🌟 Modern Periodic Law & Today’s Periodic Table
🔬 How the Periodic Law Evolved
Mendeleev didn’t know about atomic structure when he made his table! Later, in 1913, physicist Henry Moseley discovered something crucial using X-rays:
- He plotted \(\sqrt{\nu}\) (nu = X-ray frequency) vs. atomic number (\(Z\)) → got a straight line 📈
- Same plot using atomic mass didn’t work → proving \(Z\) is more fundamental than mass!
⚛️ The Modern Periodic Law
“Physical & chemical properties of elements are periodic functions of their atomic numbers.”
This works for all 94 natural elements + synthetic ones!
🔑 Why Atomic Number Rules
- \(Z\) = nuclear charge (proton count) = electron count in neutral atoms
- Electron arrangements (configurations) repeat periodically → causing property patterns!
📊 Structure of the Modern Periodic Table
Long form table (most used today):
- ➡️ Rows = Periods (7 total):
- Period 1: 2 elements
- Period 2 & 3: 8 elements each
- Period 4 & 5: 18 elements each
- Period 6 & 7: 32 elements each (7th is incomplete)
- Period number = highest \(n\) (principal quantum number) 🎯
- ⬇️ Columns = Groups (IUPAC system):
- Numbered 1 to 18 (replaced old IA, IIA, etc.)
- Same group = similar outer electron configurations → similar chemistry!
- ✨ Special placement: Lanthanoids & actinoids (elements 58-71 & 90-103) sit in separate panels at the bottom → keeps table compact!
🧪 Fun Fact: Element Naming
For elements beyond \(Z = 100\):
- Discoverers suggest names → approved by IUPAC
- Controversial sometimes!
- These elements are super unstable (exist for tiny fractions of a second!) ⏳
🎖️ NEET Super Focus
Must-Know Concepts:
- 💡 Modern Periodic Law statement (properties depend on \(Z\))
- 💡 Moseley’s experiment (\(\sqrt{\nu}\) vs \(Z\) plot is linear)
- 💡 Period number = Highest principal quantum number (\(n\))
- 💡 Groups 1-18 (IUPAC) and why elements in same group behave similarly
- 💡 Lanthanoids/actinoids placement at bottom + reason
💎 Bonus History
Glenn Seaborg reconfigured the table in the 1940s-50s by placing actinoids below lanthanoids. He discovered plutonium and elements 94-102, winning the 1951 Nobel Prize! Element 106 is named Seaborgium (Sg) in his honor 🏆
