Is Classification of Elements and Periodicity in Properties part of the EAPCET Chemistry syllabus?

Yes. Classification of Elements and Periodicity in Properties 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 Classification of Elements and Periodicity in Properties?

Saying fluorine has the most negative electron gain enthalpy — it is chlorine, because fluorine’s compact 2p shell repels the added electron. Applying the across-period trend blindly to ionisation enthalpy — remember Be > B and N > O in period 2 (and Mg > Al, P > S in period 3). Ranking isoelectronic ions by atomic number in the wrong direction — the ion with more protons is smaller, so Mg²⁺ < Na⁺ < F⁻ < O²⁻. Confusing electronegativity (a property of a bonded atom, no units on the Pauling scale) with electron gain enthalpy (an energy change of an isolated gaseous atom in kJ mol⁻¹).

EAPCET Chemistry · Intermediate first year

Classification of Elements and Periodicity in Properties

Q: How should I prepare Classification of Elements and Periodicity in Properties for EAPCET?

The trends are easy; the marks are in the exceptions — list them separately and revise them often.

This chapter covers the modern periodic table and how properties such as atomic radius, ionisation enthalpy, electron gain enthalpy and electronegativity vary across it.

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

Key formulas and results

Properties are periodic functions of atomic number
Atomic radius decreases across a period and increases down a group
Ionisation enthalpy generally increases across a period
Fluorine has the highest electronegativity
Chlorine has the most negative electron gain enthalpy
Moseley’s law: $\sqrt{\nu} \propto Z$ , where $\nu$ is the frequency of the characteristic X-rays
Mulliken electronegativity: $\chi = \frac{\text{IE} + \text{EA}}{2}$ ; Mulliken values (in eV) are roughly 2.8 times the Pauling values
Pauling scale: $\chi_A - \chi_B = 0.208\sqrt{\Delta}$ , where $\Delta = E(\text{A--B}) - \sqrt{E(\text{A--A}) \times E(\text{B--B})}$ in kcal/mol
Radii of the same atom: covalent radius < metallic (crystal) radius < van der Waals radius

What this chapter covers

Modern periodic law
s, p, d and f blocks
Atomic and ionic radii
Ionisation enthalpy
Electron gain enthalpy
Electronegativity

How it is usually asked

Order elements by a property
Explain exceptions such as N vs O or Be vs B in ionisation enthalpy
Identify the block of an element

Understanding Classification of Elements and Periodicity in Properties

The modern periodic law says that properties of elements are periodic functions of their atomic numbers, not their atomic masses. Elements are arranged in 18 groups and 7 periods, and the last electron decides the block: s-block (groups 1–2), p-block (groups 13–18), d-block (groups 3–12) and f-block (lanthanoids and actinoids). A period number equals the outermost shell, so an element with configuration [Ar]3d¹⁰4s²4p³ is in period 4, group 15, p-block.

Two ideas explain almost every trend: effective nuclear charge and shell number. Across a period, electrons enter the same shell while nuclear charge rises, so atoms shrink, ionisation enthalpy increases and electronegativity increases. Down a group, a new shell is added each time and inner electrons shield the outer ones, so atoms grow and ionisation enthalpy falls. Cations are smaller than their parent atoms and anions larger; in an isoelectronic series such as N³⁻ > O²⁻ > F⁻ > Na⁺ > Mg²⁺, more protons means a smaller ion.

The exceptions come from subshell stability. Be (2s²) and N (2p³, half-filled) have higher first ionisation enthalpies than B and O, so across period 2 the order is Li < B < Be < C < O < N < F < Ne. Electron gain enthalpy is most negative for chlorine, not fluorine, because fluorine’s small 2p shell repels the incoming electron; noble gases and N have positive or near-zero values. EAPCET ordering questions usually hinge on one of these exceptions.

Worked example

EAPCET-style question

Which of the following is the correct order of first ionisation enthalpy for B, C, N and O?

(1)B < C < N < O
(2)B < C < O < N
(3)C < B < O < N
(4)O < N < C < B

Solution

All four are in period 2, so the general trend is an increase from left to right as nuclear charge rises.
Nitrogen (2s²2p³) has a stable half-filled 2p subshell, so removing an electron from it needs extra energy.
In oxygen (2s²2p⁴), the fourth 2p electron is paired, and pairing repulsion makes it easier to remove — so O is lower than N.
B (2p¹) and C (2p²) follow the normal trend: B < C.
Hence B < C < O < N (approximate values 801, 1086, 1314 and 1402 kJ mol⁻¹).

Answer: B < C < O < N

Shortcuts and results to remember

Electron gain enthalpy exceptions: Cl > F > Br > I, and S is more negative than O, because the small, crowded 2p shell repels the added electron.
Ionisation enthalpy across period 2 is Li < B < Be < C < O < N < F < Ne and across period 3 is Na < Al < Mg < Si < S < P < Cl < Ar (filled s and half-filled p cause the dips).
Group 13 exceptions: ionisation enthalpy B > Tl > Ga > Al > In and atomic radius Ga < Al, because d and f electrons shield poorly.
Zr and Hf have almost the same atomic radius because of the lanthanoid contraction.
Bond dissociation energy of the halogens is $\ce{Cl2} > \ce{Br2} > \ce{F2} > \ce{I2}$ ; $\ce{F2}$ is low because of lone pair repulsion in the short F–F bond.

Common mistakes to avoid

Saying fluorine has the most negative electron gain enthalpy — it is chlorine, because fluorine’s compact 2p shell repels the added electron.
Applying the across-period trend blindly to ionisation enthalpy — remember Be > B and N > O in period 2 (and Mg > Al, P > S in period 3).
Ranking isoelectronic ions by atomic number in the wrong direction — the ion with more protons is smaller, so Mg²⁺ < Na⁺ < F⁻ < O²⁻.
Confusing electronegativity (a property of a bonded atom, no units on the Pauling scale) with electron gain enthalpy (an energy change of an isolated gaseous atom in kJ mol⁻¹).

How to prepare it

The trends are easy; the marks are in the exceptions — list them separately and revise them often.

Once the formulas feel familiar, test them the way the exam does: practise Classification of Elements and Periodicity in Properties 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 Classification of Elements and Periodicity in Properties 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 Classification of Elements and Periodicity in Properties part of the EAPCET Chemistry syllabus?: Yes. Classification of Elements and Periodicity in Properties 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 Classification of Elements and Periodicity in Properties for EAPCET?: The trends are easy; the marks are in the exceptions — list them separately and revise them often.
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 Classification of Elements and Periodicity in Properties?: Saying fluorine has the most negative electron gain enthalpy — it is chlorine, because fluorine’s compact 2p shell repels the added electron. Applying the across-period trend blindly to ionisation enthalpy — remember Be > B and N > O in period 2 (and Mg > Al, P > S in period 3). Ranking isoelectronic ions by atomic number in the wrong direction — the ion with more protons is smaller, so Mg²⁺ < Na⁺ < F⁻ < O²⁻. Confusing electronegativity (a property of a bonded atom, no units on the Pauling scale) with electron gain enthalpy (an energy change of an isolated gaseous atom in kJ mol⁻¹).