Q.Which important property did Mendeleev use to classify the elements in his periodic table and did he stick to that?

Concept: Mendeleev's Classification Principle and Flexibility

Mendeleev classified elements primarily by atomic mass (atomic weight), arranging them in order of increasing mass. This was the fundamental organizing principle of his 1869 periodic table, as atomic number was unknown at the time.

However, he did not stick rigidly to this criterion. When strict adherence to atomic mass disrupted the periodicity of chemical properties, Mendeleev prioritized chemical similarity over mass order.

Key instances of deviation:

1. Tellurium (Te, mass ~127.6) placed before Iodine (I, mass ~126.9) — reversed mass order to keep Te with oxygen-family elements and I with halogens
2. Left gaps for undiscovered elements (Ga, Sc, Ge), predicting their properties based on periodic trends rather than filling by mass alone

This flexibility — valuing chemical periodicity over strict mass sequence — was Mendeleev's genius and validated his table when the predicted elements were discovered.

The Foundation of Mendeleev's Periodic Table

When Dmitri Mendeleev sat down in 1869 to organize the known elements, he needed a measurable, fundamental property that could serve as the backbone of his system. The property he chose was atomic mass (or atomic weight, as it was called then). This was the only reliable quantitative property available at the time—the concept of atomic number and protons wouldn't emerge for another four decades.

But here's what made Mendeleev a genius rather than just a compiler: he didn't treat atomic mass as an inflexible rule. He understood that the pattern of chemical properties was the deeper truth the periodic table was trying to reveal.

How Mendeleev Actually Built His Table

1. Primary organizing principle: Increasing atomic mass

   Mendeleev arranged elements in order of increasing atomic mass. This gave him a rough framework—lighter elements first, heavier ones later.

2. Overriding criterion: Chemical similarity

   When he noticed that certain elements with similar chemical behavior (valency, reactivity, compound formation) were falling into vertical columns, he recognized this as the fundamental pattern. Elements in the same group formed similar compounds, showed similar physical properties, and exhibited the same valency.

3. Breaking the mass rule when necessary

   Here's where Mendeleev showed true scientific insight. He encountered several problems:

   • Tellurium (Te) and Iodine (I): Tellurium has atomic mass ~127.6, while iodine has ~126.9. Strictly by mass, tellurium should come after iodine. But chemically, tellurium behaves like sulfur and selenium (Group 16), while iodine behaves like chlorine and bromine (Group 17). Mendeleev reversed their order, placing Te before I, trusting chemistry over mass.

   • Cobalt (Co) and Nickel (Ni): Similar reversal based on chemical properties.

   • Argon (Ar) and Potassium (K): Another pair where properties trumped mass order.

4. Leaving gaps for undiscovered elements

   When the mass sequence would place an element in a group where it didn't chemically belong, Mendeleev left blank spaces. He predicted these gaps represented undiscovered elements and even predicted their properties. The later discoveries of gallium (eka-aluminum), scandium (eka-boron), and germanium (eka-silicon) with properties matching his predictions validated this approach spectacularly.

Why This Worked (and Why It Had Limits)

Mendeleev's approach worked because atomic mass roughly correlates with atomic number (the number of protons), which we now know is the true organizing principle. Most of the time, heavier atoms have more protons. But isotopes and nuclear binding energy mean the correlation isn't perfect—hence the reversals.

The modern periodic table, reorganized by Moseley in 1913 using atomic number, resolved all these anomalies. Tellurium (atomic number 52) now correctly sits before iodine (atomic number 53) without any contradiction, because we're counting protons, not mass.