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What is diagonal relationship? Give one example.

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Last updated date: 28th Mar 2024
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MVSAT 2024
Answer
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Hint: A relationship exists between certain diagonally adjacent elements of the s-block due to which they have similar properties. Such relationships exist because moving rightward and descending the periodic table have opposite effects.

Complete answer:
Some pairs of diagonally adjacent elements of s-block in the second and third periods of the periodic table exhibit similar properties. Most of the properties follow opposite trends when moving along a period and down a group. So, the changes more or less cancel each other out, and elements with similar properties can be found. On moving along a period, the elements become more covalent, more electronegative and less basic. On the other hand, on moving down a group, the elements become more ionic, less electronegative and more basic. So, the atomic radius, electronegativity, properties of compounds, etc. of the diagonal elements are similar.
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The properties of the elements of first-group second-period are often similar to the properties of the elements of second-group third-period. Thus, the properties of Li are similar to Mg, the properties of Be are similar to that of Al and B and Si also have the same properties.
For the Li-Mg pair:
- Lithium reacts slowly with water as compared to other Group 1 elements, alike Mg.
- The hydroxides of both the elements are weak bases and decompose on heating.
- Li is the only element of Group 1 which forms a stable nitride . Mg and other Group 2 also form nitrides.
- Li and Mg both form covalent organometallic compounds while the rest of Group 1 and Group 2 compounds analogues are ionic and very reactive.
- Metals of Group 1, excluding Li form peroxides and superoxides when reacted with oxygen under standard conditions. Li, on the other hand, forms normal oxides like Mg.
- Heating properties of carbonates of Li and Mg are similar. [Both are unstable and produce corresponding oxides and carbon dioxide upon heating].

Note: Some might consider diagonal relationship past B and Si too which is wrong. Practically, after B-Si pair the diagonal relationship becomes less dominant.
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