The first Periodic system of the elements was proposed in 1869 by the Russian chemist D.I. Mendeleev — Physical Chemistry Chemistry Question
On the borders of the periodic system
The first Periodic system of the elements was proposed in 1869 by the Russian chemist D.I. Mendeleev, who arranged all the known chemical elements in the order of increasing atomic mass. In 1871 Mendeleev published the article «The natural system of the elements and its application to the prediction of properties of yet undiscovered elements » in the «Journal of the Russian Chemical Society». In that article Mendeleev described in detail the properties of three unknown elements that were ekaboron (Eb), ekaaluminum (Ea), and ekasilicon (Es). All of them were discovered in the next 15 years.
1.1 What are the present names of the three elements predicted by Mendeleev? Interestingly, all three names have a geographical origin.
Model Answer
1.1 In 1875 the French chemist Paul-Emile Lecoq de Boisbaudran studied the spectra of zinc ore and discovered the traces of a new element, which he called “gallium” from the Latin word "Gallia" meaning "France" and perhaps also from the Latin word "gallus" (the cock, a translation of Lecoq). In the same year Lecoq de Boisbaudran obtained the free metal by electrolysis of a solution of the hydroxide Ga(OH)3 in KOH. When Mendeleev knew about this discovery he understood that the properties of gallium resemble those of ekaaluminum. Moreover, he wrote to Boisbaudran that he obtained the wrong value for the density of gallium (4.7 g cm–3) whereas Mendeleev predicted the density to be 5.9 - 6.0 g cm–3). Indeed, more accurate measurements gave the correct value 5.904 g cm–3.
Scandium (from the Latin word "Scandia" meaning "Scandinavia") was discovered by Swedish chemist Lars Frederick Nilson in 1876 in the minerals euxenite and gadolinite, which had not yet been found anywhere except in Scandinavia. He and his coworkers were actually looking for rare earth metals. By processing 10 kg of euxenite and other residues of rare-earth minerals, Nilson was able to prepare about 2 g of scandium oxide (scandia, Sc2O3) of high purity. Per Theodor Cleve found scandium oxide at about the same time. He noted that the new element was the element ekaboron predicted by Mendeleev in 1871.
Germanium (from the Latin word "Germania" meaning "Germany") was discovered in a mineral called argyrodite by Clemens Alexander Winkler in 1886. The properties of germanium became remarkably close to those predicted by Mendeleev.
The first Periodic system listed 66 elements only, of which three were unknown. In the present-day system there are 118 elements. The last, 118th element was discovered in 2005 during the collaborative studies by the Joint Institute for Nuclear Research (Russia) and the Livermore National Laboratory (USA). After the collisions of calcium-48 nuclei with the target containing californium-249 nuclei three cascades of α-decay were detected, that started from the 118th element with the mass number 294.
1.2 Write the balanced equations of the nuclear reactions of: i) the synthesis and ii) the α-decay of the 118th element.
Model Answer
1.2 Nuclear synthesis of the 118th element led to formation of three neutrons:
249 98 Cf + 48 20 Ca → 294 118 + 3 n
The α-decay of the obtained nuclide gave the nuclei of the 116th element:
294 118 → 290 116 + 4 2 He
1.3 To which group of the Periodic system does the 118th element belong? Give its electron configuration using a noble gas with the spdf notation.
Model Answer
1.3 The 118-th element completes the 7-th period. It belongs to the inert gases (group 18). Its electron configuration is [Rn] 5f14 6d10 7s2 7p6.
1.4 Based on the properties of the same-group analogs of the 118th element and using extrapolation predict the following properties of the 118th element: i) melting point; ii) boiling point, iii) atomic radius, iv) first ionization energy, v) the formula of the oxide of the 118th element in its highest oxidation state.
Model Answer
1.4 For the extrapolation we will consider inert gases of the periods 3 – 6 because helium and neon differ significantly in their properties from other inert gases.
i) Melting points:
Z Tm, K
18 84
36 116
54 161
86 202
The dependence of boiling point on atomic number is almost linear. Linear extrapolation gives Tm(118) = 263 K = –10 °C.
ii) Boiling points:
Z Tb, K
18 87
36 120
54 165
86 211
On average, boiling points are 4 degrees higher than the corresponding melting points, hence we predict that Tb(118) = 267 K = –6 °C.
iii) Covalent atomic radii:
Z r, nm
18 0.097
36 0.110
54 0.130
86 0.145
Linear extrapolation gives: r(118) = 0.171 nm.
iv) Ionization energies:
Z IE, eV
18 15.8
36 14.0
54 12.1
86 10.7
Ionization energy is a non-linear function of atomic number. Linearization in coordinates ln Z – IE gives for Z = 118 the ionization energy IE = 9.7 eV.
Compare these data with the values predicted for the 118th element by American chemists 40 years ago: tm = –15 °C, tb = –10 °C, r = 0.23 nm, I = 9.8 eV. Of course, these results obtained by extrapolation are approximate. Moreover, bulk properties such as melting and boiling points can be measured only for significant amounts of an element, whereas only three atoms of the 118-th element were obtained and they decayed during milliseconds. For this reason, our predictions may hardly be confirmed in future.
v) The highest oxidation state for the 118-th element is VIII, and the corresponding oxide should be RO4 as for xenon (for radon neither oxide, nor any other compounds have been obtained).