Mastering the periodic table class 11 notes pdf is the first step toward a perfect score in Inorganic Chemistry for NEET. The chapter “Classification of Elements and Periodicity in Properties” serves as the foundation for chemical bonding, p-block, d-block, and s-block elements. In this guide, we break down the complexities of electronic configurations, periodic trends, and exceptions that frequently appear in the exam.

With 118 elements known today, studying them individually is impossible. The classification organizes elements into groups and periods based on their similarities, allowing chemists to predict properties. The modern periodic table is not just a list but a logical map of atomic structure.

Dobereiner’s Triads

In 1817, Johann Dobereiner arranged elements in groups of three (triads). The atomic mass of the middle element was approximately the arithmetic mean of the other two.

Newlands’ Law of Octaves

John Newlands proposed that when elements are arranged in increasing order of atomic masses, every eighth element has properties similar to the first, similar to musical notes. However, this law only worked up to Calcium (Ca).

Mendeleev’s Periodic Table

Dmitri Mendeleev formulated the Periodic Law: “The properties of elements are periodic functions of their atomic masses.”

• Achievements: He left gaps for undiscovered elements like Eka-boron (Scandium), Eka-aluminium (Gallium), and Eka-silicon (Germanium).
• Limitations: It could not explain the position of isotopes, the placement of Hydrogen, or the anomalous pairs like Cobalt (58.9) before Nickel (58.7).

The Modern Periodic Law, proposed by Henry Moseley, states that the physical and chemical properties of elements are periodic functions of their atomic number (Z). This resolved the discrepancies found in Mendeleev’s mass-based table.

The table is divided into four main blocks based on the orbital being filled by the last electron:

Block	Groups	General Configuration	Properties
s-block	1 & 2	ns1-2	Highly reactive metals, low IE
p-block	13 to 18	ns2np1-6	Contains metals, non-metals, metalloids
d-block	3 to 12	(n-1)d1-10ns1-2	Transition metals, colored ions
f-block	Bottom Rows	(n-2)f1-14(n-1)d0-1ns2	Inner transition elements (Lanthanoids/Actinoids)

Understanding trends is crucial for solving conceptual questions in your periodic table class 11 notes pdf. These trends are governed by Effective Nuclear Charge (Z_eff) and Shielding Effect.

1. Atomic Radius

Atomic radius generally decreases across a period due to increased Z_eff pulling electrons closer. It increases down a group as new shells are added.

2. Ionization Enthalpy (Δ_iH)

The energy required to remove an electron from an isolated gaseous atom.

3. Electron Gain Enthalpy (Δ_egH)

Across a period, Δ_egH becomes more negative (except noble gases which are positive). Down a group, it usually becomes less negative.

Trends in chemical reactivity are also predictable. Metallic character increases down the group while non-metallic character increases across the period.

• Oxide Nature: Basic oxides (e.g., Na₂O) are formed by metals on the left. Acidic oxides (e.g., Cl₂O₇) are formed by non-metals on the right. Amphoteric oxides include Al₂O₃.
• Valency: Across a period, it increases from 1 to 4 and then decreases to 0 (for noble gases) with respect to hydrogen.

The first element of each group (Li, Be, B) shows different behavior than its peers due to extremely small size, high IE, and absence of d-orbitals. Furthermore, some second-period elements show similarities with third-period elements placed diagonally.

Download Periodic Table Notes PDF

NEET focuses heavily on exceptions. Ensure you have these noted down in your periodic table class 11 notes pdf:

Why does Chlorine have more negative electron gain enthalpy than Fluorine?

Fluorine is very small. When an electron is added, it experiences significant repulsion from the existing 7 electrons in its compact 2p subshell. Chlorine has a larger 3p subshell, accommodating the extra electron more easily.

What is the main difference between Mendeleev and Modern Periodic Law?

Mendeleev’s Law is based on atomic mass, whereas the Modern Periodic Law is based on the atomic number, which is a more fundamental property of an element.

What are isoelectronic species?

These are atoms or ions that have the same number of electrons (e.g., O^2-, F^–, Ne, Na⁺, Mg²⁺). For these, the radius decreases as nuclear charge increases.

What is the diagonal relationship?

It is the similarity in properties between certain elements of the second period and the elements of the third period that are diagonally opposite to them, such as Lithium and Magnesium.

Which element has the highest electronegativity?

Fluorine (F) has the highest electronegativity with a value of 4.0 on the Pauling scale.