Mendeleev put tellurium before iodine despite higher atomic mass, so how did he justify it and what did Moseley change?
Aarav Kulkarni11th Sample
27 Aug 2026 · 1 min read
Doing the Genesis of Periodic Classification part of NCERT and my notes are a mess. I have Dobereiner's triads, Newlands' octaves, Lothar Meyer's curve, Mendeleev and Moseley all squeezed on one page and I keep mixing up who did what.
The specific thing bugging me: Mendeleev's law says properties of elements are a periodic function of atomic mass. But in his own table Te (127.6) sits before I (126.9). If he was strictly going by mass that is backwards. NCERT mentions he left gaps and swapped a few elements but never clearly says why that was allowed.
Then Moseley's law says properties are a periodic function of atomic number. I get that this "fixed" the Te–I problem, but how exactly? Did it also fix Ar–K and Co–Ni?
Also my memory for the eka-elements is zero. Mendeleev predicted eka-aluminium, eka-silicon... what are they in today's table?
I made a small table for triads (Li–Na–K, Ca–Sr–Ba, Cl–Br–I) and checked that the middle mass is roughly the average, so that part is fine. Rest is confusion. Is there a clean way to remember all this for the exam?
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[+][–]Nisha Verma12thSample· 1mo ago
He trusted chemical behaviour over mass. Te behaves like Se and S (group 16), I behaves like Cl and Br (group 17), so he just put them where they fit and assumed the mass values were off. Not a very scientific excuse tbh, but it worked.
Moseley (1913) bombarded elements with electrons and looked at the characteristic X-rays. The frequency was related to the atomic number, so basically he could count nuclear charge. Te is Z = 52, I is Z = 53, so order by Z and the problem disappears. Same for Ar (18) and K (19), Co (27) and Ni (28). All three pairs fixed in one go. That's why the modern law is in terms of atomic number.
[+][–]Aarav Kulkarni11thSample· 1mo agoAuthor
the "count nuclear charge" line finally made it click, thank you. making the timeline table tonight
[+][–]Rahul DubeyDropperSample· 1mo ago
For the eka-elements, three are enough for NEET: eka-aluminium is gallium, eka-silicon is germanium, eka-boron is scandium. "Eka" just means "one" in Sanskrit, it was his placeholder name for a missing element sitting in that group.
The point examiners care about: he left blank spaces and predicted properties of the missing elements, and when they were discovered the properties matched closely. That is why people trusted the table over the rival ones. Gaps being a feature and not a bug.
[+][–]Diya Menon11thSample· 1mo ago
omg i love this chapter. Also Mendeleev's table already had the noble gases in a zero group from the start, that's why it was so successful, he predicted them too 😄
[+][–]Kavya NaikDropperSample· 1mo ago
Small correction diya, noble gases were not known when Mendeleev made his table. Ramsay and others discovered them in the 1890s and they were fitted in later as a new group (group 0 then, group 18 now) without disturbing the rest of the table. That actually became a point in favour of the periodic law.
[+][–]Diya Menon11thSample· 1mo ago
ahh right right, i mixed it with the predictions part. thanks kavya!
[+][–]Kavya NaikDropperSample· 1mo ago
Quick one-liners for the older attempts, because these come as match-the-column:
Dobereiner: triads, middle element's mass is about the average of the other two (Li 7, Na 23, K 39: (7+39)/2 = 23). Newlands: law of octaves, every eighth element resembles the first like musical notes, worked only up to calcium. Lothar Meyer: plotted atomic volume against atomic mass, got a periodic curve. Mendeleev: periodic law with mass, gaps, predictions. Moseley: atomic number.
Amma made me recite this in the kitchen once, never forgot it 😅
[+][–]Tanvi Deshmukh12thSample· 1mo ago
Make it a timeline table, it sticks so much better!!
Who Idea Weakness Dobereiner triads only a few triads found Newlands octaves fails after Ca Mendeleev mass + properties Te/I, Ar/K, position of H, isotopes Moseley atomic number none, it is the modern basis And the modern table is 7 periods and 18 groups. Add hydrogen's odd position in the weakness column, examiners love that!
