Is p- Block Elements Group 13 (Boron Family) part of the EAPCET Chemistry syllabus?

Yes. p- Block Elements Group 13 (Boron Family) is taught in Intermediate first year Chemistry, and the EAPCET Engineering syllabus is based on the Intermediate first- and second-year syllabus. Check the official notification each year for any changes.

How many Chemistry questions are there in EAPCET?

40 of the 160 questions in the Engineering paper are Chemistry, spread across 24 chapters.

What mistakes do students make in p- Block Elements Group 13 (Boron Family)?

Calling boric acid a tribasic protic acid because it has three OH groups — it is a monobasic Lewis acid that accepts OH⁻. Ranking BF₃ as the strongest Lewis acid because fluorine is most electronegative — back-bonding makes the order BF₃ < BCl₃ < BBr₃. Expecting atomic radius and ionisation enthalpy to change smoothly down the group — Ga is smaller than Al because of poor shielding by 3d electrons. Saying Tl³⁺ is more stable than Tl⁺ — the inert pair effect makes the +1 state of thallium more stable, so Tl³⁺ is a good oxidising agent.

EAPCET Chemistry · Intermediate first year

p- Block Elements Group 13 (Boron Family)

Q: How should I prepare p- Block Elements Group 13 (Boron Family) for EAPCET?

Draw the structure of diborane until you can do it from memory — it is asked repeatedly.

This chapter covers the boron family: trends, the inert pair effect and the structures of compounds such as diborane and boric acid.

Subject: Chemistry
Taught in: Intermediate first year
Chemistry in the paper: 40 of 160 questions

Key formulas and results

Configuration ns²np¹
Stability of the +1 state increases down the group (inert pair effect), so $\ce{Tl+}$ is more stable than $\ce{Tl^3+}$
Boric acid is a weak monobasic Lewis acid
Diborane has two three-centre two-electron (banana) bonds
Aluminium chloride exists as the dimer $\ce{Al2Cl6}$

What this chapter covers

General trends in group 13
Inert pair effect
Anomalous properties of boron
Boric acid, borax and diborane
Aluminium chloride

How it is usually asked

Structure of diborane
Acidic nature of boric acid
Inert pair effect
Anomalous behaviour of boron

Understanding p- Block Elements Group 13 (Boron Family)

Group 13 elements (B, Al, Ga, In, Tl) have three valence electrons, ns²np¹. Boron is a non-metal that forms only covalent compounds, while the rest are metals. Trends are irregular: gallium is smaller than aluminium and has a slightly higher ionisation enthalpy, because the poorly shielding 3d electrons raise the effective nuclear charge. Down the group, the ns² pair becomes reluctant to take part in bonding (inert pair effect), so the +1 state becomes more stable and Tl⁺ is preferred over Tl³⁺.

With only six electrons around boron, compounds such as BF₃ are electron-deficient and act as Lewis acids. Boric acid, H₃BO₃, does not release its own protons; it accepts OH⁻ from water to form [B(OH)₄]⁻, so it is a weak monobasic Lewis acid. The Lewis acid strength of boron halides is BF₃ < BCl₃ < BBr₃, because back-bonding from fluorine’s 2p orbital into boron’s empty 2p orbital is most effective.

Diborane, B₂H₆, has four terminal B–H bonds (normal two-centre bonds) and two bridging B–H–B bonds, each a three-centre two-electron (banana) bond; each boron is roughly sp³ hybridised. Aluminium chloride dimerises to Al₂Cl₆ so that aluminium completes its octet. EAPCET commonly asks about diborane’s bonds, boric acid’s basicity and the inert pair effect.

Worked example

EAPCET-style question

In the diborane ( $\ce{B2H6}$ ) molecule, the numbers of three-centre two-electron bridge bonds and two-centre two-electron terminal B–H bonds are, respectively:

(1)2 and 4
(2)4 and 2
(3)2 and 6
(4)0 and 6

Solution

$\ce{B2H6}$ has 12 valence electrons ( $3 \times 2$ from B and 6 from H) — too few for seven ordinary two-electron bonds.
Each boron holds two terminal hydrogens in ordinary B–H bonds: $2 \times 2 = 4$ terminal bonds using 8 electrons.
The remaining 4 electrons form two B–H–B bridges, each a three-centre two-electron bond.
So there are 2 bridge bonds and 4 terminal bonds; the bridging hydrogens lie above and below the plane of the other four.

Answer: 2 and 4

Common mistakes to avoid

Calling boric acid a tribasic protic acid because it has three OH groups — it is a monobasic Lewis acid that accepts OH⁻.
Ranking BF₃ as the strongest Lewis acid because fluorine is most electronegative — back-bonding makes the order BF₃ < BCl₃ < BBr₃.
Expecting atomic radius and ionisation enthalpy to change smoothly down the group — Ga is smaller than Al because of poor shielding by 3d electrons.
Saying Tl³⁺ is more stable than Tl⁺ — the inert pair effect makes the +1 state of thallium more stable, so Tl³⁺ is a good oxidising agent.

How to prepare it

Draw the structure of diborane until you can do it from memory — it is asked repeatedly.

Once the formulas feel familiar, test them the way the exam does: practise p- Block Elements Group 13 (Boron Family) questions chapter-wise, then attempt a full previous year paper to see whether it holds up under a 3-hour timer.

Related Chemistry chapters

See every chapter in the EAPCET Chemistry syllabus.

Practise p- Block Elements Group 13 (Boron Family) chapter-wise

Chapter-wise quizzes show exactly which topics in this chapter you still get wrong — before a full paper does.

Frequently asked questions

Is p- Block Elements Group 13 (Boron Family) part of the EAPCET Chemistry syllabus?: Yes. p- Block Elements Group 13 (Boron Family) is taught in Intermediate first year Chemistry, and the EAPCET Engineering syllabus is based on the Intermediate first- and second-year syllabus. Check the official notification each year for any changes.
How should I prepare p- Block Elements Group 13 (Boron Family) for EAPCET?: Draw the structure of diborane until you can do it from memory — it is asked repeatedly.
How many Chemistry questions are there in EAPCET?: 40 of the 160 questions in the Engineering paper are Chemistry, spread across 24 chapters.
What mistakes do students make in p- Block Elements Group 13 (Boron Family)?: Calling boric acid a tribasic protic acid because it has three OH groups — it is a monobasic Lewis acid that accepts OH⁻. Ranking BF₃ as the strongest Lewis acid because fluorine is most electronegative — back-bonding makes the order BF₃ < BCl₃ < BBr₃. Expecting atomic radius and ionisation enthalpy to change smoothly down the group — Ga is smaller than Al because of poor shielding by 3d electrons. Saying Tl³⁺ is more stable than Tl⁺ — the inert pair effect makes the +1 state of thallium more stable, so Tl³⁺ is a good oxidising agent.