Arrangement of Elements in the Periodic Table

Lessons

Senior 1 · Chemistry

Arrangement of Elements in the Periodic Table

4 lessons

Lesson 2 of 4

Mendeleev's Periodic Table and the Modern Periodic Law

22 min

A table that made predictions

In 1869 Dmitri Mendeleev, a chemistry professor in St Petersburg, was writing a textbook and needed a sensible order in which to describe the 63 elements then known. He wrote each element's name, atomic mass and properties on a separate card and moved the cards around until similar elements lined up. The result, printed on the page below, did something no earlier arrangement had done: it predicted elements that nobody had yet found.

Mendeleev's four big decisions

He arranged the elements in order of increasing atomic mass.
He put elements with similar properties in the same group, for example elements whose oxides have the same type of formula. Where atomic mass and properties disagreed, he trusted the properties.
He left gaps where no known element fitted, instead of forcing the wrong element into the space.
He predicted the properties of the missing elements from those of their neighbours.

He summed this up in his periodic law: the properties of the elements repeat periodically when the elements are arranged in order of atomic mass. Figure 2.1 shows part of the improved table he published in 1871, turned the way we draw it today.

Figure 2.1: Part of Mendeleev's 1871 table (atomic masses as he gave them). R stands for any element in the group; the oxide formulae show the pattern he used to sort them. The dashed yellow boxes are his gaps.

The test: eka-silicon

Mendeleev called the missing element below silicon eka-silicon ("one below silicon"). In 1871 he predicted its properties from those of silicon above it and tin below it. Fifteen years later, in 1886, the German chemist Clemens Winkler found a new element in a silver ore and named it germanium. Table 2.1 compares the two.

Table 2.1: Mendeleev's prediction (1871) and germanium (found 1886)

Property	Eka-silicon (predicted)	Germanium (measured)
Atomic mass	72	72.6
Density	$5.5\,\mathrm{g\,cm^{-3}}$	$5.32\,\mathrm{g\,cm^{-3}}$
Colour	grey	grey
Melting point	high	$938\,^\circ\mathrm{C}$
Formula of oxide	$\mathrm{EsO_2}$	$\mathrm{GeO_2}$
Formula of chloride	$\mathrm{EsCl_4}$	$\mathrm{GeCl_4}$
Boiling point of chloride	below $100\,^\circ\mathrm{C}$	$86.5\,^\circ\mathrm{C}$
Density of chloride	$1.9\,\mathrm{g\,cm^{-3}}$	$1.88\,\mathrm{g\,cm^{-3}}$

Worked example

Problem. How close was Mendeleev's prediction of (a) the atomic mass and (b) the density of germanium? Give each error as a percentage of the measured value.

Solution. Use $\text{percentage error} = \frac{\text{predicted} - \text{measured}}{\text{measured}} \times 100\%$

(a) Atomic mass: $\frac{72 - 72.6}{72.6} \times 100\% \approx -0.8\%$

(b) Density: $\frac{5.5 - 5.32}{5.32} \times 100\% \approx +3.4\%$

Both predictions were within a few per cent, made fifteen years before anyone had seen the element. Correct predictions of gallium (1875) and scandium (1879) convinced chemists everywhere that Mendeleev's table was right.

Worked example

Problem. Tellurium has an atomic mass of 127.6 and iodine 126.9. Strict order of mass would put iodine first. Why did Mendeleev put tellurium first, and who turned out to be right?

Solution. Iodine behaves like chlorine and bromine: it forms the ion $\mathrm{I^-}$ and salts such as potassium iodide, $\mathrm{KI}$ . Tellurium behaves like sulfur and selenium: it forms $\mathrm{H_2Te}$ , like $\mathrm{H_2S}$ . To keep each in its proper family, Mendeleev swapped them and suspected that tellurium's mass had been measured wrongly. The mass was in fact correct, but the swap was right, for a reason nobody knew in 1869: iodine has more protons. Tellurium's atomic number is 52 and iodine's is 53.

From atomic mass to atomic number

Two later discoveries fitted Mendeleev's table without breaking it. Between 1894 and 1898 William Ramsay, working first with Lord Rayleigh and then with Morris Travers, discovered a whole family of unreactive gases, the noble gases: helium, neon, argon, krypton and xenon. They simply formed a new group at the edge of the table.

Then in 1913 Henry Moseley, using X-rays, measured the positive charge on the nucleus of each element, its atomic number. When the elements are ordered by atomic number, the odd pairs disappear: tellurium (52) comes before iodine (53), and argon (18, mass 39.9) comes before potassium (19, mass 39.1). The periodic law was restated.

Definition

Modern periodic law: the properties of the elements are a periodic function of their atomic numbers: when the elements are arranged in order of increasing atomic number, elements with similar properties recur at regular intervals.

Figure 2.2: A timeline of how the periodic table was built.

Warning

Common mistake: saying that the modern periodic table is arranged by atomic mass. Mendeleev used atomic mass; the modern table is arranged by atomic number. That is why argon (39.9) comes before potassium (39.1).

Tip

What made Mendeleev different from Newlands can be remembered as G-P-S: he left Gaps, made Predictions, and Swapped elements to fit their properties.

TED-Ed: The genius of Mendeleev's periodic table, by Lou Serico (4 min). Watch for the gaps in his table and what happened when gallium was discovered.

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Practice

State Mendeleev's periodic law and the modern periodic law. What is the difference?
Give two ways in which Mendeleev's table was better than Newlands' octaves.
What did Mendeleev call the undiscovered element below aluminium, and what is it called today?
Use Table 2.1 to find the percentage error in Mendeleev's prediction of the density of germanium's chloride.
Argon (atomic mass 39.9) comes before potassium (39.1) in the modern table. Explain why.
How did the discovery of the noble gases test Mendeleev's table?
Gallium melts at $30\,^\circ\mathrm{C}$ . Mendeleev predicted only that eka-aluminium would have a "low" melting point. Was his prediction good? Explain.

Show answersHide answers

Mendeleev: properties repeat periodically when elements are arranged by atomic mass. Modern: properties are a periodic function of atomic number. The ordering quantity changed from atomic mass to atomic number.
Any two: he left gaps for undiscovered elements; he predicted their properties; he put elements in groups by properties even when this meant swapping the mass order; his table worked for all the known elements, not only the light ones.
Eka-aluminium; gallium.
$\frac{1.9 - 1.88}{1.88} \times 100\% \approx 1\%$ .
The modern table is ordered by atomic number. Argon has 18 protons and potassium 19, so argon comes first, even though it is slightly heavier.
They were a whole new family not predicted by Mendeleev, yet they fitted as a complete new group at the edge of the table without disturbing any other element's place.
Yes. $30\,^\circ\mathrm{C}$ is very low for a metal (gallium melts in your hand), so "low" was correct.

Sources and credits

Mendeleev's predicted elements (the predicted and measured properties of eka-silicon and germanium), Wikipedia contributors, Wikipedia, licensed under CC BY-SA 4.0. Adapted by Ganzaa.
History of the periodic table (Mendeleev's 1869 and 1871 tables, the noble gases and Moseley's atomic numbers), Wikipedia contributors, Wikipedia, licensed under CC BY-SA 4.0. Adapted by Ganzaa.
Physical Sciences Grade 10, 5.1: The arrangement of the elements (Mendeleev's gaps and predictions), Siyavula Education and volunteers, Siyavula, licensed under CC BY 3.0. Adapted by Ganzaa.
Image: 1869-periodic-table.jpg by Dmitri Mendeleev (1869), via Wikimedia Commons, in the public domain.
Video: The genius of Mendeleev's periodic table - Lou Serico, TED-Ed, embedded from YouTube.
The redrawn table, timeline, worked examples, Practice items and exercises were written and drawn by Ganzaa.

This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1A table that made predictions

2Mendeleev's four big decisions

3The test: eka-silicon

4From atomic mass to atomic number

A table that made predictions

Mendeleev's four big decisions

The test: eka-silicon

From atomic mass to atomic number