Ionic radius — cation - parent atom, anion - parent atom; isoelectronic series

2.2.3 · Chemistry › Periodic Trends

Ionic radius change kyun hota hai?

KYA compare karte hain: ek ion ka size uske neutral parent atom ke size se.

KYUN badalta hai: radius is baat se tay hoti hai ki outermost electrons kitni tightly hold hote hain. Do competing effects hain:

Nuclear charge $Z$ (protons) — electrons ko andar kheenchta hai.
Electron–electron repulsion aur number of occupied shells — electrons ko bahar dhakelta hai.

Jab aap ionize karte ho, $Z$ fixed rehta hai lekin electrons ki sankhya badal jaati hai, isliye balance shift ho jaata hai.

Cation < parent atom

KAISE electrons hatane se atom shrink hota hai (derivation-from-scratch reasoning):

Start: neutral atom, $Z$ protons, $Z$ electrons.
Ek electron hatao → ab $Z$ protons hain lekin $Z - 1$ electrons hain.
Reason A (fewer shells): jo electron remove hota hai woh usually outermost hota hai. Jaise $Na = [Ne] 3 s^{1} \to Na^{+} = [Ne]$ . Ek poori valence shell gayab ho jaati hai, isliye ion dramatically smaller ho jaata hai.
Reason B (less repulsion / higher $Z_{eff}$ ): kam electrons hone se shielding $S$ drop hoti hai, toh baaki electrons zyada bada $Z_{eff} = Z - S$ feel karte hain aur andar kheench jaate hain.

Anion > parent atom

KAISE electrons badhane se atom bada hota hai:

Start: neutral atom, $Z$ protons, $Z$ electrons.
Ek electron badhao → $Z$ protons lekin $Z + 1$ electrons.
Wohi nucleus ab apni pull zyada electrons mein share karta hai: $Z_{eff}$ per electron drop hoti hai.
Extra electron–electron repulsion cloud ko bahar dhakelta hai.

Isoelectronic series — same electrons, different nuclei

KYUN size ordering aisi hoti hai: sabke paas same 10 electrons aur identical shielding structure hai, isliye $S$ roughly constant hai. Lekin $Z$ (protons) left→right increase karta hai:

$Z_{eff} = Z - S, S \approx const \Rightarrow Z_{eff} \propto Z$

Zyada protons same electron cloud ko squeeze kar rahe hain ⇒ tighter ⇒ smaller.

Figure — Ionic radius — cation - parent atom, anion - parent atom; isoelectronic series

Recall Feynman: ek 12-saal ke bachche ko explain karo

Socho ek papa (nucleus) apne bacchon (electrons) ka haath pakad ke circle mein khade hain. Agar ek bachcha chala jaata hai (isse cation banta hai), toh papa baaki bacchon ko zyada tightly andar kheeench sakte hain — circle chhota ho jaata hai. Agar ek extra bachcha aa jaata hai (isse anion banta hai), toh papa ki grip patli pad jaati hai aur bachche ek doosre ko dhakelte hain, toh circle bada ho jaata hai. Ab isoelectronic game ke liye: same 10 bachche rakho, lekin ek stronger papa swap karo (zyada protons). Zyada strong papa ⇒ tighter circle ⇒ smaller. Sabse weak papa ⇒ sabse bada circle.

Flashcards

Cation apne parent atom se chhota kyun hota hai?

Kam electrons (aksar poora shell remove ho jaata hai) aur kam shielding, isliye baaki electrons higher

Z_{eff}

feel karte hain aur zyada tightly andar kheeench jaate hain.

Anion apne parent atom se bada kyun hota hai?

Zyada electrons same nuclear charge share karte hain → lower

Z_{eff}

per electron + extra electron–electron repulsion → cloud expand hota hai.

Isoelectronic series define karo.

Atoms/ions ka ek set jisme electrons ki sankhya same ho (aur same configuration bhi ho), jaise N³⁻, O²⁻, F⁻, Ne, Na⁺, Mg²⁺, Al³⁺ (har ek mein 10 electrons).

Isoelectronic series mein size kya determine karta hai?

Protons ki sankhya (

Z

): zyada protons → smaller ion, kyunki electron count aur shielding fixed hai isliye

Z_{eff} \propto Z

10-electron series ko size ke hisaab se rank karo.

N³⁻ > O²⁻ > F⁻ > Ne > Na⁺ > Mg²⁺ > Al³⁺ (sabse zyada protons = smallest).

Cl⁻ aur K⁺ mein kaunsa bada hai, aur kyun?

Cl⁻ — dono 18-electron hain, lekin Cl mein sirf 17 protons hain jabki K mein 19 hain, isliye K⁺ zyada strongly kheenchta hai aur smaller hai.

Size ko shielding se jodne wala formula.

Z_{eff} = Z - S

; higher

Z_{eff}

⇒ smaller radius.

Na (186 pm) → Na⁺ roughly kitna bada hai, aur itna chhota kyun?

~102 pm; single 3s electron khone se poora n=3 shell remove ho jaata hai, aur ek compact [Ne] core reh jaata hai.

Connections

Effective Nuclear Charge (Z_eff) — har size trend ke peeche ka master variable.
Atomic Radius Trends — neutral-atom baseline jisse yeh note compare karta hai.
Shielding and Penetration — $S$ set karta hai, isliye $Z_{eff}$ bhi.
Ionization Energy — smaller cations ↔ aur electrons remove karna mushkil.
Lattice Energy — ionic sizes directly Coulombic lattice calculations mein jaati hain.
Electron Affinity — yahan discuss ki gayi anions banane ki energetics.