Why does Beryllium show a diagonal relationship with Aluminum?

1. Why does Beryllium show a diagonal relationship with Aluminum? +

Correct Answer

Be and Al have similar charge density, polarising power, electronegativity and covalent character.

Detailed Explanation

Beryllium (Be) belongs to Group 2, Period 2, while Aluminium (Al) belongs to Group 13, Period 3. Although they are in different groups and periods, they show several similar chemical properties. This is called a diagonal relationship.

Be ↘ Al

The main reason is that moving downward increases atomic size, while moving towards the right decreases atomic size. These opposing effects partly compensate each other.

Why Be and Al Become Similar?

1. Similar Charge Density Be²⁺ is a very small ion with a +2 charge, while Al³⁺ has a higher charge but a somewhat larger ionic size. Both therefore have relatively high charge density and strong polarising power.

High charge density → Strong polarising power → Greater covalent character

2. Similar Polarising Power Both Be²⁺ and Al³⁺ strongly distort the electron cloud of anions. Hence, their compounds often show appreciable covalent character. 3. Similar Electronegativity The electronegativities of Be and Al are relatively close compared with what might be expected from their positions in the periodic table. 4. Similar Nature of Oxides Both form amphoteric oxides:

BeO and Al₂O₃ are amphoteric.

They react with both acids and strong bases.

Important Similarities

Property	Be	Al
Oxide	BeO	Al2O3
Nature of oxide	Amphoteric	Amphoteric
Hydroxide	Be(OH)2	Al(OH)3
Hydroxide nature	Amphoteric	Amphoteric
Covalent character	High	High

Important Reactions

BeO with acid:

BeO + 2HCl → BeCl₂ + H₂O

BeO with base:

BeO + 2NaOH + H₂O → Na₂[Be(OH)₄]

Al₂O₃ with acid:

Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O

Al₂O₃ with base:

Al₂O₃ + 2NaOH + 3H₂O → 2Na[Al(OH)₄]

These reactions demonstrate their common amphoteric behaviour.

JEE / NEET Shortcut

Be ↘ Al = Diagonal Relationship

Remember the three important diagonal pairs:

Li – Mg
Be – Al
B – Si

For Be–Al, remember:

Small size + High charge density + Strong polarising power → Covalent character + Amphoteric oxides

30 Related MCQs with Solutions

1. Beryllium shows diagonal relationship with:

A. Mg

B. Al

C. Si

D. Na

Answer: B. Al

2. The main reason for Be–Al diagonal relationship is:

A. Same group

B. Similar charge density and polarising power

C. Same atomic number

D. Same number of shells

Answer: B

3. Which oxides are amphoteric?

A. BeO and Al₂O₃

B. BeO and Na₂O

C. Al₂O₃ and K₂O

D. Na₂O and K₂O

Answer: A

4. Be²⁺ has high polarising power mainly because it has:

A. Large size

B. Small size and high charge density

C. Low charge

D. Many electron shells

Answer: B

5. Which pair is a standard diagonal pair?

A. Be–Al

B. Be–Mg

C. Be–Ca

D. Al–Ga

Answer: A. Be–Al

6. BeO is:

A. Strongly acidic

B. Strongly basic

C. Amphoteric

D. Neutral

Answer: C. Amphoteric

7. Al₂O₃ is:

A. Acidic

B. Basic

C. Amphoteric

D. Neutral

Answer: C. Amphoteric

8. Which property is similar for Be and Al?

A. Amphoteric nature of oxides

B. Same valency only

C. Same group number

D. Same atomic radius

Answer: A

9. Diagonal relationship occurs because of compensation between:

A. Size and effective nuclear charge trends

B. Mass and volume

C. Neutrons and protons

D. Melting and boiling points

Answer: A

10. Which ion has very high charge density?

A. K⁺

B. Na⁺

C. Be²⁺

D. Cs⁺

Answer: C. Be²⁺

11. Be–Al similarity is associated with greater:

A. Ionic character

B. Covalent character

C. Metallic radius

D. Basicity only

Answer: B. Covalent character

12. Which hydroxides show amphoteric behaviour?

A. Be(OH)₂ and Al(OH)₃

B. NaOH and KOH

C. Ca(OH)₂ and Ba(OH)₂

D. LiOH and NaOH

Answer: A

13. Which element is in Group 2?

A. Be

B. Al

C. Si

D. B

Answer: A. Be

14. Aluminium belongs to:

A. Group 1

B. Group 2

C. Group 13

D. Group 14

Answer: C. Group 13

15. Which is NOT a reason for Be–Al similarity?

A. Similar charge density

B. Similar polarising power

C. Same atomic number

D. Similar covalent character

Answer: C. Same atomic number

16. BeO reacts with NaOH because BeO is:

A. Acidic only

B. Basic only

C. Amphoteric

D. Neutral

Answer: C. Amphoteric

17. Al₂O₃ reacts with acids and bases because it is:

A. Neutral

B. Amphoteric

C. Only acidic

D. Only basic

Answer: B. Amphoteric

18. Which diagonal relationship is correct?

A. Li–Mg

B. Be–Al

C. B–Si

D. All of these

Answer: D. All of these

19. The high polarising power of Be²⁺ leads to:

A. Greater covalent character

B. Only ionic character

C. Lower charge density

D. Larger ionic radius

Answer: A

20. Be and Al show similarity particularly in the nature of their:

A. Oxides and hydroxides

B. Noble gases

C. Halides only as gases

D. Atomic masses

Answer: A

21. Which oxide is amphoteric?

A. BeO

B. Al₂O₃

C. Both A and B

D. Neither

Answer: C. Both A and B

22. Which property increases covalent character in Be and Al compounds?

A. High polarising power

B. Very large ionic size

C. Low charge density

D. Low nuclear attraction

Answer: A

23. Diagonal relationship is most prominent in:

A. Early elements of the periodic table

B. Noble gases only

C. Lanthanides only

D. Actinides only

Answer: A

24. Be and Al both form:

A. Amphoteric oxides

B. Only acidic oxides

C. Only strongly basic oxides

D. Noble gases

Answer: A

25. Which is the best explanation of diagonal relationship?

A. Same electronic configuration

B. Similar physical and chemical properties due to compensating periodic trends

C. Same atomic number

D. Same group

Answer: B

26. Which pair has similar amphoteric hydroxides?

A. Be(OH)₂ and Al(OH)₃

B. NaOH and KOH

C. Mg(OH)₂ and Ca(OH)₂

D. KOH and Ba(OH)₂

Answer: A

27. Be is diagonally related to Al because:

A. Be is small and Al has relatively high charge density

B. Both are in Group 2

C. Both are in Period 2

D. Both have identical configurations

Answer: A

28. Which pair represents the Group 2 → Group 13 diagonal relationship?

A. Be → Al

B. Mg → Ga

C. Ca → In

D. Sr → Tl

Answer: A. Be → Al

29. Which factor is common to the diagonal relationship between Be and Al?

A. High charge density

B. Same number of shells

C. Same atomic mass

D. Same number of neutrons

Answer: A. High charge density

30. The most important conclusion about Be–Al diagonal relationship is:

A. Their properties are completely identical

B. Their similar charge density and polarising power produce several similar chemical properties

C. They belong to the same group

D. They have the same electronic configuration

Answer: B

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