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1.4.1 · D5Periodic Table — First Look

Question bank — Mendeleev's periodic table — based on atomic mass

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Before you start, four anchor ideas we will lean on:

  • Atomic mass — how heavy one atom is (Mendeleev's ordering ruler in 1869).
  • ==Atomic number ()== — the count of protons, discovered later; the true ordering ruler. See Atomic number and mass number.
  • Valency — how many bonds an atom forms, read off its oxide/hydride formula. See Valency and oxide formulas.
  • ==== — a placeholder letter standing for "any element in that group" (like in algebra). So means "two atoms of the element joined to one oxygen".

Everything here builds on the parent topic.

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

True or false — justify

Mendeleev's law says properties are a periodic function of atomic number.
False — the original 1869 law used atomic mass. Atomic number came with Modern Periodic Law — based on atomic number after Moseley (1913).
In Mendeleev's table the elements are in strictly increasing order of atomic mass everywhere.
False — he broke mass order in a few places (Ar before K, Co before Ni, Te before I) so chemically alike elements stayed in the same group.
Two elements in the same group have similar chemical properties.
True — a group is a vertical column chosen to hold chemical twins; that shared valency/behaviour is the whole reason the column exists.
Across a period, atomic mass increases from left to right with no exceptions.
False — it generally rises, but a handful of pairs invert (Ar/K, Co/Ni, Te/I). Some inversions were also blurred by 1869 mass measurement error, so "rises left to right" is a trend, not an ironclad rule.
Mendeleev's table already contained the noble gases in 1869.
False — noble gases were not discovered until the 1890s; they were added later as an entirely new group.
Because he ordered by mass, Mendeleev could give each isotope its own box.
False — isotopes are the same element with different masses; mass-ordering wrongly demands separate boxes, so it simply couldn't handle them. See Isotopes.
Mendeleev used only atomic mass to decide where an element goes.
False — mass set the queue order, but valency (from highest-oxide and hydride formulas) decided which group it joined.
The prefix "eka" meant a brand-new chemical family.
False — "eka" is Sanskrit for "one", meaning "one place below" a known element (eka-silicon = the box just under silicon = germanium).
Hydrogen has one obvious, undisputed position in Mendeleev's table.
False — H acts like an alkali metal (, valency 1) and like a halogen (), so no single mass-based slot fits it cleanly.
Correcting beryllium's mass shows Mendeleev trusted data over pattern.
False — the reverse: he trusted the pattern, repositioned Be by its chemistry, and corrected its mass from ~14 to ~9.
The lanthanoids all fitted neatly into Mendeleev's original columns.
False — the rare-earth (lanthanoid) elements have very similar masses and similar chemistry, so they crowded one region and could not be spread cleanly across groups; the actinoids weren't even mostly known yet.

Spot the error

"Ar (39.9) sits before K (39.1), so mass ordering is never violated."
The two facts contradict: a heavier Ar comes before a lighter K, which is a violation — an anomalous pair kept for chemical reasons.
"Mendeleev left gaps because he ran out of paper / ran out of known elements to write."
The gaps were deliberate placeholders for undiscovered elements the pattern demanded, letting him predict their properties (e.g. eka-silicon → germanium).
"Oxide valency and hydride valency both rise across a period."
Only oxide valency rises (1→7, …, with = any element in that group); hydride valency falls 4→1 (…) on the right side.
"The anomalous pairs prove Mendeleev made careless mistakes."
They were deliberate, correct choices to preserve chemistry; the real lesson is that mass isn't the true ordering variable — atomic number is.
"Since Te (127.6) is heavier than I (126.9), Te must sit after I."
By mass yes, but by chemistry Te belongs with the oxygen family and I with the halogens, so Mendeleev put Te before I — a justified inversion (see Figure 2).
"Mendeleev knew protons existed and chose to ignore them."
The proton wasn't identified until ~1919; in 1869 atomic number simply didn't exist, so mass was genuinely the only universal ruler available.
"Germanium confirmed the table by accident."
Not accident — its measured mass (~72.6), density (~5.35), oxide and chloride matched Mendeleev's pre-published predictions, showing the table is a genuine law.
"The lanthanoids sit inside the main body of the table exactly where mass puts them."
No — because so many rare-earths share near-identical mass and chemistry, they resist clean column-by-column placement and are drawn as a separate strip; mass-ordering alone cannot separate them.

Why questions

Why did Mendeleev order by atomic mass rather than atomic number?
In 1869 the proton and therefore atomic number were unknown; atomic mass was the only measurable quantity that gave every element a unique place.
Why does breaking the line at Na (starting a new row) matter?
It stacks Na directly under Li so their shared valency-1, violent-reaction chemistry lines up in one column — that vertical grouping is the entire point of the table.
Why did Mendeleev use both oxide and hydride formulas instead of just one?
Rising oxide valency and falling hydride valency form a complementary fingerprint that pins a group's identity far more tightly than either formula alone.
Why can't mass-ordering resolve where isotopes go?
Isotopes differ in mass but are the same element with the same chemistry; mass says "different boxes" while chemistry says "same box", and mass has no way to reconcile that. Atomic number does (isotopes share ).
Why do the anomalous pairs vanish under atomic number?
Ordering by (proton count) is strictly monotonic and matches chemistry, so Ar<K, Co<Ni, Te<I all fall into correct, non-inverted order — no exceptions needed.
Why were correct predictions stronger evidence than merely sorting known elements?
Sorting could be a lucky filing trick; predicting an unknown element's mass, density and formulas in advance can only work if the arrangement reflects a real natural law.
Why was hydrogen so hard to place using valency alone?
Its valency of 1 matches alkali metals via and halogens via , so a single valency clue points to two different groups at once.
Why do the lanthanoids strain a mass-ordered table?
They form a long run of elements with almost the same mass and almost the same chemistry, so neither the mass ruler nor the valency fingerprint separates them into distinct groups — they had to be handled as a special block.

Edge cases

What happens at the very first element, hydrogen, before any pattern has "repeated"?
There is no earlier twin to align it with, and its dual behaviour matches two groups — so it becomes a permanent placement exception rather than a clean case.
What does mass-ordering do when two elements have almost equal mass (e.g. Co 58.9 vs Ni 58.7)?
The tiny mass gap makes ordering ambiguous, and chemistry overrides it — Mendeleev placed Co before Ni despite Co being heavier.
If a group had a missing element, what filled the "hole"?
Nothing physically — a deliberate gap was left, and its neighbours' averaged properties let Mendeleev predict the missing element (e.g. eka-aluminium → gallium).
What would ordering purely by mass, with zero chemical judgement, produce?
A sequence with Ar after K, I before Te, etc., that breaks up chemical families — showing mass alone is insufficient and chemistry must guide the layout.
How does the table handle an element whose accepted mass is simply wrong?
Its chemistry won't fit its mass-implied slot; Mendeleev repositioned it by properties and re-estimated the mass (as with Be, ~14 → ~9).
Where do the lanthanoid and actinoid series go, and why are they an edge case?
They are pulled out as a separate strip below the main table because their members share nearly identical mass and chemistry; forcing them into ordinary groups would swell the rows and blur the pattern — a limit case mass-ordering cannot resolve, and the actinoids were largely undiscovered in Mendeleev's time.
Recall One-line summary of every trap here

Mass was Mendeleev's ruler, but chemistry (valency) was his judge — wherever the two disagreed (anomalous pairs, isotopes, hydrogen, lanthanoids, noble gases) the table showed its cracks, and atomic number later healed them.

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