Definition:

An atom is the smallest unit of matter that retains the chemical properties of an element.

• Composed of subatomic particles: electrons, protons, and neutrons.

• Study of atoms includes structure, models, and energy levels.

📌 1️⃣ Structure of Atom

• Nucleus → contains most of the mass of the atom

• Electrons → occupy energy levels / shells

📌 2️⃣ Atomic Models

1️⃣ Thomson’s Model (Plum Pudding)

• Atom is sphere of positive charge

• Electrons embedded like plums in pudding

• Explains neutrality, but not spectral lines

2️⃣ Rutherford’s Model

• Atom has a tiny, dense, positively charged nucleus

• Electrons move around nucleus

• Explains scattering experiments

• Could not explain stability and discrete spectral lines

3️⃣ Bohr’s Model

• Electrons move in fixed orbits (energy levels) around nucleus

• Each orbit → specific energy

• Explains hydrogen spectrum and stability of atom

📌 3️⃣ Energy Levels and Spectra

• Electrons occupy discrete energy levels

• Absorption of energy → electron jumps to higher level (excited state)

• Emission of energy → electron falls back → emits photon

• Explains line spectra of hydrogen and other elements

Key Spectra:

• Lyman series → ultraviolet

• Balmer series → visible

• Paschen series → infrared

📌 4️⃣ Quantum Nature of Electrons

• Electrons behave according to Quantum Mechanics

• Have wave-particle duality

• Described by orbitals → probability of finding electron in a region

Quantum Numbers:

1. Principal (n) → energy level

2. Azimuthal (l) → shape of orbital

3. Magnetic (m) → orientation of orbital

4. Spin (s) → electron spin

📌 5️⃣ Important Concepts

1. Atomic Number (Z) → number of protons

2. Mass Number (A) → protons + neutrons

3. Isotopes → same Z, different A

4. Isobars → same A, different Z

5. Valency → combining capacity of an element

🌟 Quick Notes for Revision

• Atom = electrons + nucleus (protons + neutrons)

• Nucleus → dense, positive, most mass

• Electrons → occupy discrete energy levels

• Bohr model → explains spectral lines of hydrogen

• Line spectra → result of electron transitions

• Quantum numbers → define electron arrangement