Chemistry Classification & Periodicity Syllabus for NEET 2026

The classification of elements forms the foundation of modern chemistry by helping students understand recurring patterns in their properties. Instead of studying each element separately, the periodic table arranges elements systematically based on atomic number and electronic configuration, making it easier to predict trends in atomic size, reactivity, and bonding behaviour. This chapter is important for NEET because many questions are based on periodic trends, exceptions, and property-based comparisons.

Genesis of Periodic Classification

This section explains how scientists gradually developed the idea of classifying elements by identifying patterns in their properties long before the modern periodic table was established.

Dobereiner’s Triads (1829):
Elements were grouped in threes, where the middle element had an atomic weight approximately equal to the average of the other two.
Example: Li (7), Na (23), K (39) → (7 + 39)/2 = 23

Newlands’ Law of Octaves (1865):
Elements arranged in increasing atomic weight showed repetition of properties every 8th element, similar to musical notes. This law was found to be applicable mainly for lighter elements and showed limitations beyond calcium.

Mendeleev’s Periodic Law:
Physical and chemical properties of elements are periodic functions of their atomic weights.
He left gaps for undiscovered elements like eka-aluminium (Ga) and eka-silicon (Ge) and successfully predicted many of their properties before their discovery.

Modern Periodic Law and Structure of the Periodic Table

The modern periodic system is based on atomic number, which correctly explains periodic trends and electronic configuration.

Modern Periodic Law (Moseley):
Properties of elements are a periodic function of their atomic numbers (Z).

The long form periodic table contains 7 periods and 18 groups.
Period number corresponds to the principal quantum number (n).

IUPAC naming for elements with Z > 100 uses digit roots:
0 = nil, 1 = un, 2 = bi, 3 = tri, 4 = quad, 5 = pent, 6 = hex, 7 = sept, 8 = oct, 9 = enn + “ium”

Division of Elements into s, p, d, and f Blocks

This classification is based on the subshell into which the last electron enters.

s-Block:
Groups 1 and 2, configuration ns¹ or ns². Highly reactive metals.

p-Block:
Groups 13–18, configuration ns²np¹ to ns²np⁶. Includes non-metals, metalloids, and noble gases.

d-Block:
Transition elements generally have valence electronic configuration (n−1)d¹–¹⁰ ns¹–² and show variable oxidation states and catalytic properties.

f-Block:
Lanthanoids and actinoids involve progressive filling of the (n−2)f subshell and are known as inner transition elements. 

Periodic Trends in Atomic and Ionic Radius

Atomic size changes systematically across periods and groups due to nuclear charge and shielding effects.

Across a period, atomic radius decreases due to increasing effective nuclear charge.
Down a group, atomic radius increases due to the addition of new shells.

Order of radii types:
Covalent < Metallic < Van der Waals

Isoelectronic series example:
N³⁻ > O²⁻ > F⁻ > Na⁺ > Mg²⁺ > Al³⁺

Lanthanoid contraction leads to similar sizes of 4d and 5d elements like Zr ≈ Hf.

Ionization Enthalpy and Shielding Effect

Ionization enthalpy is the enthalpy required to remove the most loosely bound electron from an isolated gaseous atom. 

Trend: increases across a period and decreases down a group.

Penetration power:
s > p > d > f

Exceptions due to stability:
Be > B (stable filled s-subshell)
N > O (half-filled p-subshell stability)

Shielding or screening effect refers to the reduction in effective nuclear attraction experienced by outer electrons due to the presence of inner shell electrons.

Electron Gain Enthalpy and Electronegativity

This section explains how atoms gain electrons and attract shared electrons in bonds.

Electron gain enthalpy becomes more negative across a period and less negative down a group.

Exception: Cl has more negative electron gain enthalpy than F due to lower electron-electron repulsion.

Electronegativity measures the ability to attract bonding electrons; fluorine is the most electronegative element (4.0 on the Pauling scale).

PW Resources for Classification & Periodicity

Physics Wallah offers a range of study and revision resources for chapter-wise NEET preparation. These resources help improve conceptual understanding, formula revision, and numerical-solving skills.