The Development of the Periodic Table

Newlands and De Chancourtois provided the groundwork which Mendeleev improved upon in 1869. He realised that Newlands anomalies were due to undiscovered elements, and re-drew the table leaving gaps. Mendeleev was able to predict properties of the elements which would fit these gaps. 1

                                                                    (all photos on this page – from source 8)

Mendeleev’s Table

 Source 7

 

Source 1

The properties of Gallium are very similar to Mendeleev’s eka-Aluminium, and so prove his ideas to be accurate.  

Further proof came when Germanium was discovered. Mendeleev also made specific predictions about eka-silicium, which closely match Germanium.  

Source 1

Source 9

Gallium

Gallium was discovered in 1875 by Paul Lecoq de Boisbaudran, using atomic spectroscopy, when he noticed a faint violet line at 416 nm on the spectrum of zinc sulphide ore.  1

In the modern periodic table, it is situated in period 3 and group 3, which denotes that Gallium is a metal. It has several unusual properties, some usually attributed to non-metals. 1

For a metal, Gallium has a fairly low melting point, 29.78 oC, meaning it is liquid at room temperature in hot countries. This is strange when compared to elements surrounding Gallium. The only explanation for this anomaly is that in liquid Gallium there are G2 molecules. 1

        Gallium                                   29.78 oC                                   2403 oC

Source 10

The boiling point of Gallium is high, as expected from a metal, but because of the low melting point, gallium has the largest liquid range of all elements – 2372.22  oC. 1 Another unusual property of Gallium is that it is denser as a solid than as a liquid, consequently the top will solidify before the bottom. The explanation for this is that the structure of solid Gallium is more open, and so the atoms cannot pack as close together. 1

Source 1

Dissolving Gallium in Acid 1

2Ga (s) + 6H+ (aq)                       2Ga3+ (aq) + 3H2(g)

Dissolving Gallium in Alkali 1

2Ga (s) + 2OH- (aq) + 6H2O (l)                    2[Ga(OH)4]- (aq) + 3H2(g)

Both of these reactions evolve hydrogen, and show that Gallium has amphoteric hydroxides, an unusual property. 1

Gallium’s final unusual property is its bonding with chlorine. It would be expected to bond ionically with Ga3+ ions. However, with chlorine, the Gallium atom covalently and datively bonds with four chlorines, two of which are bonded to another Gallium. The formula of this molecule is Ga2Cl6, with empirical formula GaCl2 or GaCl.GaCl3 1

Atomic Spectroscopy and the UNILAC Accelerator

There are two types of atomic spectroscopy, absorption and emission. Both types are based around the equation                                        E = hv

Emission Spectroscopy

1. The atoms in a sample of an element are excited, using an ICP (inductively coupled plasma) source. 11 
2. An electron is promoted to a higher, and more unstable energy state
3. The electron returns to its ground state
4. The light emitted by the excited electrons is passed  through a prism 3 and 4

Source 12

Absorption Spectroscopy

1.  The atoms are excited and promoted as before
2.  The light emitted from the cooler sample (not the excited atoms) is passed through a prism. 3 and 4

           Source 13 and 16

The lines and gaps match up in these emission and absorption spectrums of hydrogen.

The energy required to promote an electron is quantised, and therefore a fixed amount of energy is absorbed and emitted. Using the equation the frequency of the radiation emitted and absorbed can be calculated. 1

Spectroscopy is responsible for the discovery of new elements, such as Gallium. Each element’s atoms have unique spectrums, so scientists were able to examine samples lines at anomalous frequencies which could indicate a new element. 1

Pioneered by Neils Bohr, 3 spectroscopy can also support quantum and electronic structure theories. The movement of electrons when they have absorbed a specific amount of energy shows that they must exist in orbitals and energy levels. Spectroscopy has furthered knowledge on the movement of atomic particles, and the internal arrangement of atoms. 2

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Source 15

Only 92 elements occur naturally, any heavier ones have to be artificially created. The UNILAC accelerator is a new development, required because the heavier an element is, the more difficult it is to synthesise. 2

The UNILAC accelerator fires a beam of heavy metals ions at a rotating target disc of a heavy, stable element. This eventually overcomes the repulsive forces of the nucleus, and produces radioactive nuclei which are separated using electromagnetic fields. 18

The main aim of the UNILAC project is to synthesis a stable isotope of Element 114. Scientists at GSI in Darmstadt believe that an isotope of it could be stable enough for them to examine, unlike other synthesised elements with very short half-lives. 2 UNILAC’s successes has been in creating elements 107 to 112

24Cr + 83Bi              107Bh

30Zn + 82Pb              112Uub

Source 2

The Work of Chemists

This was originally to discover new elements, name them, and to explain the similarities between small groups of them. This extended to explaining the patterns found between all elements. 1

Mendeleev explained patterns between existing elements and used them to predict unknown elements. Chemists then began using spectroscopy to discover them. Between the period 1829 and 1940, around 45 elements were discovered. 6

Once all 92 naturally occurring elements had been found, chemists realised that they could extend the number of elements, synthesising increasingly heavy and unstable atoms. The aim for chemists now is to create a heavy atom which is stable enough for in-depth study.  1 and 18