Modern periodic law — based on atomic number

1.4.2 · Chemistry › Periodic Table — First Look

Modern Periodic Law KYA hai?

Purani idea se key change:

• Mendeleev ka law (purana): properties atomic mass ki periodic function hain.
• Modern law (Moseley, 1913): properties atomic number ki periodic function hain.

Atomic number kyun, atomic mass kyun nahi?

Teen famous anomalies (mass ek order kehta hai, chemistry ulta kehti hai):

Moseley ne $Z$ ko sahi basis kyun prove kiya?

Moseley's law scratch se derive karna

Step 1 — Koi bhi relation kyun hoga? X-ray tab emit hoti hai jab ek inner electron lower shell mein drop karta hai. Uski frequency is baat par depend karti hai ki nucleus electrons ko kitni strongly pull karta hai — yaani nuclear charge par.

Yeh step kyun? Nuclear charge = protons ki sankhya = $Z$. To X-ray frequency mein $Z$ encoded hona chahiye.

Step 2 — Bohr-type energy of a transition. Ek one-electron-like transition ke liye, energy $\propto (\text{effective nuclear charge}{)}^2$. Effective charge ko $(Z-\sigma )$ likhte hain jahan $\sigma$ ek screening constant hai: $h\nu \propto (Z-\sigma {)}^2$

Yeh step kyun? Inner electrons nucleus ko partly shield karte hain, isliye felt charge $Z-\sigma$ hoti hai, $Z$ nahi.

Step 3 — Square root lo (frequency hi measure hoti hai): $\sqrt{\nu }=a(Z-\sigma )$

Step 4 — Isne debate kyun settle ki. $\sqrt{\nu }$ vs $Z$ ek perfect straight line deta hai, jabki $\sqrt{\nu }$ vs atomic mass NAHI deta. Isliye $Z$ — mass nahi — atom ki sacchi, ordering identity hai.

Worked Examples

Common Mistakes

Active Recall

Connections

• Mendeleev's Periodic Table — purana mass-based law jise isne replace kiya
• Atomic Number and Atomic Mass — $Z$ ka meaning
• Moseley's X-ray Experiment — experimental proof
• Effective Nuclear Charge and Shielding — constant $b$ ki origin
• Periodic Trends — "repeating" properties (ionization energy, radius)
• Bohr Model of the Atom — $(Z-\sigma {)}^2$ energy dependence ka source

Concept Map