Diagonal Relationship in the Periodic Table (Li-Mg, Be-Al)
Chemistry · Periodic Classification Of Properties · NEET
The diagonal relationship means the first element of a group is more like the element one step down and one step to the right, than like the rest of its own group. So lithium (Li) behaves like magnesium (Mg), and beryllium (Be) behaves like aluminium (Al). Memory hook: "go one down, one right" - Li points to Mg, Be points to Al, like a staircase step.
Move one box down and one box right: Li points to Mg and Be points to Al. The size increase going down and the size decrease going right cancel, so each diagonal pair shares similar size, polarising power and chemistry.
Your doubts, answered
Why does lithium resemble magnesium and not the other alkali metals?
Lithium is very small. As you move down group 1 (Na, K...), atoms get much bigger, so Li stands apart from them. But if you move one step to the right into group 2, atoms get smaller again. These two changes cancel out, so Li ends up close in size and charge-density to Mg (the second element of group 2). Because of this, Li and Mg share many properties, like forming a nitride (Li3N, Mg3N2) and having a hard, less soluble carbonate.
Why does beryllium show a diagonal relationship with aluminium?
Be is in group 2 and Al is in group 13. Moving down a group increases size (weaker pull), and moving right across a period decreases size (stronger pull). Going one step down AND one step right from Be lands on Al, and the two effects roughly balance. So Be2+ and Al3+ have similar 'polarising power' (charge divided by size). Both form covalent, amphoteric oxides (BeO, Al2O3) and both dissolve in strong alkali.
What is the actual reason behind the diagonal relationship?
It comes from two periodic trends working against each other. Down a group, atomic/ionic size increases. Across a period (left to right), size decreases and effective nuclear charge increases. When you go diagonally (one down, one right), the size increase and size decrease nearly cancel. The result is that the diagonal pair has nearly the same size, same charge/size ratio (polarising power) and same electronegativity - so they react in similar ways.
Which pairs actually show the diagonal relationship in NEET?
The two pairs you must know are Lithium-Magnesium (Li-Mg) and Beryllium-Aluminium (Be-Al). A third often-mentioned pair is Boron-Silicon (B-Si). NEET questions almost always test Li-Mg and Be-Al, so memorise those two first.
Do Li and Be form ionic or covalent compounds? This confuses me.
This is a favourite NEET trap. Unlike the rest of their groups, Li and Be form compounds with pronounced COVALENT character, not ionic. Their ions are tiny with high charge density (high polarising power), so they distort the electron cloud of the anion and make the bond covalent. NEET 2023 marked an assertion FALSE for saying Li and Be form 'pronounced ionic' compounds - remember it is covalent.
Is Zr and Hf having similar radii an example of diagonal relationship?
No. Zr and Hf have almost the same radius because of the LANTHANOID CONTRACTION, not the diagonal relationship. NEET 2021 used this exact trap - 'diagonal relationship' was a wrong option. Diagonal relationship is only between the first-period elements and their lower-right neighbour (Li-Mg, Be-Al).
⚠️ The NEET trap ✗ Since Li and Mg show a diagonal relationship, Li (like all alkali metals) forms strongly ionic compounds. ✓ Li and Be are the exceptions in their groups: they form compounds with pronounced COVALENT character because their tiny ions have very high polarising power. The diagonal relationship (Li-Mg) is real, but it does NOT make Li ionic. 🧠 First element = odd one out. Li and Be break their group's ionic habit and turn covalent. If a statement says 'Li/Be form pronounced ionic compounds', it is FALSE.
Real NEET questions
NEET 2023 Phase 2
Assertion (A): Lithium and beryllium, unlike their other respective group members, form compounds with pronounced ionic character. Reason (R): Lithium and magnesium have similar properties due to the diagonal relationship.
A · Both (A) and (R) are true and (R) is the correct explanation of (A).
B · Both (A) and (R) are true but (R) is not the correct explanation of (A).
C · (A) is true but (R) is false.
D · (A) is false but (R) is true. ✓
Solution: Li and Be have very small ions with high polarising power, so they form compounds with pronounced COVALENT (not ionic) character. So the Assertion is false. The Reason is true: Li and Mg really do show similar properties because of the diagonal relationship. False (A) + true (R) gives option D. This question mixes the diagonal relationship with the covalent-character trap - the two together are a classic NEET combo.
NEET 2022 / 2026
Identify the incorrect statement from the following:
A · The largest and the smallest species among Mg, Mg2+, Al and Al3+ are Al and Mg2+ respectively. ✓
B · The IUPAC name of the element with atomic number 107 is Unnilseptium.
C · The similarity in behaviour of Li with Mg is referred to as the 'diagonal relationship'.
D · The oxidation state and covalency of Al in [AlCl(H2O)5]2+ are 3 and 6, respectively.
Solution: Statement C is correct - the Li and Mg similarity IS the diagonal relationship, so it is not the answer. The incorrect statement is A: among Mg, Mg2+, Al and Al3+, the largest is neutral Mg (most electrons, weaker nuclear pull than Al) and the smallest is Al3+ (highest charge, fewest electrons). A wrongly claims Al is largest and Mg2+ smallest, so A is the incorrect statement. This confirms C as a true, testable definition of diagonal relationship.
NEET 2021
Zr (Z=40) and Hf (Z=72) have similar atomic and ionic radii because of:
A · lanthanoid contraction ✓
B · having similar chemical properties
C · belonging to same group
D · diagonal relationship
Solution: Hf comes right after the lanthanoids. The poor shielding by the intervening 4f electrons raises the effective nuclear charge and cancels the expected size increase down the group, so Zr and Hf end up with nearly equal radii - this is the LANTHANOID CONTRACTION (option A). 'Diagonal relationship' (option D) is a deliberate trap: it only links first-period elements to their lower-right neighbour (Li-Mg, Be-Al), never Zr-Hf.
Solved Periodic Classification Of Properties NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is the close similarity between an element and the element placed one step down and one step to the right of it in the periodic table - mainly Li with Mg and Be with Al.
Why does the diagonal relationship happen?
Two trends cancel out: size increases down a group but decreases across a period. Going diagonally balances them, so the pair ends up with similar size, charge/size ratio (polarising power) and electronegativity.
Which are the main diagonal pairs for NEET?
Li-Mg and Be-Al are the two you must know. B-Si is a third, less-tested pair.
Do Li-Mg share real chemical properties?
Yes. Both form nitrides (Li3N, Mg3N2), both have carbonates that decompose on heating, and both form more covalent compounds than the rest of group 1 and 2.
Why do Be and Al behave alike?
Be2+ and Al3+ have almost the same polarising power (charge divided by size). Both form amphoteric oxides (BeO, Al2O3) and both dissolve in strong alkali to give complex ions.
Is the diagonal relationship the same as the anomalous behaviour of second-period elements?
They are linked but not the same. The first element of each group is anomalous (different from its group). The diagonal relationship explains WHY that anomalous first element instead matches its lower-right diagonal neighbour.
