Question

Lithium shows diagonal relationship with

• A. $\mathrm{Mg}$
• B. $\mathrm{Al}$
• C. $\mathrm{Be}$
• D. $\mathrm{Na}$

Hint:

Remember the three classic diagonal pairs of the periodic table: Lithium ($\mathrm{Li}$) with Magnesium ($\mathrm{Mg}$), Beryllium ($\mathrm{Be}$) with Aluminum ($\mathrm{Al}$), and Boron ($\mathrm{B}$) with Silicon ($\mathrm{Silicon}$).

Answer 1

The Correct Option is A

Step 1: Recall the diagonal relationship.
Certain Period 2 elements resemble the element placed one step down and one step right in Period 3, owing to similar size and polarising power.
Step 2: Place lithium.
Lithium is in Group 1, Period 2. Moving diagonally down-right lands on Group 2, Period 3.
Step 3: Identify the diagonal partner.
Group 2, Period 3 is magnesium, so $\mathrm{Li}$ pairs diagonally with $\mathrm{Mg}$.
Step 4: Justify with ionic size.
The radii are close, $r_{\mathrm{Li^+}} \approx 76\ \mathrm{pm}$ and $r_{\mathrm{Mg^{2+}}} \approx 72\ \mathrm{pm}$, giving nearly equal polarising power.
Step 5: Cite shared behaviour.
Both form normal oxides rather than peroxides, both have covalent-leaning compounds, and both decompose their carbonates on heating, confirming the kinship.
Step 6: Conclude.
Lithium shows a diagonal relationship with magnesium, option (A).
\[ \boxed{\mathrm{Mg}} \]