⚛️ Atomic Number, Proton Number and Nucleon Number
Moseley’s Law (1913)
When different elements were bombarded with cathode rays, X-rays of characteristic frequencies were produced. The square root of the frequency of these X-rays is directly proportional to the atomic number (Z) of the element.
Mathematical Relation: √ν ∝ Z
Atomic Number (Z) / Proton Number
The atomic number Z is a fundamental property of an element, equal to the number of protons in the nucleus. It determines the identity of the element.
Nucleon Number (A) / Mass Number
The total number of protons and neutrons in the nucleus of an atom is called the nucleon number. It is related to the atomic number by the equation: A = Z + N, where N is the number of neutrons.
⚡ Effect of Electric Field on Fundamental Particles
Three Fundamental Particles
| Particle | Charge (Coulomb) | Relative Charge | Mass (Kg) | Mass (amu) |
|---|---|---|---|---|
| Proton | +1.6022 × 10⁻¹⁹ | +1 | 1.6726 × 10⁻²⁷ | 1.0073 |
| Neutron | 0 | 0 | 1.6750 × 10⁻²⁷ | 1.0087 |
| Electron | -1.6022 × 10⁻¹⁹ | -1 | 9.1095 × 10⁻³¹ | 5.4858 × 10⁻⁴ |
Behavior in Electric Field
Neutrons: Being neutral, they travel straight without deflection.
Protons: Positively charged, deflected toward negative plate.
Electrons: Negatively charged, deflected toward positive plate, with greater deflection due to being 1837 times lighter than protons.
🔢 Quantum Numbers
Four Quantum Numbers
A set of numerical values that gives acceptable solutions to the Schrodinger wave equation for the hydrogen atom. They describe the behavior of electrons around the nucleus.
| Quantum Number | Symbol | Values | Significance |
|---|---|---|---|
| Principal | n | 1, 2, 3, 4… | Size and energy of orbital |
| Azimuthal | l | 0 to (n-1) | Shape of orbital (s, p, d, f) |
| Magnetic | m | -l to +l | Orientation in space |
| Spin | s | +½ or -½ | Spin direction of electron |
Shapes of Atomic Orbitals
s-Orbital: Spherical shape, uniformly distributed electron density.
p-Orbitals: Dumbbell shape with three orientations (px, py, pz).
d-Orbitals: Cloverleaf shape with five orientations.
f-Orbitals: Complex shape with seven orientations.
🧬 Electronic Configuration
Aufbau Principle
Electrons fill atomic orbitals in order of increasing energy. The (n + l) rule determines the order: orbitals with lower (n + l) values are filled first. For equal (n + l) values, the orbital with lower n is filled first.
Filling Order: 1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p < 5s < 4d < 5p < 6s < 4f < 5d < 6p < 7s
Pauli Exclusion Principle
No two electrons in an atom can have the same set of all four quantum numbers. In an orbital, two electrons must have opposite spins.
Hund’s Rule
When filling degenerate orbitals (orbitals of equal energy), electrons occupy separate orbitals with parallel spins before pairing up.
📊 Electronic Configuration and Periodic Table
Blocks in Periodic Table
s-block: Groups 1 & 2, outer configuration ns¹⁻²
p-block: Groups 13-18, outer configuration ns² np¹⁻⁶
d-block: Transition metals, filling (n-1)d orbitals
f-block: Lanthanides & Actinides, filling (n-2)f orbitals
Ionization Energy Trends
Down a group: Ionization energy decreases due to increasing atomic size and shielding effect.
Across a period: Ionization energy increases due to increasing nuclear charge and decreasing atomic size.
🚀 Study Strategies
1
Master Quantum Numbers
Create a chart showing the relationship between n, l, m, and the number of orbitals and electrons. Practice determining quantum numbers for specific electrons.
2
Electronic Configuration Patterns
Learn the Aufbau order using the diagonal rule. Practice writing configurations for elements 1-36, noting exceptions like Cr and Cu.
3
Periodic Trends
Draw graphs showing how ionization energy, atomic size, and electronegativity change across periods and down groups. Understand the reasons behind these trends.