Diagonal Relationship of B and Si
Why do boron and silicon exhibit remarkably similar chemical properties and belong to different periods? What is the reason behind this unusual resemblance between two elements placed diagonally apart in the periodic table? This phenomenon is known as the diagonal relationship, an important concept in inorganic chemistry that explains the similarities in the physical and chemical behaviour of certain pairs of elements. The diagonal relationship between boron and silicon arises due to their comparable atomic size, electronegativity, and charge-to-radius ratio. In this article, we will explore the meaning, causes, and examples of the diagonal relationship between boron and silicon and understand its significance in the study of periodic trends.
This Story also Contains
- Resemblance of Boron (B) with Silicon (Si)
- Diagonal Relationship of Boron and Silicon
- Similarities Between Boron and Silicon
- Some Solved Examples
Resemblance of Boron (B) with Silicon (Si)
Diagonal Relationship of Boron and Silicon
The diagonal relationship refers to the similarity in properties between certain pairs of diagonally adjacent elements in the periodic table. Boron (B) of Group 13 and Silicon (Si) of Group 14 exhibit a pronounced diagonal relationship due to their comparable atomic size, electronegativity, and charge/radius ratio.
Reasons for the Diagonal Relationship
Similar atomic sizes: Boron and silicon have comparable atomic radii.
Similar electronegativity: Boron (2.04) and silicon (1.90) have nearly similar electronegativities.
High charge density: Both possess relatively high charge-to-radius ratios, leading to similar chemical behaviour.
Similarities Between Boron and Silicon
| Property | Boron (B) | Silicon (Si) |
|---|---|---|
| Nature | Metalloid | Metalloid |
| Oxides | $\mathrm{B}_2 \mathrm{O}_3$ (acidic) | $\mathrm{SiO}_2$(acidic) |
| Halides | $\mathrm{BCl}_3$ | $\mathrm{SiO}_4$ |
| Hydrides | Boranes | Silanes |
| Bonding | Predominantly covalent | Predominantly covalent |
| Reaction with alkalis | Liberates H2 | Liberates H2 |
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Some Solved Examples
Question 1: Which of the following elements does not show a diagonal relationship?
1) B – Si
2) Li – Mg
3) Be – Al
4) Li – Na
Solution:
Li–Mg, B–Si, and Be–Al show a diagonal relationship, but Li and Na do not, as both belong to the same group and are not placed diagonally.
Hence, the correct answer is option (4).
Question 2:Which of the following shows a diagonal relationship with B?
1) C
2) Si
3) Mg
4) P
Solution:
Si shows a diagonal relationship with B due to their similar physical and chemical properties.
Hence, the correct answer is option (2).
Question 3: Which of the following pairs of elements form acidic oxides?
1) B and Fe
2) B and Si
3) Mg and Si
4) B and Mg
Solution:
Oxides of B and Si are both acidic and can be reduced by a limited amount of Mg.
Both are readily soluble in alkalies:
$\begin{aligned} & \mathrm{B}_2 \mathrm{O}_3+3 \mathrm{Mg} \rightarrow 3 \mathrm{MgO}+2 \mathrm{~B} \\ & \mathrm{SiO}_2+2 \mathrm{Mg} \rightarrow 2 \mathrm{MgO}+\mathrm{Si}\end{aligned}$
Hence, the correct answer is option (2).
Question 4: Which of the following properties is primarily responsible for the diagonal relationship between boron and silicon?
1) Same number of valence electrons
2) Similar atomic mass
3) Similar atomic size and electronegativity
4) Same oxidation state in all compounds
Solution:
Diagonal relationship arises because diagonally adjacent elements possess similar atomic radii, electronegativity, and charge density, leading to similarities in their chemical behavior.
- Boron and silicon do not have the same number of valence electrons.
- Their atomic masses are quite different.
- They do not always exhibit the same oxidation states.
Hence, the correct answer is option (3).
Question 5: Which one of the following statements correctly illustrates the diagonal relationship between boron and silicon?
1) Both form ionic chlorides that conduct electricity in the molten state.
2) Both form acidic oxides and predominantly covalent compounds.
3) Both show a strong tendency to form +3 ions in aqueous solution.
4) Both readily form stable cations in water.
Solution:
- B₂O₃ and SiO₂ are acidic oxides.
- BCl₃ and SiCl₄ are covalent compounds.
- Boron and silicon do not readily form simple cations in aqueous solution.
- Their chlorides are covalent, not ionic.
Hence, the correct answer is option (2).
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Frequently Asked Questions (FAQs)
When some elements on the periodic table that happen to be diagonally adjacent to each other exhibit similar properties, then there is a diagonal relationship. This results from the balancing of atomic size versus electronegativity across different periods and groups coming out to be similar, thus causing similar chemical properties.
Their diagonal relation could be explained by the fact that Boron and Silicon exhibit electronegativities, ionization energies, and a possibility of forming covalent bonds of similar magnitude. All these chemical properties result from their diagonal position in the periodic table.
Examples include emissions in the same structure, such as borates B2O3 and silicates SiO2, common in glass and ceramics. These elements exist in tetrahedral structures within their compounds.
Both Boron and silicon are semiconductors such that in industry, boron dopes silicon, a process that increases the semiconductor's conductance, and it acts as the basis for making transistors, and hence, eventually computers, which form part of most modern electronics
