Mission 100 NEET 2026 Free Physical Chemistry Class by Sudhanshu Sir

Preparing for NEET 2026 requires a strong conceptual foundation, especially in Physical Chemistry, where numerical accuracy and conceptual clarity play a decisive role. Under Mission 100 NEET 2026, Sudhanshu Sir’s free Physical Chemistry classes aim to strengthen students’ understanding from the very basics. The programme focuses on building concepts step by step, ensuring that aspirants develop problem-solving skills aligned with the NEET examination pattern. One of the most important starting points of this initiative is the chapter Basic Concepts of Chemistry, which lays the groundwork for all advanced topics in chemistry.

Mission 100 NEET 2026 Free Physical Chemistry

The Mission 100 NEET initiative is designed to cover Physical Chemistry in a structured and student-friendly manner. It emphasizes clarity of fundamentals such as atoms, molecules, chemical calculations, and atomic structure. These topics are not only essential for Physical Chemistry but also frequently tested in NEET through direct questions and application-based problems. Understanding these basics enables students to confidently approach mole concept, stoichiometry, and atomic theory-related questions.

Chapter 1: Basic Concepts of Chemistry

This chapter, often called "Mole Concept," collects many small, fundamental concepts that build upon each other. Mastering these basics is crucial, as they form the foundation for more advanced topics appearing later.

The Definition of Chemistry

Chemistry is fundamentally the science of Atoms and Molecules. The entire discipline revolves around understanding these two entities.

Atoms vs. Molecules

1. Atom

• An atom is the smallest unit of a substance.

• It may or may not exist independently.

• Examples of atoms that can exist independently: Noble gases like Helium (He), Neon (Ne), Argon (Ar).

• Examples of atoms that cannot exist independently: Hydrogen (H), Oxygen (O). These are reactive and do not persist in isolation.

2. Molecule

• A molecule is also a small unit of a substance, but it is defined by its ability to always exist independently.

• A molecule consists of more than one atom chemically bonded together.

• The formation of molecules allows unstable atoms (like H or O) to achieve stable existence.Classification of Molecules

Molecules are classified based on the types of atoms they contain.

Classification of Molecules

Molecules are classified based on the types of atoms present in them.

• Homoatomic molecules consist of atoms of the same element, such as oxygen (O₂), phosphorus (P₄), and sulphur (S₈).

• Heteroatomic molecules contain atoms of different elements, such as water (H₂O), carbon dioxide (CO₂), and sucrose (C₁₂H₂₂O₁₁).

Atomicity

Atomicity is the total number of atoms present in a single molecule. The subscript in a chemical formula indicates the number of those atoms.

• Example 1 (Homoatomic): O₂ has an atomicity of 2. P₄ has an atomicity of 4. S₈ has an atomicity of 8.

• Example 2 (Heteroatomic): H₂O has 2 atoms of Hydrogen and 1 atom of Oxygen, so total atomicity is 2 + 1 = 3.

Linking Concepts: Elements and Compounds

The classification of molecules relates to elements and compounds:

• Elements: Pure substances existing as individual atoms (e.g., He) or homoatomic molecules (e.g., O₂).

• Compounds: Pure substances existing as heteroatomic molecules, formed when different elements combine in a fixed ratio.

Calculating Atoms and Molecules

The chemical formula of a molecule provides direct information about its composition.

Example: Sucrose (C₁₂H₂₂O₁₁)

• One molecule of sucrose contains 12 Carbon atoms, 22 Hydrogen atoms, and 11 Oxygen atoms.

To find the Number of Atoms: Number of Atoms = Number of Molecules × Atomicity

To find the Number of Molecules: Number of Molecules = Number of Atoms / Atomicity

(Memory Tip: To remember which operation to use, think of 'molecules to atoms' as multiplying by how many atoms are in each molecule, and 'atoms to molecules' as dividing by that number.)

Worked Example:

• Question: 1200 atoms of Carbon are present in how many molecules of sucrose (C₁₂H₂₂O₁₁)?

• Solution: Atomicity of Carbon in sucrose is 12.
  Number of Molecules = 1200 / 12 = 100 molecules of sucrose.

The Structure of an Atom

Atoms are divisible and consist of three fundamental subatomic particles: electrons, protons, and neutrons. Protons and neutrons are present in the nucleus and are collectively known as nucleons, while electrons revolve around the nucleus. The mass of electrons is negligible compared to protons and neutrons, which have nearly equal masses.

• The mass of an electron is negligible compared to that of a proton or a neutron.

• Mass of Proton ≈ Mass of Neutron.

Location of Subatomic Particles

• Protons and Neutrons are located in the central part of the atom, the nucleus. They are collectively called nucleons.

• Electrons exist in the region around the nucleus.

Standard Representation of an Atom

Any atom can be represented using a standard notation: # ᴬZX

• X = Symbol of the element (e.g., C for Carbon)

• A = Mass Number (superscript)

• Z = Atomic Number (subscript)

Defining Mass Number and Atomic Number

• Mass Number (A): Total number of nucleons (protons + neutrons) in the nucleus.

• A = (Number of Protons) + (Number of Neutrons)

• Atomic Number (Z): Number of protons in the nucleus. This uniquely identifies an element.

• Z = Number of Protons

Calculating Subatomic Particles from the Notation

• Number of Protons = Z

• Number of Neutrons = A – Z (Mass Number – Atomic Number)

Calculating the Number of Electrons

Case 1: Neutral Atom

For an uncharged atom: Number of Electrons = Number of Protons (Z)

• Example: Iron (⁵⁶₂₆Fe) has 26 protons and 26 electrons.

Charged Atoms: Ions

In a neutral atom, electrons equal protons. However, in a charged atom (ion), this equality changes due to electron gain or loss.

Comparative Structure: Cations vs. Anions

In-Depth Analysis of Cations

Cations form when an atom loses electrons, resulting in a positive charge.

• Example: Sodium Ion (Na⁺)

• Neutral Na (Z=11) has 11 protons, 11 electrons.

• Na⁺ (loses 1 electron) has 11 protons, 10 electrons.

In-Depth Analysis of Anions

Anions form when an atom gains electrons, resulting in a negative charge.

• Example: Oxide Ion (O²⁻)

• Neutral O (Z=8) has 8 protons, 8 electrons.

• O²⁻ (gains 2 electrons) has 8 protons, 10 electrons.

Relative Charge vs. Absolute (Exact) Charge

It is a critical mistake to confuse the charge number on an ion with its actual charge in Coulombs.

• Relative Charge: The number in the superscript (e.g., +3 on Al³⁺). It is unitless and indicates electrons lost/gained. Al³⁺ indicates a loss of 3 electrons.

• Absolute (Exact) Charge: The actual charge in Coulombs. Calculated by multiplying the number of excess protons/electrons by the fundamental charge (1.6 x 10⁻¹⁹ C).

• Charge on Al³⁺: 3 * (+1.6 x 10⁻¹⁹ C) = +4.8 x 10⁻¹⁹ C.

• Charge on S²⁻: 2 * (-1.6 x 10⁻¹⁹ C) = -3.2 x 10⁻¹⁹ C.
  A common error is stating "S²⁻ has -2 Coulombs" which is incorrect.

Key Rules for Subatomic Particles & Reactions

1. Fundamental Particle Counts are Whole Numbers: Electrons, protons, neutrons, atomic number (Z), and mass number (A) are always whole numbers.

2. Origin of Chemical vs. Nuclear Reactions:

• Chemical Reactions: Involve changes in electrons (loss, gain, sharing). The nucleus is unaffected.

• Nuclear Reactions: Involve changes within the nucleus (protons and/or neutrons).

3. Ion Formation and the Nucleus: During ion formation, there is no change in the number of nucleons (protons and neutrons); only electrons change.

Worked Examples (Based on NEET Exam Questions)

Question 1 (NEET 2020)

Find the number of protons, neutrons, and electrons in Lutetium-175 (¹⁷⁵Lu₇₁).

• Protons (Z) = 71

• Neutrons (A-Z) = 175 - 71 = 104

• Electrons = Protons (neutral atom) = 71

Question 2

An element has 11 protons and an atomic mass 23 times that of the lightest element. Find protons, neutrons, and electrons.

• The lightest element is H (mass ~1). Atomic mass = 23. Protons (Z) = 11. Element is Sodium (Na). ²³Na₁₁.

• Protons = 11

• Neutrons = 23 - 11 = 12

• Electrons = 11 (neutral atom)

Question 3

Element M has atomic number 25, mass number 52. Find electrons, protons, and neutrons in M²⁺ ion. (⁵²M₂₅²⁺).

• Protons (Z) = 25

• Neutrons (A-Z) = 52 - 25 = 27

• Electrons = Protons - charge = 25 - 2 = 23

• Answer: Electrons, Protons, Neutrons: 23, 25, 27.

Average Atomic Mass

The atomic mass on the periodic table is an average atomic mass, a weighted average based on the natural abundance of an element's isotopes.

Formula for Average Atomic Mass (AAM):

AAM = Σ [ (Isotopic Mass) × (% Abundance) ] / 100

Example: Chlorine (Cl)

Chlorine has ³⁵Cl (approx. 35 amu, 75% abundance) and ³⁷Cl (approx. 37 amu, 25% abundance).

AAM of Cl = [ (35 × 75) + (37 × 25) ] / 100 = 35.5

No single chlorine atom has this mass; it is a weighted average.

Dalton's Atomic Theory (1808)

Dalton's theory provided the first scientific framework for understanding matter. Though some postulates are now disproven, it was revolutionary.

Key Postulates of Dalton's Theory

1. Atoms of the Same Element: All atoms of a single element possess identical properties (size, mass, shape).

2. Atoms of Different Elements: Atoms of different elements have different masses and properties.

3. Formation of Compounds: Atoms combine in a fixed, simple whole-number ratio to form compounds (e.g., H₂O is 2:1).

4. Formation of Multiple Compounds: Two elements can combine in different ratios to form more than one compound (e.g., CO and CO₂). This is the basis of the Law of Multiple Proportions.

5. Conservation of Atoms: In chemical reactions, atoms are neither created nor destroyed; they are only reorganized. This aligned with the Law of Conservation of Mass.

Limitations of Dalton's Atomic Theory

Later discoveries revealed shortcomings in Dalton's theory:

1. Sub-atomic Particles: It did not account for electrons, protons, and neutrons, as Dalton believed atoms were indivisible.

2. Isotopes: It couldn't explain isotopes (atoms of the same element with different masses), contradicting the idea that all atoms of an element are identical.

3. Isobars: It couldn't explain isobars (atoms of different elements with the same mass), contradicting that different elements have different masses.

4. Non-Stoichiometric Compounds: It didn't account for compounds where elements don't combine in simple whole-number ratios (e.g., Fe₀.₉₄O₁).

5. Allotropes: It did not explain allotropy, where an element exists in multiple forms with different physical properties (e.g., carbon as diamond and graphite).

Application: Practice Problems (Based on NEET Exams)

These examples highlight how Dalton's theory is tested.

Question 1 (NEET 2023): Identify the correct statement.

• A correct statement would reflect an accurate aspect of atomic structure or chemical principles. Incorrect options often misstate basic facts about subatomic particles (e.g., the number of fundamental particles or what constitutes a nucleon).

Question 2: Identify the incorrect statement about Dalton's Theory.

• A common incorrect statement regarding Dalton's theory involves the ratio of combination. Dalton stated a fixed ratio, so any statement suggesting "any ratio" would be incorrect.

Question 3: Identify the incorrect postulates from a given list.

• Incorrect postulates contradict Dalton's original assertions, such as "Matter consists of divisible atoms" (Dalton said indivisible) or "All the atoms of a given element have different properties" (Dalton said identical). It requires evaluating statements against what Dalton believed, not modern science.