Q.In what manner is the long form of periodic table better than Mendeleev's periodic table? Explain with examples.

Concept: Modern Periodic Law and Electronic Configuration

Mendeleev arranged elements by atomic mass and left gaps for undiscovered elements, but his table had anomalies. The long form (modern periodic table) is based on atomic number and reflects electronic configuration, removing those inconsistencies.

Key improvements:

1. Resolves inversion anomalies. Mendeleev placed Co (58.9 u) before Ni (58.7 u) and Te (127.6 u) before I (126.9 u) based on properties, contradicting his mass-based order. The modern table uses atomic number: Co ($Z=27$) before Ni ($Z=28$), Te ($Z=52$) before I ($Z=53$) — no contradiction.

2. Proper placement of isotopes. Isotopes like $^{35}\text{Cl}$ and $^{37}\text{Cl}$ have different masses but identical atomic number ($Z=17$), so they occupy one position. Mendeleev's mass criterion would have required separate slots.

3. Justifies group similarities. Elements in a group share the same valence-shell configuration. Group 1 (Li: $2s^1$, Na: $3s^1$, K: $4s^1$) all have one $s$-electron, explaining identical +1 oxidation states. Mendeleev noted patterns but could not explain them. …

The heart of the improvement lies in the organizing principle. Mendeleev arranged elements by increasing atomic mass and grouped them by similar chemical properties, which worked remarkably well but created contradictions. The long form (modern periodic table) arranges elements by increasing atomic number — the number of protons — and groups them by identical valence electronic configurations. This shift from mass to nuclear charge as the fundamental property removes ambiguity and aligns the table perfectly with quantum mechanics.

Why does atomic number work better? Because chemical properties arise from how electrons are arranged, and electron count equals proton count in a neutral atom. Elements in the same group now share the same outer-shell configuration, so their chemical behavior follows naturally from a single principle: the Aufbau filling order.

Key improvements, with examples

1. Resolution of position anomalies

Mendeleev had to place some elements "out of order" by mass to keep similar properties together. Argon ($\text{Ar}$, mass ≈ 40) comes before potassium ($\text{K}$, mass ≈ 39) in his table, and cobalt ($\text{Co}$, mass ≈ 59) before nickel ($\text{Ni}$, mass ≈ 58). He trusted chemistry over mass, but couldn't explain why.

The long form resolves this instantly: Ar has atomic number 18, K has 19; Co is 27, Ni is 28. Arranged by $Z$, the order is natural and no exception is needed. The mass inversions occur because isotopes and nuclear binding energy affect mass independently of chemical identity.

2. Proper placement of isotopes

Mendeleev's table had no clear place for isotopes. If you organize by mass, where do you put $\ce{^{35}Cl}$ versus $\ce{^{37}Cl}$? They have identical chemistry but different masses.

The long form solves this trivially: isotopes share the same atomic number, so they occupy the same position. Chlorine is element 17 regardless of neutron count. The table reflects chemical identity, and isotopes are simply mass variants of one element.

3. Systematic accommodation of all elements

Mendeleev left gaps (correctly predicting some elements) but had no framework for the lanthanides and actinides — the $f$-block elements piled up awkwardly. His table also couldn't predict how many elements should exist.

The long form, built on electronic structure, has a place for every element determined by quantum numbers. The $f$-block (lanthanides, actinides) fits naturally as the filling of $4f$ and $5f$ orbitals. We know there are exactly 118 elements discovered so far (up to oganesson, $Z = 118$) because each atomic number corresponds to one unique electron configuration.

The table's shape — 2, 8, 18, 32 elements in successive shells — emerges directly from the $2(2\ell + 1)$ capacity of each subshell.

4. Elimination of the need for multiple groups

Mendeleev used subgroups (Group I-A, I-B, etc.) to fit elements with vaguely similar properties into the same column, creating confusion. Copper and sodium were both in "Group I" despite vastly different chemistry.

The long form separates them cleanly: sodium is Group 1 ($s^1$ valence), copper is Group 11 ($d^{10}s^1$). Each group now has a single, unambiguous valence configuration. Group 17 (halogens) all have $ns^2 np^5$; Group 2 (alkaline earths) all have $ns^2$. Periodicity is no longer approximate — it is exact.

5. Correct placement of hydrogen and helium

Mendeleev placed hydrogen with alkali metals (similar $+1$ oxidation state) but it also resembles halogens (forms $\ce{H-}$, needs one electron to complete a shell). Helium's position was unclear.

The long form places hydrogen in Group 1 (one valence electron, $1s^1$) and helium in Group 18 (complete shell, $1s^2$, noble gas behavior). The placement reflects electronic structure, not just reactivity, so the logic is transparent.

6. Prediction of properties from position …