonally, not C.
Why questions
Why does moving right across a period raise polarising power?
Across a period increases and decreases (higher Effective Nuclear Charge (Zeff) pulls electrons in), so climbs from both the numerator rising and the denominator shrinking.
Why does moving down a group lower polarising power?
Ionic charge stays the same but radius grows (a new shell), so falls; the ion's surface field weakens.
Why does a diagonal step leave the pair with similar chemistry despite the trends not cancelling?
The right-step and down-step push in opposite directions, so at least one input — size or — lands near-matched, and chemistry (lattice/hydration balance, covalency) tracks that matched input. See Fajans' Rules.
Why is LiF sparingly soluble while LiCl is very soluble?
With the tiny F⁻, the small Li⁺ gives lattice energy that outweighs hydration energy (low solubility); with the larger, more polarisable Cl⁻ the lattice is weaker and hydration wins, so LiCl dissolves. This is a Lattice Energy vs Hydration Energy competition.
Why are BeCl₂ and AlCl₃ covalent and Lewis-acidic while MgCl₂ is much more ionic?
Be²⁺ and Al³⁺ have high, closely matched that strongly polarises Cl⁻ into covalent, electron-deficient chlorides; Mg²⁺'s lower leaves MgCl₂ predominantly ionic.
Why do Li₂CO₃ and MgCO₃ decompose on heating while Na₂CO₃ is stable?
The small, high- Li⁺/Mg²⁺ polarise the carbonate ion, weakening a C–O bond and favouring loss of CO₂ to the more stable oxide; the large, low- Na⁺ cannot do this, so Na₂CO₃ survives.
Why do we compare covalent radii for B/Si but ionic radii for Li/Mg?
Li/Mg chemistry is genuinely ionic, so ionic potential (ionic radius) applies; B/Si is covalent with no real cation, so only covalent size and electronegativity are meaningful measures.
Edge cases
Is the diagonal effect strongest for the ionic pair (Li/Mg) or the covalent pair (B/Si)?
The mechanism differs, not the strength: Li/Mg is driven by matched size (ionic, energetics-based), B/Si by matched covalent size + electronegativity; both are real, just via different inputs.
Does the diagonal relationship extend past B/Si (e.g. C/P, N/S)?
It is prominent mainly for the three period-2/3 pairs at the left where the anomalous first-element effect and -matching are strongest; further right the analogy weakens and is not usually invoked. See Anomalous Behaviour of First Element.
What happens to for a hypothetical degenerate ion with radius approaching zero?
As , , meaning infinite polarising power — a limiting reminder that smaller ions are always more polarising, which is why the smallest period-2 ions (Li⁺, Be²⁺) show the sharpest anomalies.
Why can't we just use the neutral atomic radius to argue the Li/Mg match?
The relevant species in salts are the cations, so ionic radii (Li⁺ 76, Mg²⁺ 72 pm) are what govern lattice and hydration energies; neutral radii would misrepresent the near-match. See Periodic Trends — Atomic and Ionic Radii.
Is BeO's amphoterism a coincidence, or does it follow from the same logic?
It follows — Be²⁺'s high makes Be–O bonds covalent enough that the oxide reacts with base as well as acid, the same reasoning that makes Al₂O₃ amphoteric. See Amphoterism.
If Mg²⁺ has higher than Li⁺, shouldn't Mg salts always be less soluble?
Not always — higher raises lattice energy but also raises hydration energy, so solubility depends on which grows faster with the anion; that's why only Mg carbonate/phosphate/fluoride are insoluble while MgCl₂/MgSO₄ dissolve.
Recall Two-line self-test before you leave
The diagonal driver is ::: opposing period/group trends in leaving one input (size or ) matched — never exact cancellation. The one claim you must never make ::: that φ cancels exactly for Li→Mg (it doubles; the radius is what matches).