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1.4.2 · D5Periodic Table — First Look

Question bank — Modern periodic law — based on atomic number

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True or false — justify

Each statement is either true or false. Say which, and give the one-sentence because.

The Modern Periodic Law arranges elements by atomic mass.
False. Mendeleev used mass; the Modern Law arranges by atomic number , which is why the mass anomalies vanish on their own.
The Ar–K pair is a genuine exception to the Modern Periodic Law.
False. It only looked odd when ordered by mass; since , the Modern Law places them correctly with no swap needed.
Moseley's plot of against atomic number is a straight line.
True. From , plotting (y) versus (x) is a line of slope , proving is a clean integer property.
"Periodic function" means properties increase steadily and never fall.
False. Periodic means they repeat in cycles — they rise and fall over and over, like a wave, not a one-way climb.
Moseley's line for vs passes exactly through the origin.
False. It hits the -axis at (the screening constant, for the K-series), not at , because of the term.
Under the Modern Law, cobalt is placed before nickel even though cobalt is heavier.
True. Chemistry demanded Co first; delivers exactly that ordering, so mass being larger is irrelevant.
Atomic number equals the number of neutrons in the nucleus.
False. counts protons; neutron count varies between isotopes and does not define the element's identity or its place in the table.
A plot of against atomic mass would also give a perfect straight line.
False. Mass rises irregularly (isotopes, differing neutron numbers), so that plot scatters — this is precisely how Moseley showed mass is not the true ordering property.

Spot the error

Each line contains a mistake. Name it and correct it.

"Moseley's law is ."
The screening term is dropped; it must be , because inner electrons shield the nucleus so the felt charge is , not .
"Argon comes before potassium because argon is lighter."
Wrong reason — argon is actually heavier (39.9 vs 39.1); it comes first because its atomic number is smaller ().
"The Modern Law is a periodic function of atomic mass."
Swap "mass" for "atomic number"; the whole point of the Modern Law is that , not mass, drives the periodicity.
"Since depends on , we plot vs for a line."
We plot vs ; taking the square root of is what linearises it, and itself would give a parabola.
"Moseley disproved the periodic law."
He confirmed and repaired it — he replaced the shaky mass basis with the exact atomic-number basis, removing every anomaly.
"The constant in Moseley's law is the atom's mass number."
is the screening (shielding) constant from inner electrons, unrelated to mass number.

Why questions

Answer each in one or two sentences of real reasoning.

Why does ordering by atomic number fix the Ar–K, Co–Ni and Te–I anomalies automatically?
Chemistry always agreed with the smaller--first ordering, so once you rank by instead of mass every one of these pairs falls into its correct chemical group with no manual swap.
Why must X-ray frequency encode atomic number rather than mass?
X-rays are emitted when an inner electron drops shells, and that energy is set by the nuclear pull — i.e. the number of protons — so the frequency directly reports , not the neutron-loaded mass.
Why do we take the square root of frequency in Moseley's law?
Energy (hence ) goes as ; taking undoes the square so the relation becomes linear in , giving the tell-tale straight line.
Why is the felt nuclear charge instead of just ?
Inner electrons sit between the nucleus and the emitting electron and partly cancel the pull, so the emitting electron "feels" a reduced charge — this is Effective Nuclear Charge and Shielding.
Why was Mendeleev forced to swap some pairs by hand while the Modern Law does not?
His mass ordering genuinely put a few pairs in the chemically wrong order, so he patched them; the Modern Law's ordering never produces those clashes, so nothing needs patching.
Why is a law that explains its exceptions considered stronger than one that patches them?
Patching (Mendeleev's swaps) hides a flaw with no reason; explaining (the basis) shows the "exceptions" were never exceptions, marking as the true underlying cause.
Why does the periodic pattern come "back around" as increases?
Because outer-electron arrangements repeat as shells fill (see Bohr Model of the Atom and Periodic Trends), and it is the outer electrons that govern chemical behaviour.

Edge cases

Boundary and degenerate situations the topic quietly invites.

For hydrogen () with in the K-series, what does Moseley's law predict for ?
, so this K-line frequency collapses to zero — a reminder that Moseley's straight-line fit is meant for the heavier target metals he actually measured, not literally .
Two isotopes of the same element have different masses — do they sit in different places in the Modern table?
No; they share the same (same proton count), so they occupy the same box — proof that , not mass, fixes an element's identity (Atomic Number and Atomic Mass).
If two elements had the identical atomic mass, could the Modern Law still order them?
Yes; their atomic numbers must differ (no two elements share a ), so still gives a unique, unambiguous order where mass alone would be stuck.
What happens to the anomalies if you insist on keeping the mass order?
You are forced back into Mendeleev's manual swaps — Ar/K, Co/Ni, Te/I all sit in chemically wrong groups until you override mass by hand.
Is the screening constant the same for every spectral line of an element?
No; and are fixed only for a given line (e.g. the K-series), so a different line has its own constants — always compare like with like.
Could Moseley's law be tested with a single element?
Not meaningfully; its power is the straight line across many elements — one point can't reveal a line, so the proof needs a range of values (Ca, Ti, Fe, …).
At exactly, what is the predicted , and why is that the x-intercept?
, so the line crosses the -axis at — this nonzero intercept is exactly why the "" shortcut is wrong.
Recall The single trap that catches everyone

Every misconception above traces to one confusion: mass vs. atomic number. Whenever you feel unsure, ask "is this claim secretly using mass where it should use ?" — nine times out of ten, that's the error.

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