Topic 5 Atomic Theory
Figure 5.1: A Bust of Democritus
If one recalls from the first topic of this website, the four Greek philosophers Abdera, Thrace, Leucippus, and Democritus stated that everything in the universe were made up of atomos: fundamental, indivisible entities that are eternal and unchanging (notwithstanding their motions).
Long after, two scientists: Issac Newton and Robert Boyle proposed that matter in the universe was made up of corpuscles: untouchable, discrete, and invisible units. However, such an idea was still far too general (i.e., even though the general idea was there, it did not bother to differentiate between molecules and atoms and for that reason, wasn’t very helpful).
“All that can be said upon the number and nature of elements is, in my opinion, confined to discussions entirely of a metaphysical nature. The subject only furnishes us with indefinite problems, which may be solved in a thousand different ways, not one of which, in all probability, is consistent with nature. I shall therefore only add upon this subject, that if, by the term elements, we mean to express those simple and indivisible atoms of which matter is composed, it is extremely probable we know nothing at all about them; but, if we apply the term elements, or principles of bodies, to express our idea of the last point which analysis is capable of reaching, we must admit, as elements, all the substances into which we are capable, by any means, to reduce bodies by decomposition. Not that we are entitled to affirm, that these substances we consider as simple may not be compounded of two, or even of a greater number of principles; but, since these principles cannot be separated, or rather since we have not hitherto discovered the means of separating them, they act with regard to us as simple substances, and we ought never to suppose them compounded until experiment and observation has proved them to be so.”
– Lavoisier in page XXV of his magnum opus Traité Élémentaire de Chimie
Lavoisier - on the contrary - felt that it was pointless trying to speculate the particles that matter is made up of. He thought that what we knew about elements and we could decompose (using the elements) was what we could analyze.
5.1 John Dalton
Figure 5.2: Artist Impression of John Dalton
Dalton (1766 - 1814) was an English scientist who started out as a meteorologist. He lived in Manchester during the British industrial revolution and made a living as an industrial consultant and as a private teacher (one of his students was James Prescott Joule).
Nonetheless, Dalton thought that atoms were not some fictional or metaphysical entity like what Lavoisier and others thought. In other terms, Dalton thought that there were connections between atoms (i.e., the hypothetical) and experiments (i.e., reality).
5.1.1 Dalton on atomic theory
Figure 5.3: Dalton’s Atomic Theory
In Dalton’s mind, all matter was composed of solid, indestructible atoms that re-arrange during chemical reactions.
“No new creation or destruction of matter is within the reach of chemical agency. We might as well attempt to introduce a new planet into the solar system, or to annihilate one already in existence, as to create or destroy a particle of hydrogen”
– John Dalton, Chapter III of Atomic Theory4
Furthermore, Dalton proposed that each element had its own kind of atom. Hence, all atoms in an element are identical, and other atoms of other elements have different weights and other properties.
Dalton’s law of multiple proportions state that atoms of different elements combine in whole-number ratios.
5.1.2 Atom hypothesis
Figure 5.4: Proponents of the Atom Hypothesis
Dalton’s law of multiple proportions state that atoms of different elements combine in whole-number ratios.
Interestingly, Dalton went a step further (which turned out to be wrong): suppose that atoms A and B formed a binary system AB (e.g., O and H should have formed OH-, but we know that OH- doesn’t exist by itself).
\[\begin{equation*} \text{Hydrogen + Oxygen} \rightarrow \text{Water} \end{equation*}\]
In the past, it was thought that 87.4 parts of oxygen combined with 12.6 parts of hydrogen. From this, Dalton hypothesized that if one atom of hydrogen combined with one molecule of oxygen to yield one molecule of water, then oxygen’s atomic weight5 to hydrogen’s atomic weight would be 87.4 : 12.6 or 7.
However, we now know that 2 atoms of hydrogen and one atom of hydrogen yields one water molecule. Furthermore, the oxygen : hydrogen weight ratio is about 8 : 1.
5.1.2.1 Dalton’s law of partial pressures
Figure 5.5: Dalton’s Mention of his Atomic Theory in Writing
Dalton was a meteorologist by trade - he was concerned with the atmosphere, so he wondered if different gases in the atmosphere were in a mixture or if they were compounds in their own rights?
His thoughts would eventually lead him to the invention of the law of partial pressures: each gas exerts its own pressure as if other gases do not exist (i.e., their partial pressures). The total pressure is then the sum of all these partial pressures.
Dalton’s (mostly incorrect) attempt to explain gases were as follows:
Figure 5.6: Dalton’s Original Explanation of Why Gases Repel One Another
- Like atoms repel each other to give rise to partial pressures (though why is it that unlike atoms do not repel each other).
- Different kinds of atoms have a different effective size. Dalton also thought that atoms were surrounded by an atmosphere of caloric; gas contains a larger atmosphere of heat than solid state as a result. Furthermore, Dalton also thought that atoms of different sizes would ignore one another and not repel.
From the above idea, Dalton then believed that different atoms have different masses.
5.1.2.2 Dalton’s atomic symbols
Figure 5.7: Dalton’s Original Elements and Atoms
Dalton thought that there were initially two kinds of elements: simple and compound.
The simple elements were as such:
- Hydrogen
- Oxygen
- Nitrogen (known as Azote back in Dalton’s times)
- Carbon
- Sulfur
The compound elements had few points worth noting:
- His geometric interpretation of compound elements implied that Dalton had a rough idea of what these molecules would look like.
- His interpretations was an innovation to present abstract, theoretical atoms and abbreviations. In some sense, Dalton’s drawings were a precursor of our modern-day chemical symbols.
5.2 Davy and Berzelius
Figure 5.8: Napolean and His Final Defeat in the Battle of Waterloo
Between 1815 and 1914, there weren’t anymore major, protracted wars in Continental Europe.
Figure 5.9: Exile of Naoplean and the Congress of Viena
Industrialization accelerated the growth of the world and colonization also happened.
5.2.1 Industrial revolution during the 1760s to the 1850s
Figure 5.10: Manchester During the 1850s
During John Dalton’s days, Manchester was in the early stages of an industrial revolution that began in the UK.
Several notable events happened during that time frame:
James Watt’s invention of the steam engine
Figure 5.11: James Watt at his Bench
This happened in 1781 - the engine used coal to empower machineries, hence resulting in higher productivities throughout the nation.
Increased textile production
This was a result of the invention of the steam engine.
Increased iron production
Iron making was much more efficient with English coal.
The above events all propelled England (and eventually, North America) to the leading powers of the world.
5.2.2 Voltaic pile
Figure 5.12: A Voltaic Pile on Display
The above device as invented by Alessandro Volta in 1800 - the device was made up of alternating slices of zinc and silver.
Figure 5.13: Electrolysis of Water to Yield Hydrogen and Oxygen Gas
Immediately after the invention of the pile, it was discovered that water can be split into hydrogen and oxygen at the anodes and the cathodes respectively.
5.2.3 Humphry Davy
Figure 5.14: Two Artists’ Rendition of Humphry Davy
Davy (1778 - 1829) was an English inventor, poet, and the president of the Royal Society.
Davy’s chemical demonstrations were often public and attracted a lot of audiences - Davy once almost died from CO poisoning and was addicted to laughing gas (i.e., NO gas).
5.2.3.1 Davy’s idea of an element
Via experimentation from Volta’s pile, Davy speculated at the possibility of chemical affinity: the idea that electrical attractions are responsible for different substances being attracted to one another to form compounds. In 1806, Davy wrote the following:
“Is not what has been called chemical affinity merely the union or coalescence of particles in naturally opposite states. And are not chemical attractions of particles and electrical attractions of masses owing to one property and governed by one simple law?”
– Humphry Davy in writing
Conversely, the voltaic pile showed that compounds can be “detached” using electricity - like H2O being disassociated into H2 gas and O2 gas.
For this reason, Davy also thought that one could decompose some of Lavoisier’s “elements” to even simpler substances. Because of this, Lavoisier also used stronger batteries for his experiments to attempt to decompose substances.
5.2.3.1.1 Davy’s experiment
Figure 5.15: Davy’s Experiments
In 1807, Davy used galvanic fluid (i.e., electricity) to decompose soda and caustic potash to sodium and potassium respectively.
The biggest batteries that Davy used had about 2500 metal plates; Davy also discovered the elements Barium, Strontium, Calcium, and Magnesium.
5.2.3.2 Do all acids contain oxygen?
In the past, Lavoisier had suggested that all acids contained oxygen, hence the root name oxygen. For that reason, the belief used to be such that muriatic acid (what we call HCl in modern chemistry) should also contain oxygen.
Figure 5.16: A Vial of Oxymuratic Gas (i.e., Chlorine Gas)
In 1810, Davy tried to burn oxymruriatic gas (see above image) using carbon to yield carbon oxide, but to no avail. So, he says:
“One of the singular facts I have observed on this subject, and which I have before referred to, is, that charcoal, even when ignited to whiteness in oxymuriatic or muriatic acid gases, by the Voltaic battery, effects no change in them; if it has been previously freed from hydrogen and moisture by intense ignition in vacuo. This experiment, which I have several times repeated, led me to doubt the existence of oxygen in that substance.”
– Humphry Davy
Hence, Davy believed that this so-called “oxymuriatic gas” was not a compound, but an element; he then decided to name this gas chlorine (i.e., Cl2). The new name for muriatic acid - hydrochloric acid - was then used soon after. Davy also discovered iodine and fluorine shortly after (important for the development of acid-base theory).
5.2.4 Jöns Jacob Berzelius
Figure 5.17: A Black-and-White Image of a Painting of Berzelius
Berzelius (1779 - 1848) was a Swedish chemist and Davy’s rival. He was an excellent experimentalist (even more so than Davy).
He performed the following feats:
- Discovering thorium, selenium, and cerium - in his laboratory, his also found lithium and vanadium.
- Characterizing numerous salts.
- Finding accurate atomic weights for more than 50 elements.
- Developing the idea of dualism in electrochemistry.
- Came up with the following terms: catalyst, polymer, isomer, and allotrope.
- Wrote textbooks that were highly influential.
Figure 5.18: Berzelius’ Chemical Notations in Writing
Berzelius’ chemical notation is also the basis of our modern day chemical notations.
5.2.4.1 Combustion analysis
Figure 5.19: Berzelius’ Sketch of his Combustion Analysis
The careful, yet quantitative nature of Berzelius’ experiments allowed him to write down formulas for the composition of many substances - for instance, CuSO4 to be one part copper and one part sulfate (i.e., sulfur as it was called at the time).
5.2.4.2 Electrochemical dualism
Berzelius played around a lot Alessandro Volta’s voltaic pile. However, he wasn’t interested in finding new element, but rather understanding the nature of compounds’ constitutents.
He found that every chemical compound had a positive and a negative constituent that were combined because of their electrochemical affinities. When a chemical reaction happens, their positive / negative pairs become re-arranged:
Figure 5.20: Berzelius’ Idea of Chemical Reactions
Obviously, this worked very well for ionic compounds (e.g., salts) - these are compounds that are especially prevalent in inorganic chemistry: a field of chemistry that dominated chemical research in the 1800s - 1850s.
However, Berzelius’ idea cannot be applied to organic compounds - a field that mostly contains organic chemistry.
5.2.4.3 Other contributions of Berzelius
Figure 5.21: Berzelius’ Ordering of Elements and his Essay
Berzelius also ordered 49 elements in order of their electronegativities; he also wrote an essay titled “Essay on the theory of chemical proportions and on the influence of electrochemical” in 1818: a comprehensive list of the properties of molecules and atoms.
Figure 5.22: Berzelius’ Table of Atomic Weights
Berzelius also created a table of atomic and molecular weights for all the elements of his day.
5.3 Atomic and Molecular Proportionality
At the time, many atomic weights were very close to integers - was there a reason?
Dalton also assumed water to be OH, but many other chemists (e.g., Berzelius) thought that it was H2O. Why was this?
While we know that natural oxygen, hydrogen, and nitrogen are diatomic gases (i.e., these gases exist as O2, H2, and N2 respectively), what did the above people know then?
5.3.1 William Prout
Figure 5.23: Painting of William Prout
Prout (1786 - 1850) was an English chemist and a physician. By the early 1800s, about 50 elements are known.
Scientists like Humphry Davy believed that the universe could not be so complicated to the point that it had 50 different elements
Nonetheless, Prout’s experiments were based on various others’ experiments
5.3.1.1 What did Prout’s hypothesis sound like?
Prout stated that heavier elements are made up of multiple units of hydrogen; hence, heavier elements’ atomic weights were whole numbers of hydrogen’s weight.
However, the final understanding of the above only came around in the late 19th and the early 20th century.
Atoms are made up of protons and neutrons (and very light electrons).
The variations away from whole numbers are mainly due to the presence of isotopes (i.e., the same element with differences in neutron numbers).
For reference, the modern theory of elements has been provided in the above quoteblock.
5.3.2 Joseph Louis Gay-Lussac
Gay-Lussac (1778 - 1850) was a French physicist and a chemist. He discovered boron during electrolysis.
Figure 5.24: Painting of Gay-Lussac and His Discoveries
In 1804, Gay-Lussac took hot air balloon rides to over 7000m - an altitude that wasn’t broken in 50 years. He also found out that the content of air is the same as sea level.
5.3.2.1 Law of combining volumes
During Gay-Lussac’s time, it was already very well known that about two volumes of hydrogen reacted with one volume of oxygen to yield water.
Figure 5.25: Gay-Lussac’s Further Experimetns Involving Gas Reactions
Gay-Lussac not only confirmed the volumes of hydrogen and oxygen needed to yield water, but also performed further experiments (see above).
5.3.3 Avogadro and the Avogadro hypothesis
Figure 5.26: Artist’s Rendition of Avogadro
In 1811, Italian physicist Amedeo Avogadro stated the following that would eventually become known as the Avogadro hypothesis:
Equal volume of gases contain equal number of particles at a given temperature and a pressure.
For this reason, one can also state the following relationship between volume \(V\) and pressure \(P\):
\[\begin{equation*} V = p \end{equation*}\]
Hence, if one volume of oxygen reacts with two volumes of hydrogen to yield water, then two hydrogen particles react with one oxygen particle to yield two particles of water.
However, this had serious implications: if one particle of oxygen must split to yield two water particles, how is this possible? Atoms cannot be divided!
5.3.3.1 Avogadro’s solution
…It seems that a molecule composed of two or more elementary molecules should have its mass equal to the sum of the masses of these molecules; and that in particular, if in a compound one molecule of one substance unites with two or more molecules of another substance, the number of compound molecules should remain the same as the number of molecules of the first substance. Accordingly, on our hypothesis, when a gas combines with two or more times its volume of another gas, the resulting compound, if gaseous, must have a volume equal to that of the first of these gases. Now, in general, this is not actually the case. For instance, the volume of water in the gaseous state is, as M. Gay-Lussac has shown, twice as great as the volume of oxygen which enters into it, or, what comes to the same thing, equal to that of the hydrogen instead of being equal to that of the oxygen. But a means of explaining facts of this type in conformity with our hypothesis presents itself naturally enough; we suppose, namely, that the constituent molecules of any simple gas whatever are not formed of a solitary elementary molecule, but are made up of a certain number of these molecules united by attraction to form a single one; and further, that when molecules of another substance unite with the former to form a compound molecule, the integral molecule which should result splits itself into two or more parts (or integral molecules) composed of half, quarter, &c., the number of elementary molecules going to form the constituent molecule of the first substance, combined with half, quarter, &c., the number of constituent molecules of the second substance that ought to enter into combination with one constituent molecule of the first substance (or, what comes to the same thing, combined with a number equal to this last of half-molecules, quarter-molecules, &c., of the second substance); so that the number of integral molecules of the compound becomes double, quadruple, &c., what it would have been if there had been no splitting up, and exactly what is necessary to satisfy the volume of the resulting gas.
Thus, for example, the integral molecule of water will be composed of a half-molecule of oxygen with one molecule, or what is the same thing, two half-molecules of hydrogen."
– Avogadro himself (?)
However, there was a problem:
Figure 5.27: Gas Postulates Proposed by Avogadro
In order to explain experimental observations, diatomic gases (see above) had to be postulated first.
5.3.4 Politics
John Dalton heard about Avogadro’s - he did not like what he heard, even going as far as to say that Avogadro was wrong.
However, Berzelius accepted the following equation:
\[\begin{equation*} 2H + O \rightarrow H_2O \end{equation*}\]
Berzelius was also willing to forgo Dalton’s rule of binary simplicity. However, he did not believe that oxygen would form a dimer. He thought that compounds are formed between two unlike parts; an oxygen atom is unable to form diatomic molecules with another oxygen atom.