Tuesday, March 23, 2010

The indivisible atom

These atoms move in the infinite gap,
separated some of others and different between themselves
in figures, sizes, position and order;
after surprises some others they clash
and some of them are expelled
by means of shaken at random in any direction,
while others,
interlacing mutually in consonance
with the congruity of his figures, sizes,
positions and arrangings,
they stay close
and this way they cause the birth of the compound bodies.

Democrito – Simplicio, Of it fall down 242, 21


Atoms as model átomico of Dalton

The chemistry was already beginning having fundamental laws that were allowing to study much better the chemical reactions, the compounds and elements. But, the question that us still had not been solved was: the elements, these substances that could not decompose, of that they were formed? What was differing to the hydrogen of the sulfur? Or to the iron or of the oxygen?

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In the XIVth Century aC, Mosquito of Sidón, a thinker of Phoenician origin raised for the first time the thought atomista that then the preSocratic Greek philosophers Leucipo would recover, 1000 years later, (Vth century) and his beginner founding Demócrito de Abdera (470/460aC - 360/370aC) of the school atomista. This school was defending the concept that the whole matter is formed by a miscellany of unchangeable and eternal original elements, infinitely small, imperceptible for the senses and indivisible. This concept is born of the vision of the world that tapeworm Democrito, for which the reality splits into two elements or causes: What is (το ον), formed by atoms (of the Latin atomum, and this one of the Greek , that it means without parts) eternal and indivisible; and what is not (το μηον), represented by the gap. In spite of well directed that his ideas were going, it did not have any scientific foundation and I remain eclipsed by the theory aristotélica of four elements.Es known also, that in parallel in the India, the philosopher and alchemist Kanada (~600aC) was already representing the thought atomista and the school founded the philosophical school Vaisheshika at the end of the VIIth century, which although it had differences in some points of view, like the theological one, also many similarities had atomista of Demócrito, as for example, from the physical point of view.

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The centuries happened and, how we have seen in the chapter of the elements, the model aristotélico did not begin staggering up to good brought in the XVIIIth century thanks to the advances of Boyle, Lavoisier and Proust. At the beginning of 1800, it became fashionable in England to inhale nitrous and enclosed oxide there were done “parties of the gas of the laugh”, in that that volunteers were inhaling the gas to entertain the public with the staggerings. The curious thing of this, the fact is that they did not realize that the nitrous oxide, to part of being an "entertaining" drug, also it was possible to use anesthetic how until 1846. At the end of 1799, the young man Humphry Davy (1778 - 1829) deposited in the Real Institution of London how teacher of Chemistry and soon, became famous for discovering the potassium, the sodium, the magnesium, the calcium, the strontium and the aluminum, one behind other. The secret of his productivity was residing in a skill that it designed applying electricity to the substances and to which it called electrolysis. Unfortunately also his addiction went down in history to the gas of the laugh (3 or 4 meetings a day); and one believes that it was the cause of his death in 1829.

The chemistry had advanced, but the absence of scientific institutions, mass media and organization had complicated his development; and at the beginning of the XIXth, it was more for business men (you dye, carbones, …) than for scientists. In spite of this absence of communication, there was a series of personages being employed at her with more seriousness than Davy.

We recover Robert Boyle in this point, to do an incision in his studies on the gases. In 1659, with the help of Robert Hooke, it improved the vacuum pump of Otto von Guericke creating the machine Boyleana or pneumatic machine that allowed him to do multitude of experiments in the gap. The first one was to demonstrate the idea of Galilean of that, in the gap, a pen and a piece of lead they fall down at the same speed. It demonstrated also that the sound is not transmitted in the gap, but the most important discovery was that the volume occupied by a gas is inversely proportional to the pressure to which it is submitted and which if we stop exercising pressure, the gas recovers his original volume. This proportionality relation is known nowadays as the Boyle law and it led it to recovering the ideas of Demócrito on the atoms. Boyle came to the conclusion that the compressible air was formed by minuscule particles separated by empty space and that, on having applied pressure, we were reducing this space between particles and for it they were reducing the volume. All these ideas of fueron published in his work New Experiments PhysicoMechanical touching the spring of air and its effects physical-mechanical New experiments on the elasticity of the air and his effects – and I redeem an important role in the atomic conception of the matter.

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One century later, there appeared John Dalton (1766 - 1844), naturalist, chemist, mathematician, British meteorologist and teacher, between others, of James Prescott Joule (famous person for his studies on the magnetism and the conservation of energy). He was suffering a strange well-known illness as acromatopsia, the inability to distinguish the colors, and he studied it in depth going so far as to publish “extraordinary Facts relative to the Vision of Colors” in 1794. Further on, this illness was called a color blindness in his honor.

His special interest in the meteorology led him to realizing numerous remarks and measurements, topcoat, related to the ambience. It discovered that the rain was caused by a change of temperature and not of pressure, as he believed until now. Nevertheless, what more was fascinating age as the ambience had such a homogeneous appearance being a gas miscellany - a homogeneous miscellany of nitrogen, oxygen and water vapor between others, as we saw in the first chapter – of different thickness, that is to say, that a few gases were weighing more than others. Continuing in this line, it discovered that different samples of air, taken to different heights, were showing the same proportion of the every some of the gases that were composing it, when the logical thing was to think that the gases as the oxygen that were weighing less it would be floating over those who were weighing like the nitrogen, in the same way that the oil floats on the water. If Dalton could have obtained air samples to top heights, there would be account of which the composition if that changes. His interest in the topic and his numerous remarks they yielded his fruits: in 1803, there postulated the law of partial pressures that it was establishing that the pressure of a gas miscellany that they do not react between them, is equal to the sum of the partial pressures that each of the gases would exercise if they were occupying the whole space to constant temperature.

Dalton was accumulating a lot of experimental information and he tried to look for model capable of explaining them. Curiously, the first thing that was studying his information was that the elements could get together with different simple proportionality relations and that every combination was giving place to a different compound; and that it postulated like the law of multiple proportions.

Law of multiple proportions

When two elements get together, if we take a fixed quantity of one of the elements, changing the quantity of the second one we obtain different compounds and this quantity is proportional to that of the first element in a simple entire number.

If we combine the carbon with the oxygen with a proportion of masses of 3 of carbon and 8 of oxygen, we obtain the Dioxide of Carbon (CO2, the gas that we expel on having breathed); nevertheless, if we combine 3 of carbon with 4 of oxygen, we obtain the Carbon monoxide (CO, a poisonous gas).

In this moment, Dalton had to find a theory that was capable of explaining and of unifying 3 laws that were forming the base of the Chemistry of the epoch: the law of constant composition of Proust, the law of conservation of the mass of Lavoisier and his recent law of multiple proportions. But the search finished with the presentation atomic theory in 1808, a model who was explaining 3 laws and the phenomena observed in the gases; and from which we can emphasize the following assumptions:

Atomic theory of Dalton

The whole matter consists of indestructible and extremely small particles that called atoms. The chemical reactions imply atoms rearranging, neither are created atoms nor are destroyed.


The atoms of the same elements are similar in mass and other properties, but they are different from the atoms of other elements.


On having got together in compounds, the atoms support a few simple proportions; and different provide s of every type of atom they generate different compounds. The equal atoms of a certain compound sound in mass and other properties. The proportion of atoms of oxygen and of carbon in the carbon monoxide is of 1:1; in the carbon dioxide this relation is 2:1, two atoms of oxygen and one of carbon.

If we take the point one, we see that Dalton's theory proposes that during a chemical reaction, they neither the atoms nor create the mass is not even destroyed, therefore, there remains invariable and is ratified the law of conservation of mass of Lavoisier. If we join it with the second one, there is ratified the definition of element proposed by Boyle: an element is formed by the only type of atoms and but nevertheless, does not differentiate the molecules of the atoms.

Every element is formed by the same type of atoms that have a concrete mass (I) and when they form a compound, these support a proportion for every compound (III), therefore, a compound will always have the same relation of masses of his components verifying this way the law of definite proportions of Proust. Jöns Jacob Berzelius (1779 - 1848) verified with an experiment that 10g of Lead they always get together with 1,56g of Sulfur forming 11,56g of Sulfide of Lead; it realized numerous tests modifying the mass one of two elements, and it was always obtaining the same compound quantity: If it was combining 18g of Lead with 1,56g of Sulfur and kept on obtaining 11,56g of Sulfide of Lead and 8g of separate Lead; if it was combining 10g of Lead with more sulfur, for example 3g, and again it was obtaining 11,56 g of Sulfide of Lead and the rest of 1,44g of Sulfur. This experiment was verifying the law of constant composition proposed by Proust, but also it was verifying the model proposed by Dalton: If we suppose that the Lead atom weighs some 6 times more than that of Sulfur (10 / 1,56 = 6,41), we have that the Sulfide of Lead is composed by molecules formed by an atom of lead and an atom of sulfur, by the qu and if we add an excess of some of the two it will not form more compound, since it will not have in spite of whom getting together.


Today we know that the sulfur atom has an atomic mass of 32,065u and lead of 207,2u, for the calculation of which the lead atom was weighing 6,41 times more than the sulfur that we have come earlier with the Berzelius experiment was approaching very much the real value (207,2 / 32,065 = 6,461). In that moment, it was impossible to determine the real weight of the atoms because he was not getting ready of means, nevertheless if they could determine the relative weight based on the quantities of mass of an element that were getting together with a fixed mass of other. Dalton and Berzelius, determined the weight of many atoms using how unit the mass of an atom of hydrogen, which at that time one already knew that it was the lightest. Dalton prepared a table of relative masses of the atoms of each of the elements like part of his theory, and although most of the calculations was incorrect (like for example the oxygen, which according to his calculations was 7 times heavier than the hydrogen, when in fact it is 16 times heavier), it was one it managed bearing in mind the instruments which he had. On the other hand, in his book also they can be the graphic representation of the atoms some elements and the molecules of compounds (binary, ternary, …). The calculations were improving with the time, and in 1830 Berzelius published a table with the atomic masses of 54 elements that were approaching very much the one that we know today.

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Finally, the third assumption of Dalton's theory also explains the law of multiple proportions: If we consider the oxides (combination of an element with oxygen), we have that 1g of Carbon it gets together with 1,333g of Oxygen forming Carbon monoxide; but if we double the Oxygen quantity (2,666g) and supporting 1g of Carbon, the combination forms Dioxide of Carbon. At atomic level, we extract that an oxygen atom can get together with the carbon in a relation 1:1 (a carbon atom with one of oxygen) forming Carbon monoxide; but also it can get together in a relation 1:2 (a carbon atom with two of oxygen) forming Dioxide of Carbon.

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ááááá Louis Joseph Gay-Lussac (1778 - 1850) worked actively studying the behavior of the gases, his volumes and his temperatures. With his experiments, and the works of Jacques Charles that were relating the volume to the temperature; he published in 1802 that if we support to constant pressure an ideal gas, the volume and the temperature are related to a constant of direct proportionality and that it called Charles's Law. Further on, in 1805 it postulated the Law of Gay-Lussac: if we support the constant volume, the quotient between the pressure and the temperature stays constant. Together with the Boyle-Mariott law – to constant temperature, the volume of a gas is inversely proportional to the pressure, to constant temperature – they were establishing the beginning of the behavior of the ideal gases. In 1808, it demonstrated experimentally that, to equal pressure, a quantity of volume of oxygen was getting together completely with the double of volume of hydrogen forming steam it waters down; for example, two liters of hydrogen it combines with one liter of oxygen forming two liters of water vapor. Similar experiments with other gases, reached with the publication of the law of the volumes in combination that it was postulating that the gases react if getting together and obtaining volumes in proportion to small simple numbers.


Nevertheless, the experimental Gay-Lussac results were not agreeing with the atomic model of Dalton. For the last one, the gaseous elements were necessary simple and constituted by only one atom; and the current compounds like the water, they were formed only by two different atoms. Therefore, the water was a binary compound: a quantity of volume of hydrogen was getting together completely with the same oxygen volume giving place to a water vapor volume.


In 1811, an Italian called Lorenzo Romano Amedeo Carlo Avogadro (1776-1856) solved the quandary in which Gay-Lussac and Dalton were proposing the piece that was missing to the puzzle: First, the minimal unit in a chemical reaction is the molecule and it can split into atoms during the reaction; Second, two equal volumes of different gases, are of the type that there are, they contain the same number of molecules if the conditions of temperature and pressure are the same.

Hypothesis of Avogadro

ááááá Two equal volumes of different gases have they are formed by the same number of molecules if we keep the pressure and the temperature invariable. If we have two bottles with the same volume, a flood of helium and other one of oxygen, to the same temperature and with the same pressure, the two will have the same number of molecules. Although, in this case the number of atoms of oxygen will be the double, since the molecule of stable oxygen is formed by two oxygen atoms.

With to water formation he would explain himself of the following way: The oxygen molecules separate in atoms and later they get together with molecules of H2 forming molecules of H2O. If we speak about volumes, an oxygen volume they get together with two hydrogen volumes, forming two water volumes The combination reason in volumes is of 2:1:2 as it was observed in the Gay-Lussac experiments.


From this hypothesis, deduces another well-known relation how Avogardo law:

Law of Avogadro

To temperature and constant pressure, the volume of a gas is straight proportional to the gas quantity. Namely if the number of gas masses (n) doubles, the volume doubles.

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Nevertheless, Avogadro was an individual who was working only, he was supporting very few mail with other scientists, was publishing few articles and was not attending to many scientists. If there is this we add that in this moment there was no many organization in the chemistry as science; 50 years happened until the Gerhardt works came, Laurent and Willamson on the organic chemistry corroborated the law of Avogadro and the diffusion done by the Italian chemist Stanislao Cannizzaro. It was the last one, the fact that in 1858 Sunto published a titled memory “I gave a Corsican one gave Philosophy chimica” in it was using the Avogadro hypothesis to measure the molecular weight of several gases and to determine his composition from these measurements, and was telling that the exceptions that were taking place in some substances that it was not fulfilling in this hypothesis were caused because some molecules were dissociating. In 1860, four years after the Avogadro death, Cannizzaro gave one a conference on his studies and hypothesis of Avogadro, how to use it and because it was so necessary to differentiate the atoms and the molecules, managing to convince big part of the assistants that in turn, they facilitated the publication for the scientific community.

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Further on, thanks to the new chemical skills, they tried to do measurements to know the number of molecules that existed in a concrete gas quantity and that nowadays we know as number of Avogadro in his honor. The first attempt was carried out by the physicist and chemist Austrian Johann Josef Loschmidt in 1865 calculating for the first time the average value of the size of the molecules of the air and with this fact he estimated the number of molecules that exist in one cubic centimeter of air. This number indicates the thickness of the particles in an ideal gas and is known nowadays how the Loschmidt constant.

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Nevertheless, although the number of Loschmidt was approximately proportional to the Avogadro constant, it was not until 1908 when Jean Baptiste Perrin published his investigations on the Brownian movement of the particles in the water in which the Avogadro constant was calculated of exact form. The movement Browniando is the random movement that is observed in some microscopic particles when they are in a fluid way (for example, the movement of the pollen in a water drop), and it is caused by the agitation of the molecules of the liquid (we remember that in the liquids the particles can vibrate but not move freely). Perrin, was rewarded by the Nobel Prize of Physics in 1926 by his works on the discontinuity of the matter and the discovery of the balance of sedimentation.

Number of Avogadro: NA

It indicates the number of elementary units (atoms, molecules, ions, …) that are in a mol of any substance. In principle, a mol of one substances was the quantity of this substance in which there is an equal number of elementary units to the number of molecules that there is in 2,016 grams of gas hydrogen, but nowadays the equivalence is used with the number of atoms that there is in 12 grams of carbon 12. Also it is known that a mol of ideal gas has a volume of 22,4 L to 0 ºC from temperature and to 1 pressure ambience.

NA = 6,02214179 × 1023 elementary units / mol

To do an idea to us of how big this number is, we can say that it is equivalent to the quantity of necessary cups to empty the Pacific Ocean or of canisters of refreshments that we would need to cover all the ground piled up up to a height of 320km.

At the beginning of the sXIX, thanks to the electrolysis, new elements were discovered and they discarding some of the substances considered by Lavoisier. In 1830, already had been approximately 50 elements; and in the decade of 1860 this number was overcoming the 60 thanks to the use of the spectroscope of Gustav R. Kirchhoff and Robert W. Bunsen.

We will speak about how they decided to classify and arrange all these elements before jumping to divide the atom in protons, neutrons and electrons; and of exhibiting us to the radiation, the x-rays and other types of beams.

Previous chapters Introduction to the Chemistry: Properties of the matter

The elements: From the fire to the phlogiston

The new modern Chemistry

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