Wednesday 16 February 2011

AA-P1: Dalton's Atomic Theory

John Dalton was a famous British Chemist, Physicist, and Meteorologist in the late 17-1800’s. He is best known for his work into developing the atomic model and for his research into colour blindness. In 1800 he became a secretary in the Manchester Literary and Philosophical society at which point his work into the composition and properties of many mixed gases.

Before Dalton, it is accepted that two previous scientists; Antoine Lavoisier and Joseph Proust, were the basis for Daltons work on the atomic theory, and he studied theirs and elaborated on it in order to start his own research. After many experiments Dalton came up with his own theories within the first three years of the 1800’s, the most well known of these is the “Law of multiple proportions”. This law states that if two elements can together form more than one compound, then the ratios of the masses of the second element which combine with a fixed mass of the first element will be ratios of small integers. For example, he noted after doing calculations that the combination of Tin and Oxygen would produce molecules of Tin Oxide, with either one or two Oxygen’s depending on the state of the tin.
Dalton afterwards made other theories into such matters as why water can absorb some gases but not others, and hypothesized that this was because of the mass of the different gases. He also proposed that each element on its own is composed of a unique atom, which cannot be the same in any other element, and that these atoms can combine together to form other more complex molecules. This Is now recognized as the first advancement into the modern theory of the current atomic model, as we know it today.
Dalton kept his research mostly between himself and his assistant, Thomas Thomson. His first ‘announcement’ of his work was an oral presentation in a lecture where he presented his list of relative atomic weights for some of the elements he was researching at the time, using Hydrogen as the basis. Thomas Thomson first revealed how Dalton achieved these conclusions in his own third book; “A System of Chemistry”, after this, in 1808 Dalton himself published his own textbook detailing his theories; “A New System of Chemical Philosophy”, and released a later edition 2 years later in 1810.


The research in these books was flawed however, and much of his work was discredited because he was getting things wrong which didn’t fit his model. He thought that molecules always existed with the simplest number of atoms in them, so he though water was HO rather than H2O, and so on. Due to this many of his calculations and weights were incorrect; he thought that oxygen was only 5.5 times heavier than Hydrogen whereas we know it now to be 16 times heavier.
This research was some of the last he did, as pioneering scientists of the day took over his research and corrected it, just as he did with Lasoisier and Proust’s work before him. After Dalton however, Amedeo Avogadro took over his work and corrected it. His own first law was that “equal volumes of any two gases, at equal temperature and pressure, contain equal numbers of molecules”, which in short means that a gases mass does not affect the volume that it fills. From here, Avogadro developed his own theories afterwards, such as developing the ‘mole’ which is a constant (Avogadro’s constant) number, rather than writing out how many atoms or molecules there are in a substance, you could write down how many moles there are (6.023x1023 in one mole).
After both Dalton and Avogadro, many famous scientists continued to develop their work further and further and eventually have came up with the current theory of the atomic model and periodic table, so it is said that Dalton is the father for the current atomic model as we know it today.
Throughout his career, Dalton came up with other theories aswell about the current atomic model, he wrote;
1.       All matter is composed of atoms
2.       Atoms cannot be made or destroyed
3.       All atoms of the same element are the same as one another
4.       Different elements have different types of atoms
5.       Chemical reactions occur when atoms are rearranged
6.       Compounds are formed from atoms of the constituent elements
In conclusion, Dalton is commonly accepted as the father of the modern atomic model, and much of the work and theories scientists do today, could not be done without Daltons own work.

References
Figure 2 – Image from Daltons book “A New System of Chemical Philosophy (1808)”

No comments:

Post a Comment