Mendeleev's periodic table — based on atomic mass

1.4.1 · Chemistry › Periodic Table — First Look

WHAT is Mendeleev's periodic table?

Rows jo left→right jaati hain = periods (mass across badhti hai).
Columns jo similar elements ko stack karti hain = groups.

WHY atomic mass? (the historical logic)

HOW Mendeleev built it — derive the layout from scratch

Chalte hain uski reasoning ko step by step reconstruct karte hain.

Step 1 — Elements ko increasing atomic mass ke order mein likho. Li(7), Be(9), B(11), C(12), N(14), O(16), F(19), Na(23), Mg(24), ... Yeh step kyun? Mass hi woh ek universal ordering number tha jo uske paas tha.

Step 2 — Chemistry ko observe karo. Li violently react karta hai, $Li_{2} O$ banata hai (valency 1). Phir Be, B, C... aur Na par Li ka behaviour wapas aa jaata hai: Na bhi violent hai, $Na_{2} O$ banata hai, valency 1. Yeh step kyun? Kisi property ka wapas aana detect karna batata hai ki line ko nayi row mein "wrap" kab karna hai.

Step 3 — Line ko todho taaki similar elements stack ho sakein. Na par nayi row shuru karo. Ab Li, Na ke upar hai, Be, Mg ke upar hai, aur aage bhi yahi. Yeh step kyun? Vertical alignment poora point hai — yeh chemical twins ko group karta hai.

Step 4 — Valency ko (DONO oxide aur hydride formulas se) grouping key ki tarah use karo. Mendeleev ne har element ki valency uske highest oxide aur hydride ke formulas se infer ki, aur same valency pattern wale elements ko group kiya. Ek period mein highest oxides kuch is tarah chalte hain: $R_{2} O, RO, R_{2} O_{3}, RO_{2}, R_{2} O_{5}, RO_{3}, R_{2} O_{7}$ isse rising oxide valencies 1, 2, 3, 4, 5, 6, 7 milti hain. Dayi taraf ke hydrides $RH_{4}, RH_{3}, RH_{2}, RH$ hote hain — toh hydride valency groups IV→VII mein fall karti hai 4, 3, 2, 1. Dono ko saath use karna har group ka character akele kisi ek se kahin zyada tightly pin karta hai. Yeh step kyun? Formulas hard chemical facts hain; oxide-valency (rising) aur hydride-valency (falling) ek complementary fingerprint banate hain jo group ko rang ya hardness se kahin zyada reliably anchor karta hai.

Figure — Mendeleev's periodic table — based on atomic mass

The two triumphs (this is the 80/20 core)

Worked example Triumph 1 — Predicting undiscovered elements

Mendeleev ne gaps chhode wahan jahan pattern ek aisi element maangta tha jo abhi tak discover nahi hui thi. Usne unhe prefix eka (Sanskrit "one") ke saath naam diya. Usne eka-silicon predict kiya, jo baad mein mila aur germanium (1886) kahlaaya.

Property	Eka-silicon (predicted, 1871)	Germanium (found, 1886)
Atomic mass	~72	72.6
Density	~5.5 g/cm³	5.35 g/cm³
Oxide formula	$EsO_{2}$	$GeO_{2}$
Chloride	$EsCl_{4}$	$GeCl_{4}$

Yeh kyun important hai: sahi predictions = table nature ka ek real law hai, koi filing trick nahi.

Where atomic mass FAILED (the cracks)

Merits vs Defects (compact)

Mendeleev's periodic law (original 1869 form) batao.

Properties of elements are a periodic function of their atomic masses.

Mendeleev ne atomic mass use kiya, atomic number kyun nahi?

Proton/atomic number 1869 mein unknown tha; mass hi woh ek measurable property thi jo sabhi elements ko order kar sakti thi.

Mendeleev ki table mein "period" aur "group" kya hote hain?

Period = horizontal row (mass across badhti hai); group = chemically similar elements ka vertical column.

Mass ke alawa Mendeleev ne grouping ke liye kaunsi key use ki?

Valency, jo DONO highest-oxide formula aur hydride formula se infer ki gayi.

Oxide valency ek period mein kaise change hoti hai, aur hydride valency?

Oxide valency 1→7 badhti hai (

R_{2} O

...

R_{2} O_{7}

); hydride valency 4→1 ghatti hai (

R H_{4}

...

R H

) dayi taraf.

Prefix "eka" ka kya matlab hai aur ek example do.

"One" (Sanskrit); eka-silicon baad mein germanium ki tarah discover hua.

Mendeleev ne eka-silicon (germanium) ke liye do properties sahi predict ki thi, kaun si?

Atomic mass ~72 aur density ~5.5 g/cm³ (plus oxide

E s O_{2}

, chloride

E s C l_{4}

Teen anomalous pairs batao jahan heavier element lighter se pehle aata hai.

Ar before K, Co before Ni, Te before I.

Mendeleev isotopes ko properly place kyun nahi kar saka?

Isotopes ke masses alag hote hain par woh same element hain; mass-ordering alag positions maangti hai, jo galat hai.

Hydrogen ki position ambiguous kyun hai?

Yeh alkali metals (

H^{+}

, valency 1) aur halogens (

H^{-}

) dono jaisa behave karta hai, do groups se match karta hai.

Anomalous pairs ko kaun si deeper property fix karti hai?

Atomic number (Moseley, 1913);

Z

se order karne par saari mass inversions khatam ho jaati hain.

Recall Feynman: explain to a 12-year-old

Imagine karo ki LEGO logon ki ek badi pile ko hum har ek ke weight ke hisaab se sort kar rahe hain, sabse halke se sabse bhaari tak. Jaise-jaise aage badhte ho, notice karte ho: thodi der mein ek aur "firefighter" aata hai, phir aur aage ek aur "firefighter", phir dobara. Toh tum sabhi firefighters ko ek column mein stack karne lagte ho, sabhi doctors ko doosre mein. Yahi Mendeleev ki table hai — heaviness queue order hai, aur jobs (chemistry) repeat hoti hain, toh matching jobs same column mein baith jaate hain. Kabhi kabhi ek slightly heavier insaan clearly "firefighter" job wala tha, toh usne unhe firefighters ke paas rakha, chahe weight order toota — kyunki unki job zyada important thi. Baad mein logon ko pata chala ki asli queue number weight tha hi nahi, woh ek hidden ID badge (atomic number) tha.

Connections

Modern Periodic Law — based on atomic number (Moseley anomalies fix karta hai)
Atomic number and mass number (kyun $Z$ , mass se better ordering variable hai)
Isotopes (woh defect jo mass-ordering handle nahi kar sakti thi)
Valency and oxide formulas (Mendeleev ki grouping key)
Newlands' Law of Octaves (pehle ka, kamzor periodicity attempt)
Döbereiner's Triads (pehla hint ki mass properties ko link karta hai)