Electronic Configurations & The Periodic Table
🔑 Key NEET Concepts
- Period = Valence shell: Period number = Principal quantum number
\(n\)of the outermost shell. - Elements per period: Number of elements = 2 × number of atomic orbitals in the energy level being filled.
- Group similarity: Elements in the same group have identical valence-shell electron configurations → similar properties!
- Blocks = Orbital types: s, p, d, f-blocks are named after the last orbital being filled.
- Special cases: Helium (s-block but in p-block) & Hydrogen (fits both Group 1 & 17).
📊 Periods & Electron Filling
| Period | Starts at | Orbitals Filled | Key Elements | Total Elements |
|---|---|---|---|---|
| 1 (n=1) | H (1s¹) | 1s | H → He (1s²) | 2 ✨ |
| 2 (n=2) | Li | 2s → 2p | Li → Ne (2s²2p⁶) | 8 |
| 3 (n=3) | Na | 3s → 3p | Na → Ar | 8 |
| 4 (n=4) | K | 4s → 3d → 4p | Sc (3d¹) → Zn (3d¹⁰), ends at Kr | 18 ✨ |
| 5 (n=5) | Rb | 5s → 4d → 5p | Y (4d¹) → Cd, ends at Xe | 18 |
| 6 (n=6) | Cs | 6s → 4f → 5d → 6p | Ce (4f¹) → Lu (4f¹⁴) (Lanthanoids) | 32 |
| 7 (n=7) | Fr | 7s → 5f → 6d → 7p | Th → Lr (Actinoids), ends at Z=118 | Incomplete |
✨ Fun fact: Transition metals (d-block) appear because filling \((n-1)d\) orbitals becomes energetically favorable before \(np\)!
👯 Groupwise Electronic Configurations
Elements in the same group have matching valence electrons → similar chemistry! Example:
Group 1 (Alkali Metals): All end with \(ns^1\)
- Li: [He] 2s¹
- Na: [Ne] 3s¹
- K: [Ar] 4s¹
- Rb: [Kr] 5s¹
- Cs: [Xe] 6s¹
- Fr: [Rn] 7s¹
This explains why they all love to form +1 ions! 💕
🧱 Blocks in the Periodic Table
Elements split into 4 blocks based on which orbital gets the last electron:
| Block | Groups | Outer Config. | Notes |
|---|---|---|---|
| s-block | 1 & 2 | ns¹⁻² | Highly reactive metals (except H) |
| p-block | 13–18 | ns² np¹⁻⁶ | Includes metals, non-metals & metalloids |
| d-block | 3–12 | (n-1)d¹⁻¹⁰ ns⁰⁻² | Transition metals (e.g., Fe, Cu) |
| f-block | — | (n-2)f¹⁻¹⁴ | Lanthanoids & Actinoids (inner transition) |
⚠️ Important Exceptions!
- Helium (He): Has 1s² (s-block config) but placed in p-block Group 18 because it’s a noble gas 🎈.
- Hydrogen (H):
- Can behave like Group 1 (lose 1e⁻ → H⁺).
- Can behave like Group 17 (gain 1e⁻ → H⁻).
- So it sits alone above all groups! ⚛️
💡 Solved Problems
Problem: Why does Period 5 have 18 elements?
Solution: Orbitals filled: 5s (1 orbital), 4d (5 orbitals), 5p (3 orbitals). Total orbitals = 9 → 9 × 2 = 18 electrons → 18 elements! ✅
