Mendeleev's periodic table — based on atomic mass
WHAT is Mendeleev's periodic table?
- Rows running left→right = periods (mass increases across).
- Columns stacking similar elements = groups.
WHY atomic mass? (the historical logic)
HOW Mendeleev built it — derive the layout from scratch
Let's reconstruct his reasoning step by step.
Step 1 — Write elements in order of increasing atomic mass. Li(7), Be(9), B(11), C(12), N(14), O(16), F(19), Na(23), Mg(24), ... Why this step? Mass is the only universal ordering number he had.
Step 2 — Watch the chemistry. Li reacts violently, forms (valency 1). Then Be, B, C... and at Na the behaviour of Li comes back: Na also violent, forms , valency 1. Why this step? Detecting the return of a property tells you where to "wrap the line" onto a new row.
Step 3 — Break the line so similar elements stack. Start a new row at Na. Now Li sits above Na, Be above Mg, etc. Why this step? The vertical alignment is the whole point — it groups chemical twins.
Step 4 — Use valency (from BOTH oxide and hydride formulas) as the grouping key. Mendeleev inferred each element's valency from the formulas of its highest oxide and its hydride, and grouped elements sharing the same valency pattern. Across a period the highest oxides run giving rising oxide valencies 1, 2, 3, 4, 5, 6, 7. The hydrides on the right side run — so the hydride valency falls 4, 3, 2, 1 across groups IV→VII. Using both together pins each group's character far more tightly than either alone. Why this step? Formulas are hard chemical facts; oxide-valency (rising) and hydride-valency (falling) form a complementary fingerprint that anchors a group more reliably than colour or hardness.
The two triumphs (this is the 80/20 core)
Where atomic mass FAILED (the cracks)
Merits vs Defects (compact)
State Mendeleev's periodic law (original 1869 form).
Why did Mendeleev use atomic mass and not atomic number?
What is a "period" and a "group" in Mendeleev's table?
What grouping key did Mendeleev use besides mass?
How does oxide valency change across a period, and hydride valency?
What does the prefix "eka" mean and give an example?
Name two properties Mendeleev correctly predicted for eka-silicon (germanium).
Give three anomalous pairs where a heavier element precedes a lighter one.
Why couldn't Mendeleev place isotopes properly?
Why is hydrogen's position ambiguous?
What deeper property fixes the anomalous pairs?
Recall Feynman: explain to a 12-year-old
Imagine sorting a big pile of LEGO people by how heavy each one is, lightest to heaviest. As you go, you notice: every so often you hit another "firefighter", then later another "firefighter", then again. So you start stacking all firefighters in one column, all doctors in another. That's Mendeleev's table — heaviness is the queue order, and the jobs (chemistry) repeat, so matching jobs sit in the same column. Sometimes a slightly heavier person clearly had the "firefighter" job, so he put them next to firefighters even if it broke the weight order — because their job mattered more. Later people realised the real queue number wasn't weight at all, it was a hidden ID badge (atomic number).
Connections
- Modern Periodic Law — based on atomic number (Moseley fixes the anomalies)
- Atomic number and mass number (why beats mass as the ordering variable)
- Isotopes (the defect that mass-ordering couldn't handle)
- Valency and oxide formulas (Mendeleev's grouping key)
- Newlands' Law of Octaves (earlier, weaker periodicity attempt)
- Döbereiner's Triads (first hint that mass links properties)
Concept Map
Hinglish (regional understanding)
Intuition Hinglish mein samjho
Dekho, 1869 me Mendeleev ke paas 60+ elements the, aur unke paas atomic number ka koi idea hi nahi tha kyunki proton tab tak discover hi nahi hua tha (Rutherford ne hydrogen nucleus ~1919 me identify kiya, "proton" naam ~1920 me pada). Toh unhone sabse reliable cheez use ki — atomic mass. Elements ko halke se bhaari (increasing mass) me line me lagaya. Kamaal ki baat: jaise-jaise aage badho, elements ke properties repeat hote gaye — jaise Li ke baad Na, dono violent metals, dono valency 1. Isi repetition ko unhone "periodic" bola, aur similar elements ko ek hi vertical column (group) me stack kar diya. Grouping ka main key thi valency, jo unhone oxide (, ...) aur hydride (...) dono ke formula se nikali — oxide valency 1 se 7 tak badhti hai, hydride valency 4 se 1 tak ghatti hai.
Mendeleev ka asli genius do jagah dikha. Pehla — jahaan pattern kehta tha ki yahaan koi element hona chahiye par abhi discover nahi hua, wahaan unhone gap chhod diya aur uske properties predict kar diye (eka-silicon = baad me germanium nikla, bilkul sahi mass aur density ke saath). Doosra — kuch elements ki galat atomic mass ko unhone properties dekh ke theek kar diya. Yaani unhe apne pattern par itna bharosa tha ki galti data me maani, law me nahi.
Lekin mass wali approach me kuch cracks bhi the. Kuch jagah ek heavier element ko lighter se pehle rakhna pada taaki chemistry match ho — jaise Ar (39.9) before K (39.1), Co before Ni, Te before I. Ye "anomalous pairs" ishaara the ki asli order mass nahi hai. Isotopes ka bhi koi fixed ghar nahi tha, aur hydrogen kabhi alkali metal toh kabhi halogen jaisa behave karta tha. Ye saari problems baad me atomic number (Moseley, 1913) se solve hui — yahi Modern Periodic Law ka core hai. Exam ke liye yaad rakho: 3 merits (classification, prediction, mass-correction) aur 4 defects (anomalous pairs, isotopes, hydrogen, arrangement issues).