NEET Physics: Waves – NCERT Masterclass & 100 MCQ Quiz

CLASSIFICATION OF ELEMENTS & PERIODICITY

NCERT Masterclass & NEET Study Module

1. Genesis of Periodic Classification

• Dobereiner's Triads (1829): Grouped elements in threes (triads) where the middle element's atomic weight was roughly the average of the other two (e.g., Li, Na, K).
• Newlands' Law of Octaves (1865): Every eighth element had properties similar to the first, much like musical notes.
• Mendeleev's Periodic Law: The physical and chemical properties of elements are a periodic function of their atomic weights. He brilliantly left gaps for undiscovered elements like Eka-Aluminium (Gallium) and Eka-Silicon (Germanium).

2. Modern Periodic Law & Present Form

Henry Moseley (1913) observed regularities in X-ray spectra and established that Atomic Number ($Z$) is a more fundamental property.

Modern Periodic Law: The physical and chemical properties of the elements are periodic functions of their atomic numbers.

Nomenclature of Elements with Z > 100: Using numerical roots (0=nil, 1=un, 2=bi, 3=tri, 4=quad...) ending in "-ium". Example: Z=104 is Unnilquadium (Unq).

3. Electronic Configurations and Blocks

• s-Block: Groups 1 (Alkali metals) & 2 (Alkaline earth metals). Config: $ns^{1-2}$.
• p-Block: Groups 13 to 18. Config: $ns^2 np^{1-6}$. (s and p blocks together are called Representative Elements).
• d-Block (Transition Elements): Groups 3 to 12. Config: $(n-1)d^{1-10} ns^{0-2}$. They form colored ions, show variable valence, and act as catalysts.
• f-Block (Inner-Transition Elements): Lanthanoids & Actinoids. Config: $(n-2)f^{1-14} (n-1)d^{0-1} ns^2$.

4. Periodic Trends in Physical Properties

A. Atomic & Ionic Radius: Decreases across a period (due to increasing effective nuclear charge). Increases down a group (due to addition of new principal shells).
Isoelectronic Species: For ions with the same number of electrons (e.g., $O^{2-}, F^-, Na^+, Mg^{2+}$), the radius DECREASES as atomic number (positive charge) INCREASES.

B. Ionization Enthalpy ($\Delta_i H$): Energy required to remove an electron from an isolated gaseous atom. Increases across a period, decreases down a group.

🔥 NEET Exceptions in Ionization Enthalpy (Period 2):

• Be > B: Be ($2s^2$) is fully filled (stable) compared to B ($2p^1$).
• N > O: N ($2p^3$) is exactly half-filled (stable) compared to O ($2p^4$).

C. Electron Gain Enthalpy ($\Delta_{eg} H$): Enthalpy change when an electron is added to a neutral gaseous atom. Becomes more negative across a period.
Exceptions: Noble gases have large POSITIVE values. Halogens have highest negative values, but Cl is more negative than F due to the small size and inter-electronic repulsion in F.

D. Electronegativity: Ability of an atom in a chemical compound to attract shared electrons to itself. Fluorine is the most electronegative element (4.0 on Pauling scale).

5. Periodic Trends in Chemical Properties

Diagonal Relationship: Elements of the second period show similarities with elements of the third period placed diagonally opposite to them (e.g., Li-Mg, Be-Al, B-Si). This is due to their similar ionic size and identical charge/radius ratio.

Nature of Oxides: Metallic oxides are basic ($Na_2O$), non-metallic oxides are acidic ($Cl_2O_7$), and some are amphoteric ($Al_2O_3$, $ZnO$, $PbO$, $SnO$) or neutral ($CO, NO, N_2O$).

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🚀 NEET PERIODICITY MEGA QUIZ (100 MCQ)

Solve the 5 parts below to master Exceptions, Electronic Config, and Trends.

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