Topic 7 Questions on Atomic Weight
Recall that Avogadro came up with his Avogadro hypothesis in 1811: equal volume of gases contain an equal amount of particles (regardless of what kind of particles they may be - be it hydrogen, oxygen, or nitrogen).
Figure 7.1: Avogadro’s Speculated Diatomic Molecules
Avogadro also speculated at the existence of diatomic molecules!
7.1 On Diatomic Molecules
"One molecule of hydrogen gas, in being combined with one molecule of chlorine, gives 2 molecules of hydrochloric acid. In order for the combination to occur and for the compound molecules to be separated by the same distance as that in the component gases, it is necessary and sufficient for each component molecule to be divided in two…
Thus the gases hydrogen, chlorine, and hydrochloric are diatomic…"
– Marc Antoine Auguste Gaudin
French chemist Marc Antoine Auguste Gaudin (1804 - 1880) was the first to explore Avogadro’s hypothesis:
Figure 7.2: Gaudin’s Exploration of Avogadro’s Hypothesis
Furthermore, Gaudin was also among the first to make distinctions between atoms and molecules: according to him, “an atom will be for us a small, spheroidal, homogeneous substance, or essentially indivisible material point, while a molecule will be an isolated group of atoms, in any number and of any nature”.
7.1.1 Two volume formulae
After doing experiments on Avogadro’s hypothesis, Laurent - 1846 - wrote:
“Each molecule of an element is divisible at least into two parts which we will call atoms; these molecules can be divided only in the case of combinations….. My molecule represents the smallest quantity of a body which must be used to effect a combination, a quantity which is divided into two by the act of combination. Thus Cl may enter into a combination, but to do this Cl2 must be used…”
– Laurent 1846
Although Laurent’s statement wasn’t always true (i.e., not all elements exist as dimers), his two-volume formulae hints at the existence of H2, O2, Cl2, N2, and so on. Diatomic molecules and their molecular symbols were slowly accepted.
However, there was still no standardization in Chemistry during Laurent’s time. Some German chemists still believed that water was OH and not H2O and among some of them, thought that different atomic weights and molecular / atomic symbols should be used for organic and inorganic chemistry.
7.1.2 Karlsruhe conference
“Shall a difference be made between the expressions ‘molecule’ and ‘atom’ such that a molecule be named the smallest particle of bodies which can enter into chemical reactions … atoms being the smallest particles of those bodies which are contained in molecules?”
– A question during the Karlsruhe conference
In September 3 - 5, 1860, August Kekulé presided over the first international congress of chemists in Karlsruhe, Germany.
Among some of the members of the conference were 140 scientists from all over Europe (even from Mexico), including Mendeleev, Bunsen, Liebig, Erlenmeyer, and Meyer.
7.1.2.1 Stanislao Cannizaro
Figure 7.3: Black and White Photograph of Stanislao Cannizaro
Stanislao Cannizaro was an Italian chemist who was crucial in the turning point of the acceptance of Avogadro’s hypothesis.
Figure 7.4: The Cannizaro Reaction
Cannizaro has an organic reaction named after him!
7.2 Birth of the Periodic Table
For a long time, it was known that elements had similar properties with one another - for instance:
- Bromine could be extracted along with chlorine and iodine from seawater.
- Gold, silver, and copper could be found in their natural states (i.e., ores) since ancient times.
- Potassium and sodium could be isolated via Davy’s electrolysis method
- Silicon and carbon and oxygen and sulfur had similar valancies
- Selenium was often mistaken for tellurium.
7.2.1 Atomic weights and properties
Figure 7.5: Artist Impression of Johann Wolfgang Döbereiner
Döbereiner (1780 - 1849) was the first to note that there is a relationship between the atomic weights and the chemical properties of an element.
..that perhaps the atomic weight of bromine might be the arithmetical mean of the atomic weights of chlorine and iodine. This mean is (35.470+126.470)/2 = 80.470 [sic– L&K]. This number is not much greater than that found by Berzelius (78.383); ….. This idea was the motive for an attempt which I made twelve years ago to group substances by their analogies. At that time I found that the specific gravity and atomic weight of strontia are very close to the arithmetic mean of the specific gravities and atomic weights of lime and baryta, since [356.019(=Ca.)+956.880(=Ba.)]/2 = 656.449(=Sr.)and the actual value for strontia is 647.285 In the alkali group, according to this view, soda stands in the middle, since if we take the value for the atomic weight of lithia, determined by Gmelin, = 195.310, and the value for potash = 589.916, then the arithmetic mean of these numbers, (195.310+589.916)/2 = 392.613,which comes very close to the atomic value for soda, which Berzelius determined as = 390.897. …. …specific gravity of selenium is exactly the arithmetic mean of the specific gravities of sulfur and tellurium, and all three substances combine with hydrogen to form characteristic hydrogen acids, then selenium forms the middle member, since [32.239(=S) + 129.243(=Te)]/2 = 80.741and the empirically found atomic value for selenium is 79.263
– Annalen der Physik und Chemie 15, 301-7 (1829)
Figure 7.6: Triads of Elements with Trends in Atomic Weight
He noted that there were three sets of chemical elements that had similar chemical properties. He also noted that the atomic weight of the 2nd member of the “triad” was almost equal to the mean of the averages of the first and the third member.
7.2.2 Newlands’ law of octaves
Ever since Cannizaro’s interpretation of atomic weights became accepted in the 1860s, atomic weights became more consistent with quantitative analyses (i.e., nobody would debate if oxygen’s atomic weight was 16 or 8).
Figure 7.7: A Photograph of Newlands
However, John Alexander Reina Newlands (1837 - 1898) noticed that if the atoms of elements were ordered in terms of increasing weight, then every eighth element of the sequence would be similar to the first element of the group - hence his law of octaves.
Figure 7.8: A Table Demonstrating the Law of Octaves
Newlands wrote the following about his discovered law:
It will also be seen that the numbers of analogous elements generally differ either by 7 or by some multiple of seven; in other words, members of the same group stand to each other in the same relation as the extremities of one or more octaves in music. Thus, in the nitrogen group, between nitrogen and phosphorus there are 7 elements; between phosphorus and arsenic, 14; between arsenic and antimony, 14; and lastly, between antimony and bismuth, 14 also.
This peculiar relationship I propose to provisionally term the “Law of Octaves.”
– On the Law of Octaves, John A. R. Newlands, Chemical News 12, 83 (Aug. 18, 1865)
However, his proposed law was subjected to criticism.
7.2.2.1 Criticisms of the law
Professor G. C. FOSTER humorously inquired of Mr. Newlands whether he had ever examined the elements according to the order of their initial letters? For he believed that any arrangement would present occasional coincidences, but he condemned one which placed so far apart manganese and chromium, or iron from nickel and cobalt."
– Chemical News 13, 113 (March 9, 1866). PROCEEDINGS OF SOCIETIES. CHEMICAL SOCIETY. Thursday, March 1.
In other words, Foster thought that the pattern Newlands found was merely a coincidence and that a similar pattern would also emerge via coincidence in some other manner.
“Dr. GLADSTONE made objection on the score of its having been assumed that no elements remain to be discovered. The last few years had brought forth thallium, indium, caesium, and rubidium, and now the finding of one more would throw out the whole system.The speaker believed there was as close an analogy subsisting between the metals named in the last vertical column as in any of the elements standing on the same horizontal line”
– Ditto.
Gladstone thought that for the law to work, that it would assume that all elements have been discovered (hence, a newly-found element would render the law false).
7.2.3 Russia during the 1850s to the 1870s
Figure 7.9: Notable Events During Russia from 1850 - 1870
7.2.3.1 Birth of the periodic table
Figure 7.10: A Photograph of Dimitri Mendeleev
Dimitri Mendeleev (1834 - 1907) was a Russian who was also the author of Osovy Khimi: Principles of Chemistry.
While writing the aforementioned, Mendeleev was also thinking about how to introduce his students to the elements in some orderly form.
Hence, guided by atomic weights, valency, and other properties, he was able to arrange the elements in table form.
"…In conclusion, I consider it advisable to recapitulate the results of the above work…
Elements arranged according to the size of their atomic weights show clear periodic properties…
We should still expect to discover many unknown simple bodies; for example those similar to Al and Si, atomic weights of 65 to 75."
– Mendeleev during a meeting of the Russian Chemical Society in March 1869
7.2.3.2 Mendeleev as a prophet in Chemistry
Figure 7.11: Undiscovered Elements that Mendeleev Commented On
In 1871, Mendeleev provided detailed explanations about three undiscovered elements. He called them eka6-aluminum, eka-boron, and eka-silicon.
As seen in the above graphic, his explanations included predictions on atomic weights, densities, chemical properties, valencies, and other physical properties.
7.2.3.2.1 Predictions compared to reality
In 1875, the French chemist Paul Emile Lecoq de Boisbaudran discovered a new element and called it Gallium (Gallia meaning Gaul: a Roman region in present-day France).
Lecoq did not connect his discovery with Mendeleev’s predictions - he originally measured Gallium’s density to be about 4.7 g / cm3. Mendeleev then asked Lecoq to re-measure the element’s density and got 5.96 g / cm3: almost exactly as predicted.
Figure 7.12: Other Discovered Elements
As seen above, Mendeleev also made correct predictions about other elements too. However, he was clueless about the nobel gases (e.g., Helium, Neon, Argon, and Krypton).
7.2.4 Lothar Meyer
Figure 7.13: A Sketch of Meyer
Lothar Meyer (1830 - 1895) was a German chemist who had been working on ordering the elements since the early 1860s. He published his first table in 1864; his 1870 version of the table was very similar to Mendeleev’s.
“…the table gives us the conception that the properties of elements are in the great part periodic functions of the atomic weight…”
– Lothar Meyer
In 1882, Meyer shared the Davy medal with Mendeleev for “their discovery of the periodic relations of their atomic weights”.
7.2.4.1 Dispute over the first periodic table of elements
Apart from the Russians, nobody read Russian during the 19th century. So, Meyer did not read Mendeleev’s work (written in Russian). However, Mendeleev did publish a one-page abstract in an obscure German journal.
Mendeleev’s name was eventually more associated with the periodic table for several reasons:
- Russia became a superpower after the two world wars.
- Mendeleev outlived in Meyer (and hence, had the final say in things).
- Mendeleev had made earlier preictions about undiscovered elements.
7.2.4.2 John Newlands’ consolation prize
Newlands was not very happy when he found out that Mendeleev and Meyer shared the Nobel prize in 1882.
So, Newlands republished all of his major works in the Royal Society of Chemistry and in 1887, got awarded the Davy medal.
Sadly, nobody really remembers John Newlands or Lothar Meyer nowadays.
7.2.5 Spectroscopy
In 1860, Bunsen and Kirchhoff used the spectroscope to find corresponding lines between the emission spectra of atoms and the absorption of the sun.
Figure 7.14: Bunsen and Kirchhoff’s Discovered Lines
Kirchoff coined the term blackbody radiation shortly therafter; he also used the spectra shown above to identify the elements of the sun (and became blind in one eye in the process).
7.3 Noble Gases
7.3.1 Discovering helium
In August 18th, 1868, French astronomer Jules Janssen found a new, undiscovered line during a total solar eclipse in Guntur, India.
Figure 7.15: Spectrum of Helium
During the same year, British astronomer Joseph Norman Lockyer also found a similar discovery.
Nonetheless, Locket named the newly-discovered element Helium after the Greek word Helios (meaning “Sun”)
7.3.2 Discovering Argon
Figure 7.16: A Photograph of John William Strutt (i.e., Lord Rayleigh)
In 1892, Lord Rayleigh wrote the following in the Nature science magazine:
I am much puzzled by some recent results as to the density of nitrogen, and shall be obliged if any of your chemical readers can offer suggestions as to the cause. According to two methods of preparation I obtain quite distinct values. The relative difference, amounting to about 1/1000 part, is small in itself; but it lies entirely outside the errors of experiment, and can only be attributed to a variation in the character of the gas.
– Lord Rayleigh
The two different methods that Rayleigh was referring to are:
- Obtaining nitrogen from atmospheric air by removing oxygen, carbon dioxide, and water vapor.
- Obtaining nitrogen from ammonia (i.e., NH3) via a chemical reaction.
Figure 7.17: Photograph of William Ramsay
William Ramsay (1852 - 1916) was a Scottish chemist who was doing research on nitrogen. He said the following in response to Lord Rayleigh’s earlier statement in Nature:
[Rayleigh] thought that the cause of the discrepancy was a light gas in non-atmospheric nitrogen; I thought that the cause was a heavy gas in atmospheric nitrogen. He spent the summer in looking for the light gas. I spent July in hunting for the heavy one…. I have succeeded…
– Ramsay
From this, Ramsay concluded that there was some contaminant in the nitrogen that was prepared from atmospheric air. Not too long later, Ramsay collaborated with Rayleigh to explore the chemical properties of the “contaminant”.
Rayleigh noted that the emission spectra of the gas did not appear to belong to the spectrum of any known gas.
And despite Ramsay’s numerous attempts to do so, the gas did not react with hydrogen, sodium, and other elements. On May 24, 1894, Ramsay wrote the following to Rayleigh:
“Has it occurred to you that there is room for gaseous elements at the end of the first column of the periodic table? Thus: Li, Be, B, C, N, O, F, X, X, X etc. Such elements should have a density 20 or whereabouts, and 0.8 per cent (1/120th about) of the nitrogen of the air could raise so the density of nitrogen that it would stand to pure nitrogen in the ratio 230:231”
– Ramsay to Rayleigh in writing
7.3.2.1 Problems with the discovery of argon
Ramsay measured monotonic argon to have an atomic weight about 40 grams per mole. However, the question still remains - where on the periodic table should it be placed? Why is the gas’ atomic weight so close to sodium’s and potassium’s?
But more importantly, was argon actually diatomic or even an allotrope of nitrogen?
Rayleigh was tired of chemists doubting their own conclusions, so he went back to do his Physics.
7.3.3 Finding other noble gases
In 1895, Ramsay found a mineral called “cleveite” and measure its atomic weight to be about 4 grams per mole.
Figure 7.18: Cleveite Ore on Display
With the help of his assistant Morris Travers (1872 - 1961), he identified the noble gas neon in 1898 and krypton and xenon in 1903.
In 1903, the noble gas Frederick Soddy isolated radon.
In 1904, Ramsay won the Nobel prize for Chemistry for his “discovery of the inert gaseous elements in air, and his determination of their place in the periodic system.”
Lord Rayleigh also won the Nobel prize in Physics for his “investigations of the densities of the most important gases and for his discovery of argon in connection with these studies”.
7.4 Radioactivity and Transmutation
Radiation was found in January of 1896. However, X-rays were already discovered then.
Figure 7.19: A Photograph of Antoine Henri Becquerel
French professor (at the University of Paris) was studying X-ray-like radiation emitted by potassium uranyl sulfate (i.e., K2UO2(SO4)) crystals. He assumed that the aforementioned was excited by sunlight. After leaving the crystals near a window to absorb sunlight, he then exposed the crystals to photographic plates that had been wrapped in thick paper.
During a couple of non-sunny days, he then placed the experiment in a drawer - he found that the uranium in the crystals would emit radiation (which was shown on a photo plate)
Figure 7.20: Photographic Plates with Radiation
Unfortunately, there wasn’t much interest in this until Marie Curie came along
7.4.1 Marie Curie
Figure 7.21: A Photograph of Marie Curie
Marie Skłodowska Curie (1867 - 1934) was the first woman to obtain a PhD in science in 1908 and also the first woman to become a Physics professor at the University of Paris.
She was also the first scientist to obtain two Nobel prizes.
7.4.1.1 Pierre and Marie Curie working together
Both scientists carefully measured the amount of radiation that arose from the uranium crystals and concluded that the amount of radiation evolved is independent of chemical composition, but rather, dependent on the quantity of uranium.
In 1898, both scientists examined some samples of uranium ore - Pitchblende. They found that pitchblende was four times more radioactive than uranium itself. Hence, pitchblende must also contain some other element apart from uranium.
7.4.2 Elements from pitchblende
Several elements were isolated from the uranium-based ore.
In 1898, the element radium was extracted. Radium was tough to isolate one ton of pitchblende yielded 0.1 grams of radium. However, radium was a huge impact during the first international Physics conference in Pairs in 1900.
Radium was found to damage tissues and for this reason, was used as a treatment against cancer (i.e., chemotherapy).
Marie Curie did not choose to get a patent for this - she belived that science should not be used for monetary gains.
7.4.3 U-rays
Figure 7.22: A Photograph of Ernest Rutherford
Ernest Rutherford (1871 - 1731) conducted experiments to show that the “U-rays” emitted by uranium were composed of two different particles: \(\alpha\) and \(\beta\) particles.
While Curie had already shown that \(\beta\) particles are electrons (in 1899), Rutherford showed that \(\alpha\) particles are in fact hydrogen (in 1906).
In 1900, French scientist Paul Villard showed that there was a third kind of radiation in U-rays: \(\gamma\) rays (which behaved similarly to x-rays).
7.4.3.1 What happens during radioactivity?
Figure 7.23: Radioactive Decay of Thorium
Rutherford dug deeper into this, helping to discover radon in the process.
From 1899 onwards, Rutherford experimented around with thorium more to find out what happens as the element underwent radioactive decay.
eka is Sanskrit for “one”↩︎