Trends in Physical Properties of Elements in the Periodic Table

Lessons

Senior 4 · Chemistry

Trends in Physical Properties of Elements in the Periodic Table

3 lessons

Lesson 1 of 3

How the Periodic Table Was Built

25 min

A table that predicted the future

In 1869 chemists knew 63 elements, with a jumble of masses and properties. Dmitri Mendeleev, a chemistry professor in St Petersburg, arranged them in a table and did something no one had dared: he left gaps, and said that elements nobody had seen would one day fill them. He even predicted their masses, densities and compounds. Within seventeen years three of them had been found, and their properties matched his predictions closely. In S1 you met the early attempts to classify elements. This lesson looks at the evidence that turned Mendeleev's table into the modern periodic table, and at what had to be corrected along the way.

Before Mendeleev

Döbereiner's triads (1829). Johann Döbereiner noticed groups of three similar elements, such as lithium, sodium and potassium, or chlorine, bromine and iodine, in which the middle element's atomic mass is close to the average of the other two (Na: $\frac{7 + 39}{2} = 23$ ). Only a few triads could be found.
Newlands' law of octaves (1865). John Newlands listed the elements in order of atomic mass and found that every eighth element resembled the first, like notes in a musical scale. The pattern worked for the light elements but broke down after calcium, and he forced unlike elements together.
Lothar Meyer (1864–1870) plotted atomic volume against atomic mass and found a repeating curve: the first clear picture of periodicity.

Definition

Periodicity: the regular repetition of similar properties at intervals when the elements are arranged in order. Mendeleev's periodic law (1869): the properties of the elements are a periodic function of their atomic masses. Modern periodic law: the properties of the elements are a periodic function of their atomic numbers.

Mendeleev's periodic table as published in 1871. Elements are arranged in rows by atomic mass and in eight groups by the formulae of their oxides and hydrides (R₂O, RO, R₂O₃, …). Look for the dashes '— = 44', '— = 68' and '— = 72': gaps for undiscovered elements, later found as scandium, gallium and germanium. Iodine is written J (its German symbol) and placed after tellurium. Image: public domain, via Wikimedia Commons

What made Mendeleev's table different

He arranged the elements mainly by atomic mass, but put properties first. Where mass order put an element among the wrong family, he moved it. Tellurium's measured atomic mass (about 128) was larger than iodine's (127), but he placed tellurium first so that iodine sat with chlorine and bromine; he suspected the tellurium value was wrong, and his 1871 table gives it as 125.
He left gaps rather than force an element into the wrong group.
He used the gaps to predict new elements and their properties, naming them with the Sanskrit prefix eka- ("one"): eka-boron, eka-aluminium and eka-silicon.

Table 1.1: Mendeleev's eka-silicon (1871) and germanium (discovered by Clemens Winkler, 1886)

Property	Predicted for eka-silicon	Germanium, modern value
relative atomic mass	about 72	72.6
density of the element / $\mathrm{g\,cm^{-3}}$	about 5.5	5.3
formula of oxide	$\mathrm{EsO_2}$	$\mathrm{GeO_2}$
formula of chloride	$\mathrm{EsCl_4}$ , a liquid boiling below 100 °C	$\mathrm{GeCl_4}$ , a liquid boiling at 86 °C
valency	4	4

Gallium (eka-aluminium, 1875) and scandium (eka-boron, 1879) also matched his predictions. Successful predictions convinced chemists that the table described something real.

Dmitri Mendeleev (1834–1907) in the 1890s, from the frontispiece of his textbook Fundamentals of Chemistry. Photo: unknown photographer, public domain, via Wikimedia Commons

Problems that atomic mass could not solve

A few pairs are in the "wrong" order by mass: argon (39.9) comes before potassium (39.1), cobalt (58.9) before nickel (58.7), tellurium (127.6) before iodine (126.9).
The noble gases were unknown in 1869. When William Ramsay and others found argon, helium, neon, krypton and xenon (1894–1898), a whole new group had to be added.
Isotopes (Unit 1) showed that atoms of one element can have different masses, so mass cannot be what defines an element.

In 1913 Henry Moseley, working in Manchester, measured the frequencies of X-rays given off by different elements. The square root of the frequency increased by the same step from one element to the next, in a straight line. The step matched the nuclear charge, the atomic number. Ordered by atomic number, every awkward pair falls into place: argon is 18 and potassium 19, tellurium 52 and iodine 53. Moseley's line also showed exactly which atomic numbers were still missing (43, 61, 72, 75), and those elements were later found.

Henry Moseley (1887–1915), who showed in 1913 that the atomic number, the charge on the nucleus, is the true basis of the periodic table. He was killed in the First World War two years later. Photo: AIP Emilio Segrè Visual Archives, public domain, via Wikimedia Commons

Table 1.2: Mendeleev's table and the modern table

Feature	Mendeleev (1871)	Modern table
Basis of order	atomic mass (with a few swaps by properties)	atomic number (number of protons)
Number of elements	63 known, with gaps	118, all discovered and named by 2016
Groups	8 groups, some with A and B subgroups mixed together	18 groups; main groups and transition metals separated
Noble gases	absent	Group 18
Transition metals	placed among the main groups, with a Group VIII of three-element sets	a separate d-block in Groups 3–12
Lanthanides and actinides	not understood	f-block, two rows below the table
Explanation	none: an observed pattern	electron configurations (Lesson 2)

Worked example 1: testing a triad

Calcium (40.1), strontium (87.6) and barium (137.3) form a Döbereiner triad. How close is strontium's atomic mass to the average of the other two? Why could the idea of triads not be extended to the whole set of elements?

Solution.

\frac{40.1 + 137.3}{2} = 88.7 \qquad \text{difference} = 88.7 - 87.6 = 1.1

The average is within about 1% of strontium's mass. But most elements do not fall into neat sets of three similar elements, and triads say nothing about how different families are related to each other. Only a table of all the elements can show that.

Worked example 2: predicting like Mendeleev

Mendeleev predicted eka-silicon by averaging its neighbours. Silicon (above) has $A_{r} = 28.1$ and density $2.33\,\mathrm{g\,cm^{-3}}$ ; tin (below) has $A_{r} = 118.7$ and density $7.3\,\mathrm{g\,cm^{-3}}$ . Estimate the atomic mass and density of germanium from these, and compare with Table 1.1.

Solution.

A_r \approx \frac{28.1 + 118.7}{2} = 73.4 \qquad \rho \approx \frac{2.33 + 7.3}{2} = 4.8\,\mathrm{g\,cm^{-3}}

Answer: about 73 and $4.8\,\mathrm{g\,cm^{-3}}$ , against the real 72.6 and 5.3. Mendeleev also used the elements to the left and right (gallium and arsenic), which is why his density estimate, 5.5, was closer.

Warning

Common mistakes. (1) Saying Mendeleev ordered the elements by atomic number: he used atomic mass; atomic number came with Moseley in 1913. (2) Saying Mendeleev included the noble gases: they were discovered after his first table. (3) Treating his gaps as mistakes: they were his boldest and most successful step.

Tip

For a "compare Mendeleev's table with the modern one" question, use four headings: basis of order, gaps and predictions, noble gases, position of the transition and f-block elements.

SciShow: Mendeleev's Periodic Table (4 min). Watch for how Mendeleev ordered the elements, why he left gaps, and which elements later filled them.

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Practice

State Mendeleev's periodic law and the modern periodic law. What changed, and why?
Lithium (6.9) and potassium (39.1) are the outer members of a triad. Predict the atomic mass of the middle element and identify it.
Give two reasons why Newlands' law of octaves was not accepted.
Give three features of Mendeleev's table that made it more successful than earlier attempts.
Explain how Moseley's work solved the problem of argon and potassium.
Gallium was eka-aluminium. Aluminium has $A_{r} = 27.0$ and indium, below gallium, has $A_{r} = 114.8$ . Estimate the atomic mass of gallium and compare it with the real value, 69.7.
Evaluate this statement: "Mendeleev's table was correct in every detail."

Show answersHide answers

Mendeleev: properties are a periodic function of atomic mass. Modern: a periodic function of atomic number. Moseley showed that atomic number (nuclear charge), not mass, defines an element; isotopes and the Ar/K, Te/I, Co/Ni pairs confirm it.
$\frac{6.9 + 39.1}{2} = 23.0$ : sodium.
The pattern failed after calcium; he forced unlike elements into the same column (and put two elements in one place); he left no gaps for undiscovered elements.
He put properties before strict mass order; he left gaps; he predicted the properties of undiscovered elements, which were then confirmed (Ga, Sc, Ge).
By atomic mass argon (39.9) would come after potassium (39.1), putting a noble gas among the alkali metals. Moseley showed argon's atomic number is 18 and potassium's is 19, so ordered by atomic number they fall in the right groups.
$\frac{27.0 + 114.8}{2} = 70.9$ , close to 69.7 (within about 2%).
Not true: it was ordered by mass, needed swaps to fit, had no noble gases, mixed main-group and transition elements in the same groups, and could not explain the pattern. Its gaps and predictions, though, were remarkably right.

Quick check

What did Mendeleev do that made his periodic table more successful than earlier attempts?

Key points

Döbereiner's triads (1829) and Newlands' octaves (1865) found partial patterns; Lothar Meyer showed periodicity in atomic volumes.
Mendeleev (1869) ordered by atomic mass but put properties first, left gaps and predicted Ga, Sc and Ge, whose discovery confirmed his table.
Moseley (1913) showed that atomic number is the basis of the table, resolving Ar/K, Co/Ni and Te/I.
The modern table has 118 elements in 18 groups and 7 periods, with the noble gases, a separate d-block and an f-block.

Sources and credits

Image: Mendelejevs periodiska system 1871.png, Mendeleev's 1871 table, via Wikimedia Commons, in the public domain.
Image: Dmitri Mendeleev 1890s.jpg, unknown photographer, via Wikimedia Commons, in the public domain.
Image: Henry Moseley (1887-1915).jpg, AIP Emilio Segrè Visual Archives, via Wikimedia Commons, in the public domain.
Atomic masses and the properties of germanium are standard reference values; the predictions for eka-silicon are rounded from Mendeleev's 1871 paper.
Video: Mendeleev's Periodic Table, SciShow, embedded from YouTube.
The timeline, tables, worked examples and Practice items were written and drawn by Ganzaa.

1A table that predicted the future

2Before Mendeleev

3What made Mendeleev's table different

4Problems that atomic mass could not solve

A table that predicted the future

Before Mendeleev

What made Mendeleev's table different

Problems that atomic mass could not solve