Mendeleev's Periodic Table & the Modern Periodic Law

Dmitri Mendeleev: The Man Who Saw the Pattern

In March 1869, a 35-year-old Russian chemist named Dmitri Ivanovich Mendeleev was writing a textbook on general chemistry. He was stuck on one question: in what order should I introduce the 63 elements known at the time? Alphabetical? By discovery date? By their oxides?

The story goes that Mendeleev wrote each element's name, atomic mass, and key properties on a separate card — and then laid them out on his desk like a game of solitaire, shuffling and re-shuffling until a pattern emerged.

What he found changed chemistry forever. When arranged in order of increasing atomic mass, chemically similar elements fell into vertical columns. He called the law he had discovered the Periodic Law.

Mendeleev's Periodic Law (1869)

"The physical and chemical properties of elements are a periodic function of their atomic masses."

This one sentence, simple as it sounds, was revolutionary. It meant the list of elements was not a random catalogue — it had a hidden structure that repeated at regular intervals. And repeating structures allow prediction.

Big picture: Mendeleev had the structure right but the ordering parameter (mass) slightly wrong. Moseley's X-ray work showed the ordering should be by atomic number, not atomic mass. With that one fix, all anomalies vanished.

Mendeleev's Periodic Table — Structure & Layout

Mendeleev's 1869 table had eight vertical columns (groups, I to VIII) and six horizontal rows (periods). Each group was further split into sub-groups A and B — a feature that survives in modern textbooks as the distinction between 'main-group' and 'transition' elements.

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Key features

• Group I: alkali metals (Li, Na, K, Rb, Cs) — highest reactivity, form $M^+$ ions.
• Group VII: halogens (F, Cl, Br, I) — form $M^-$ ions.
• Group VIII: the triads of Fe–Co–Ni, Ru–Rh–Pd, Os–Ir–Pt — the "transition triads" placed together.
• Noble gases were missing because they hadn't been discovered yet (first isolated between 1894 and 1898). Mendeleev later added them as Group 0 or Group VIII.
• Gaps (denoted by question marks): Mendeleev deliberately left three empty cells with predicted atomic masses and properties — for elements he believed existed but had not yet been isolated. He named them eka-boron, eka-aluminium, and eka-silicon (Sanskrit eka = 'one', i.e., 'one below').

Ordering rule

Elements were placed in increasing order of atomic mass, with two important exceptions:

1. Te (127.6) placed before I (126.9) even though Te is heavier — because I's chemistry (halogen) fits Group VII and Te's chemistry (chalcogen) fits Group VI.
2. Co (58.93) placed before Ni (58.69) on similar chemistry grounds.

These anomalies bothered Mendeleev, but he trusted chemistry over arithmetic. As we'll see, Moseley later showed he was right to — the true ordering is by atomic number, not mass.

[School Exam Level] A common 3-mark CBSE question: "State Mendeleev's periodic law and describe its main features." Always mention: (a) arrangement by atomic mass, (b) 8 groups with A/B sub-groups, (c) gaps left for unknown elements, (d) chemistry-based placement (Te before I, etc.).

Key point: Mendeleev's boldest choice was to privilege chemistry over arithmetic — he let the columns be groups of chemically similar elements even when atomic-mass order was violated. This was the decision that made his table predictive.

The Famous Predictions — eka-aluminium, eka-silicon, eka-boron

Mendeleev's greatest achievement was not drawing the table — it was daring to leave gaps in it. Looking at his 1869 arrangement, three cells demanded filling: one below boron (Group III, Period 4), one below aluminium (Group III, Period 5), and one below silicon (Group IV, Period 5).

Mendeleev named these hypothetical elements eka-boron, eka-aluminium, and eka-silicon (eka = "one beyond" in Sanskrit). For each, he predicted the atomic mass, density, colour, chemical formulas, and even typical melting/boiling points — before the elements were discovered.

The eka-silicon prediction vs the reality of germanium (1886)

Property	Mendeleev's prediction (1871, 'eka-silicon')	Germanium, Ge (discovered 1886)
Atomic mass	~ 72	72.6
Density	~ 5.5 g/cm$^3$	5.47 g/cm$^3$
Colour	dark grey	grey-white
Atomic volume	13 cm$^3$/mol	13.2 cm$^3$/mol
Specific heat	0.073	0.076
Formula of oxide	$EsO_2$	$GeO_2$
Character of oxide	weakly basic	weakly basic
Formula of chloride	$EsCl_4$	$GeCl_4$
B.P. of chloride	< 100 $^\circ$C	86 $^\circ$C

The match was astonishing. When the German chemist Clemens Winkler isolated germanium in 1886 (15 years after Mendeleev's prediction) and the numbers matched to within a few percent, the scientific community was convinced: Mendeleev's table was not a curiosity but a fundamental law of nature.

Predictions and the elements that filled them

Mendeleev's name	Modern element	Discovered by	Year
eka-aluminium	Gallium (Ga)	Paul Emile Lecoq de Boisbaudran	1875
eka-boron	Scandium (Sc)	Lars Nilson	1879
eka-silicon	Germanium (Ge)	Clemens Winkler	1886

[NEET Important] NEET and AIIMS frequently ask "Which element was predicted by Mendeleev as eka-silicon?" The answer is germanium (Ge). Remember the trio: eka-Al → Ga, eka-B → Sc, eka-Si → Ge.

Key point: A good scientific theory doesn't just organise existing data — it predicts data that doesn't exist yet. Mendeleev's three successful predictions are what earned his table a permanent place in science.

Merits of Mendeleev's Table

Why did Mendeleev's arrangement beat all earlier attempts? A quick inventory of its strengths:

1. Systematic study of elements

Chemistry became a structured science. Instead of memorising 63 isolated elements, students and researchers could focus on trends within a group and trends across a period — a compression of information that still drives how chemistry is taught today.

2. Prediction of undiscovered elements

As we just saw, Mendeleev correctly predicted eka-aluminium (Ga), eka-boron (Sc), and eka-silicon (Ge). In each case he gave detailed chemical and physical properties — all verified to within a few percent.

3. Correction of wrong atomic masses

At the time, the atomic mass of beryllium was believed to be ~14 (putting it in Group IV). Mendeleev recognised that its chemistry fit Group II and boldly revised its atomic mass to 9.4. He was proved right — beryllium's correct mass is 9.01. He similarly corrected masses for indium, uranium, and a few others.

4. Accommodation of noble gases

When noble gases were discovered in the 1890s (He, Ne, Ar, Kr, Xe), they fit neatly into a new group (0, later Group VIII or 18) without disturbing the rest of the table — a sign of how robust the framework was.

5. Distinct chemistry of each group

Alkali metals, alkaline earths, halogens, transition metals — each is a cohesive family. The table made these families obvious and formalised the language we still use.

[School Exam Level] Remember: at least three merits with examples — predictions (Ga, Sc, Ge), wrong-mass corrections (Be, In, U), and the accommodation of noble gases.

Demerits (Limitations) of Mendeleev's Table

Mendeleev was honest about what his table couldn't explain. Five well-known issues:

1. Position of hydrogen — ambiguous

Hydrogen has one valence electron like the alkali metals (Group I), but it also lacks one electron to complete its shell like the halogens (Group VII). Mendeleev placed it tentatively in Group I, but noted its chemistry straddles both. This ambiguity survives in the modern table too — there's no universally agreed position for H.

2. Anomalous pairs — mass order violated

Several pairs of elements are heavier first, lighter second if placed by chemistry:

Pair	Atomic masses	Atomic numbers	Why the reversal?
Ar / K	39.95 / 39.10	18 / 19	Ar is heavier but $Z$ smaller
Co / Ni	58.93 / 58.69	27 / 28	Co is heavier but $Z$ smaller
Te / I	127.6 / 126.9	52 / 53	Te is heavier but $Z$ smaller

Mendeleev placed the lighter-atomic-mass element after the heavier one when chemistry demanded it, but he couldn't explain why. Moseley's atomic number resolved the paradox.

3. Position of lanthanoids and actinoids

The 14 lanthanoids (Ce to Lu) and 14 actinoids (Th to Lr) have nearly identical chemistry and didn't fit cleanly into any single group. Mendeleev's table didn't have a good home for them — they're now shown as two separate footnotes below the main body of the Modern Periodic Table.

4. Isotopes — same element, different masses

Isotopes of an element (e.g., $^{35}$Cl and $^{37}$Cl) have the same chemistry but different atomic masses. If masses are the organising parameter, they should occupy different positions — but clearly they don't. Mendeleev's law had no answer to this.

5. Grouping of chemically dissimilar elements

Some elements were lumped into the same group despite having very different chemistry — e.g., copper (Cu, Group IB) with sodium (Na, Group IA). The A/B sub-group split helped but didn't fully solve it.

6. No explanation of periodicity

Mendeleev observed that properties repeat periodically — but he had no theoretical explanation. Why every 2, 8, 8, 18, 18, 32 elements? The answer had to wait for quantum mechanics and the electronic-configuration picture (Section 4).

[JEE Tip] The three anomalous pairs (Ar–K, Co–Ni, Te–I) are an exam favourite. Memorise these three pairs and remember that all three are resolved by switching from mass to atomic number. Also memorise the term used: the Te–I anomaly is specifically called out in NCERT.

Moseley's Experiment & the Birth of Atomic Number (1913)

The fix came from an unlikely source: X-ray spectroscopy. In 1913, the young British physicist Henry Moseley (age 25 at the time) bombarded different metal targets with high-energy electrons and measured the frequency of the characteristic K-alpha X-rays each metal emitted.

He found an astonishingly simple relationship:

$$\boxed{\; \sqrt{\nu} = a(Z - b) \;}$$

where $\nu$ is the frequency of the K$_\alpha$ line, $Z$ is a whole number characteristic of each element (the atomic number), $a$ is a constant, and $b \approx 1$ for K$_\alpha$ transitions (it's the screening constant from the remaining 1s electron).

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What the plot showed

• Plot $\sqrt{\nu}$ against atomic mass — you get a scattered, messy curve with the three anomalous pairs out of line.
• Plot $\sqrt{\nu}$ against atomic number ($Z$) — you get a perfect straight line. Every element sits on the line.

This meant atomic number is the fundamental property that organises elements — not atomic mass. Mass happens to correlate with $Z$ because heavier nuclei usually have more protons, but it's not the controlling variable.

Why atomic number makes sense

Atomic number $Z$ = number of protons in the nucleus = number of electrons in the neutral atom. Since all chemical behaviour is determined by the electron cloud, and the electron count equals $Z$, it is $Z$ (not mass) that controls chemistry. Mass can differ between isotopes of the same element because neutrons have mass but do not affect chemistry.

Moseley's legacy — and tragedy

Moseley's discovery did four things:

1. Resolved the three anomalous pairs at a single stroke.
2. Explained why isotopes behave identically — they share $Z$.
3. Predicted four missing elements with $Z = 43, 61, 72, 75$ (later identified as Tc, Pm, Hf, Re).
4. Gave 'atomic number' its modern meaning — not just a position in a list, but a physical property.

Sadly, Moseley was killed at Gallipoli in 1915, aged 27, while serving in World War I. The Royal Society, and later Ernest Rutherford himself, mourned this as one of the greatest losses of the war.

[NEET Important] The quantity plotted on the Y-axis in Moseley's law is $\sqrt{\nu}$ (square root of frequency), not $\nu$ itself. Plotting $\nu$ vs $Z$ gives a parabola; only $\sqrt{\nu}$ vs $Z$ gives a straight line. This is a classic MCQ trap.

Key point: Atomic number is the true organising parameter of chemistry. Atomic mass is a correlated — but imperfect — proxy.

The Modern Periodic Law

Moseley's work led directly to a re-statement of the Periodic Law in its modern form:

Modern Periodic Law: "The physical and chemical properties of elements are a periodic function of their atomic numbers."

Only one word has changed — masses became numbers — but that word fixed every anomaly Mendeleev's table had.

Comparison: Mendeleev's Law vs Modern Law

Feature	Mendeleev's Periodic Law (1869)	Modern Periodic Law (post-1913)
Ordering parameter	Atomic mass	Atomic number ($Z$)
Anomalous pairs (Ar/K, Co/Ni, Te/I)	Unexplained	Naturally ordered
Isotopes	Problematic	Same $Z$ $\Rightarrow$ same position
Position of H	Ambiguous	Still ambiguous, but less so
Lanthanoids/actinoids	Awkward	Shown as separate f-block footnotes
Theoretical basis	Empirical observation	Electronic configuration (Section 4)
Number of groups	8 (I–VIII, A/B subgroups)	18 (1 to 18)

The present form of the Periodic Table

The long form of the Periodic Table that hangs in every chemistry classroom is Mendeleev's structure with Moseley's fix: elements arranged in order of increasing $Z$, split into 18 groups and 7 periods, with s-, p-, d-, f-blocks based on electronic configuration. We'll meet this in detail in Section 3.

Why the word 'periodic'?

Properties don't just vary monotonically with $Z$ — they repeat. Move from Li (Z=3) to Na (Z=11) and the chemistry starts over. That periodicity is exact only when $Z$ is the x-axis; with atomic mass, the repetition was slightly off due to isotopic mass variation. That's why Moseley's reformulation gave periodicity its true mathematical precision.

[JEE Tip] Statement form for exams:

• Mendeleev's law: "Properties are a periodic function of atomic mass."
• Modern law: "Properties are a periodic function of atomic number."

Never swap these in an answer — CBSE and JEE checkers mark strictly on the single-word difference.

Key point: Mendeleev saw the pattern; Moseley found the true parameter; together they gave us the Periodic Table we use today.

Memory Capsule: Quick Revision

• Mendeleev (1869): Arranged by Atomic Mass. Boldly used gaps for predictions.
• The Trio: eka-B (Sc), eka-Al (Ga), eka-Si (Ge).
• Moseley (1913): Arranged by Atomic Number ($Z$). Used X-ray frequency plot ($\sqrt{\nu}$ vs $Z$).
• Anomalous Pairs: Ar/K, Co/Ni, Te/I (Resolved by switching to $Z$).
• Modern Law: Properties are a periodic function of Atomic Number.
• Table Structure: 18 Groups, 7 Periods (Modern Long Form).

Solved Examples

Example 1: State Mendeleev's Periodic Law

State Mendeleev's Periodic Law and the Modern Periodic Law. What is the crucial difference between them?

Solution:

• Mendeleev's Periodic Law (1869): "The physical and chemical properties of elements are a periodic function of their atomic masses."
• Modern Periodic Law (post-Moseley 1913): "The physical and chemical properties of elements are a periodic function of their atomic numbers."

Crucial difference: The ordering parameter. Mendeleev used atomic mass; the modern law uses atomic number ($Z$, the number of protons). This change resolves the three anomalous pairs (Ar–K, Co–Ni, Te–I) and explains why isotopes of an element share the same chemistry.

[School Exam Level] A 2-mark CBSE answer needs both statements verbatim plus the one-word change (mass $\rightarrow$ number).

Example 2: Predict properties using Mendeleev-style reasoning

Using only the atomic masses Si = 28 and Sn = 119, estimate the atomic mass of 'eka-silicon' the way Mendeleev did.

Solution:

Mendeleev interpolated by averaging the atomic masses of the element above the gap (Si) and the element below the gap (Sn) in the same group:

$$\text{Estimated mass} = \dfrac{28 + 119}{2} = 73.5$$

Mendeleev's actual prediction was ~72 (he used slightly different data of the era). The real germanium has atomic mass 72.6.

Answer: Approximately 73.5 using the simplest two-element mean — remarkably close to the actual value of 72.6.

Takeaway: The idea that a missing element's properties interpolate between its group neighbours is Mendeleev's signature move. It works because atomic properties vary smoothly down a group.

Example 3: Identify the anomalous pair

Which of the following pairs of elements is NOT arranged in order of increasing atomic mass in the modern periodic table, and why?

(a) Li, Na (b) Ar, K (c) Mg, Ca (d) Be, Mg

Solution:

Check each pair:

• (a) Li (7) → Na (23): mass increases, no anomaly.
• (b) Ar (39.95) → K (39.10): mass decreases (K is lighter than Ar), yet K appears after Ar. Anomaly.
• (c) Mg (24) → Ca (40): mass increases.
• (d) Be (9) → Mg (24): mass increases.

Answer: (b) Ar and K is the anomalous pair. Ar (Z = 18) comes before K (Z = 19) in the periodic table, even though Ar is the heavier of the two, because atomic number — not mass — is the true ordering parameter.

[JEE Tip] Remember all three anomalous pairs by the acronym 'AKCN TI' or the phrase 'Ar-K, Co-Ni, Te-I' — one of each category (s-block, d-block, p-block). They come up every second year in JEE/NEET.

Example 4: Moseley's law — compare atomic numbers from frequency

The frequencies of the K$_\alpha$ X-ray lines for two elements X and Y are $1.6 \times 10^{18}$ Hz and $3.6 \times 10^{18}$ Hz respectively. Assuming Moseley's law with $b = 1$, find the ratio $(Z_X - 1):(Z_Y - 1)$.

Solution:

Moseley's law: $\sqrt{\nu} = a(Z - 1)$, so $(Z - 1) \propto \sqrt{\nu}$.

$$\dfrac{Z_X - 1}{Z_Y - 1} = \sqrt{\dfrac{\nu_X}{\nu_Y}} = \sqrt{\dfrac{1.6 \times 10^{18}}{3.6 \times 10^{18}}} = \sqrt{\dfrac{4}{9}} = \dfrac{2}{3}$$

Therefore, $$\boxed{(Z_X - 1):(Z_Y - 1) = 2:3}$$

Answer: $(Z_X - 1):(Z_Y - 1) = 2:3$.

[JEE Main] The key insight: square-root of frequency scales as $(Z-1)$, not frequency itself. A common trap is to take the ratio of frequencies directly, which gives the square of the right answer.

Example 5: Why Te is placed before I — and why that's OK

Tellurium (Te) has atomic mass 127.6 while Iodine (I) has atomic mass 126.9. Yet in Mendeleev's (and the Modern) Periodic Table, Te comes before I. Explain.

Solution:

1. By atomic mass, the order should be I (126.9) then Te (127.6). But Mendeleev's table has Te (Group VI, chalcogen) placed before I (Group VII, halogen) because of their chemistry: Te behaves like O, S, Se (Group 16) while I behaves like F, Cl, Br (Group 17).

2. By atomic number, the apparent paradox vanishes. Te has $Z = 52$, I has $Z = 53$. So Te is lighter in terms of the controlling variable (proton count). The mass reversal comes from Te having more neutrons in its abundant isotopes than I does.

Answer: Te is placed before I because the true ordering parameter is atomic number (52 < 53), not atomic mass. The apparent mass anomaly arises from isotope-mass-weighted averaging.

Takeaway: Whenever mass order and number order disagree, number always wins. Chemistry is controlled by electrons, and electron count equals $Z$.

Example 6: Predicted vs observed — eka-aluminium

Mendeleev predicted the atomic mass of eka-aluminium as ~68 and its density as ~5.9 g/cm$^3$. The element gallium (Ga), discovered in 1875, was found to have atomic mass 69.7 and density 5.91 g/cm$^3$. Calculate the percentage deviation in each prediction.

Solution:

• Atomic mass deviation: $$\% \text{ error} = \dfrac{|69.7 - 68|}{69.7} \times 100 = \dfrac{1.7}{69.7} \times 100 \approx 2.44\%$$

• Density deviation: $$\% \text{ error} = \dfrac{|5.91 - 5.9|}{5.91} \times 100 \approx 0.17\%$$

Answer: The atomic mass was predicted within ~2.4% and the density within ~0.2%.

[NEET Important] The spectacular closeness of Mendeleev's predictions is what convinced the scientific world. Remember that both properties — mass and density — were predicted, not just mass.

Example 7: Identify eka-boron and eka-silicon

Mendeleev's 'eka-boron', 'eka-aluminium' and 'eka-silicon' correspond to which modern elements? Write their atomic numbers.

Solution:

Mendeleev's name	Modern element	Symbol	Atomic number $Z$	Year discovered
eka-boron	Scandium	Sc	21	1879
eka-aluminium	Gallium	Ga	31	1875
eka-silicon	Germanium	Ge	32	1886

Answer: Sc (Z = 21), Ga (Z = 31), Ge (Z = 32).

Takeaway: Mnemonic — eka-B → Sc, eka-Al → Ga, eka-Si → Ge. Note that the modern element sits directly below Mendeleev's reference element in the same group.

Example 8: Isotopes and the old periodic law

Chlorine has two naturally occurring isotopes, $^{35}$Cl and $^{37}$Cl. Would they occupy the same position in Mendeleev's periodic table (arranged by atomic mass)? And in the Modern periodic table (arranged by atomic number)?

Solution:

• Mendeleev's table (mass-based): $^{35}$Cl and $^{37}$Cl have different masses, so strictly they should occupy different positions. But chemically they are identical — this is one of the clearest failures of the atomic-mass law.

• Modern table (atomic-number-based): Both isotopes have $Z = 17$. Hence they occupy the same position in the Modern Periodic Table. This automatically explains why they behave identically in chemistry.

Answer: Different positions under Mendeleev's scheme (a contradiction), same position under the Modern law (consistent with chemistry).

[School Exam Level] The word 'isotope' literally means "same place" in Greek — precisely because isotopes share a position in the Modern Periodic Table.

Example 9: Moseley's law numerical

The frequency of the K$_\alpha$ X-ray line of molybdenum (Z = 42) is $4.2 \times 10^{18}$ Hz. Using Moseley's law with screening constant $b = 1$, estimate the frequency of the K$_\alpha$ line of copper (Z = 29).

Solution:

Moseley's law: $\sqrt{\nu} = a(Z - 1)$, so $\nu \propto (Z - 1)^2$.

$$\dfrac{\nu_{\text{Cu}}}{\nu_{\text{Mo}}} = \left(\dfrac{29 - 1}{42 - 1}\right)^2 = \left(\dfrac{28}{41}\right)^2 \approx (0.683)^2 \approx 0.466$$

$$\nu_{\text{Cu}} \approx 0.466 \times 4.2 \times 10^{18} \approx 1.96 \times 10^{18} \text{ Hz}$$

Answer: $\nu_{\text{Cu}} \approx 1.96 \times 10^{18}$ Hz. (The experimental value is about $1.96 \times 10^{18}$ Hz — excellent agreement.)

[JEE Tip] The proportion $(Z - 1)^2$, not $Z^2$, is the key. The $-1$ comes from the screening of one inner 1s electron on the other in K-shell transitions.

Example 10: Position of hydrogen — why is it ambiguous?

In the modern periodic table, hydrogen is usually placed in Group 1 (above lithium). Give two reasons for this placement and two reasons against it (i.e., two reasons why it could alternatively sit in Group 17 with the halogens).

Solution:

For placing H in Group 1:

1. Hydrogen has one valence electron (1s$^1$), matching the alkali metals (ns$^1$).
2. Hydrogen forms a +1 ion (H$^+$) readily in many compounds (HCl, HNO$_3$, H$_2$SO$_4$).

Against (reasons to place it in Group 17):

1. Hydrogen is one electron short of the helium configuration.
2. Hydrogen can form a −1 ion (H$^-$, hydride), for example in NaH, CaH$_2$.

Furthermore, hydrogen is a diatomic gas (H$_2$) at room temperature like F$_2$, Cl$_2$ — not a metal like Li, Na.

Conclusion: Hydrogen has properties of both Group 1 and Group 17, and hence its position in the Periodic Table is ambiguous. Some modern tables place it at the top of both groups to reflect this.

[NEET Important] A favourite AIIMS question: "Why is the position of hydrogen in the periodic table controversial?" Always give electronic-configuration arguments plus examples of H$^+$ and H$^-$ ions.

Example 11: Merit-demerit comparison

List three merits and three demerits of Mendeleev's Periodic Table.

Solution:

Merits:

1. Systematic study — elements grouped into families with shared chemistry.
2. Prediction of unknown elements — eka-aluminium (Ga), eka-silicon (Ge), eka-boron (Sc) were all predicted and later discovered with properties matching closely.
3. Correction of atomic masses — e.g., Be (from ~14 to 9), In, U, etc.

Demerits:

1. Position of hydrogen ambiguous (Group I or Group VII).
2. Anomalous pairs (Ar–K, Co–Ni, Te–I) violated the atomic-mass order, with no explanation.
3. Isotopes of the same element would, by Mendeleev's rule, occupy different positions — contradicting the fact that they have identical chemistry.

Takeaway: CBSE and many JEE papers ask for three of each. Memorise this balanced list.

[School Exam Level] Short-answer CBSE Qs often go: "State any two merits and two demerits of Mendeleev's Periodic Table (2+2 = 4 marks)." Always lead with the prediction merit and the anomalous pairs demerit.

Example 12: The chronology of periodic-law discoveries

Arrange the following events in chronological order and give the year of each:

(i) Publication of Mendeleev's Periodic Table (ii) Discovery of atomic number by Moseley (iii) Discovery of germanium by Winkler (iv) Discovery of gallium by Lecoq de Boisbaudran

Solution:

Order	Event	Year
1	Mendeleev's Periodic Table published	1869
2	Gallium discovered (eka-aluminium verified)	1875
3	Germanium discovered (eka-silicon verified)	1886
4	Moseley's X-ray experiment (atomic number)	1913

Takeaway: A ~44-year gap between Mendeleev's table and Moseley's correction is what it took to get from "empirical pattern" to "physical law". The predictions (Ga 1875, Ge 1886) happened in between — progressively building confidence in the table even before its theoretical basis was understood.