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Lithium is an alkali metal, yet in many ways it behaves more like magnesium than like sodium. Beryllium, an alkaline earth metal, looks more like aluminium than like magnesium. Chemists call this the diagonal relationship. This post explains why it happens, checks the usual explanation with numbers from NCERT, and lists the similarities that questions use.
Where is this in the syllabus and the books?
The current NCERT Class 11 chapter on classification of elements describes it directly: lithium and beryllium form compounds with pronounced covalent character, unlike the rest of their groups, and their behaviour "is more similar with the second element of the following group i.e., magnesium and aluminium, respectively. This sort of similarity is commonly referred to as diagonal relationship".
The JEE (Advanced) 2026 brochure lists the s-block, including the "anomalous behaviour of lithium and beryllium". The JEE (Main) 2026 syllabus and NEET (UG) 2026 syllabus have no separate s-block unit, but they include periodic trends and the "unique behaviour of the first element in each group" of the p-block. The CBSE 2026-27 curriculum has no s-block chapter either. The detailed lists below come from the free official NIOS senior secondary lesson on the s-block. Check your exam's syllabus for the current year.
Why do diagonal neighbours resemble each other?
NCERT attributes the anomalous behaviour of the first member of each group to "small size, large charge/radius ratio and high electronegativity". NIOS adds the diagonal argument in terms of polarising power, which grows with ionic charge and falls with ionic size:
Going from lithium to sodium, the charge stays the same and the ion gets bigger, so polarising power falls.
Going from lithium to beryllium, the charge rises and the ion gets smaller, so polarising power rises.
Going from beryllium down to magnesium, it falls again and becomes comparable to lithium.
Moving right raises polarising power and moving down lowers it. A diagonal step does both, so the two changes partly cancel.
Worked example: test the idea with NCERT's radii
NCERT's table gives ionic radii of 76 pm for lithium, 102 pm for sodium, 31 pm for beryllium and 72 pm for magnesium. Use them with the usual charges: Li⁺, Na⁺, Be²⁺ and Mg²⁺. Use NIOS's measure, charge ÷ (radius)², in units of 10⁻⁴ pm⁻².
Li⁺: 1 ÷ 76² = 1 ÷ 5776 ≈ 1.73.
Na⁺: 1 ÷ 102² = 1 ÷ 10 404 ≈ 0.96.
Mg²⁺: 2 ÷ 72² = 2 ÷ 5184 ≈ 3.86.
Be²⁺: 2 ÷ 31² = 2 ÷ 961 ≈ 20.8.
Compare the neighbours of lithium. Beryllium, to its right, is about 12 times higher. Magnesium, diagonally below, is about 2.2 times higher. Sodium, directly below, is about half. So magnesium is the closest of the three in polarising power, and the sizes back this up: 76 pm for Li⁺ against 72 pm for Mg²⁺.
Notice what the numbers do not show: the values are not equal. The diagonal argument explains a direction and a closeness, not an exact match. Say "comparable", not "the same".
What do lithium and magnesium have in common?
NIOS lists nine similarities. The ones that turn up most in questions:
Both form nitrides when heated in nitrogen. Balancing with N³⁻: 6Li + N₂ → 2Li₃N and 3Mg + N₂ → Mg₃N₂.
Both form normal oxides when heated in air: Li₂O and MgO.
Their carbonates, nitrates and hydroxides decompose on heating to the oxides, for example Li₂CO₃ → Li₂O + CO₂.
Both form carbides when heated with carbon.
Their halides are appreciably soluble in organic solvents, and their oxyacid salts are sparingly soluble in water.
Our post on s-block elements puts these in the context of the whole of groups 1 and 2.
What do beryllium and aluminium have in common?
NIOS explains that "the properties in which beryllium differs from magnesium, it shares with aluminium (in general)". Three you should know:
Amphoteric behaviour. Beryllium is amphoteric, unlike magnesium: Be(OH)₂ dissolves in alkali to give [Be(OH)₄]²⁻. Aluminium dissolves in aqueous NaOH to form aluminates, which NIOS's group 13 lesson uses to show it is amphoteric.
Passivity. Both are rendered passive by concentrated nitric acid, which forms a protective oxide layer.
Covalent halides. Beryllium's high polarising power makes its compounds largely covalent; its halides fume in air and are hydrolysed. Aluminium chloride is covalent too and exists as the dimer Al₂Cl₆ at room temperature.
NIOS also notes a third pair: the chemistry of silicon "is very similar to that of boron, its diagonal neighbour".
Which mistakes cost marks?
Pairing the wrong elements. The partner is one period down and one group right: Li–Mg, Be–Al, B–Si.
Saying the charge densities are equal. They are comparable, as the calculation shows.
Writing LiN or MgN. Nitride is N³⁻: Li₃N and Mg₃N₂.
Forgetting the cause. Small size and high charge/radius ratio explain both the anomaly and the diagonal match.
If periodic trends themselves still feel like a list to memorise, read periodic trends and their exceptions first.
When a trend makes sense in one group but not across the table, the Fermi Concept Library covers every concept in chemistry, each with its own lesson and its place in the map.
A practice routine
Day 1: Draw the top-left corner of the periodic table and mark the three diagonal pairs.
Day 2: Repeat the charge-density calculation using charge ÷ radius, and compare the ranking with the one above.
Day 3: Write and balance five reactions that show Li–Mg or Be–Al similarities.
Day 4: Explain in four sentences why beryllium differs more from magnesium than lithium does from sodium.
Tonight, write the nine Li–Mg similarities from memory, then check them against the NIOS list.
