EVIDENCE THAT LED TO THE MODIFICATION OF DALTON’S ATOMIC THEORY<br />
THE ATOM<br />In 580 BC, the ancient Greek philosopher Thales suggested that water was the fundamental ‘element’ from which all matter in the universe was composed<br />
THE ATOM<br />Then, 200 years later, another philosopher by the name of Aristotle proposed that along with water, earth, air and fire were the four main elements that made up the world. In addition to this, he also proposed that there was a fifth element, ‘aether’, that made up the heavens<br />
THE ATOM<br />This suggestion made by Aristotle persisted for almost 2000 years until in 1961 a man named Robert Boyle wrote and published a book called ‘The Sceptical Chymist’ which was a turning point in chemistry. It was the first modern definition of an element as being something that cannot be changed into anything simpler rather than just merely a substance that the world was comprised of. Boyle also urged chemists to carry out practical investigations rather than just observing and making deductions as the Greeks had been doing earlier<br />
Dalton's theory was based on the premise that the atoms of different elements could be distinguished by differences in their weights. He stated his theory in a lecture to the Royal Institution in 1803. The theory proposed a number of basic ideas:All matter is composed of atomsAtoms cannot be made or destroyedAll atoms of the same element are identicalDifferent elements have different types of atomsChemical reactions occur when atoms are rearrangedCompounds are formed from atoms of the constituent elements.Using his theory, Dalton rationalised the various laws of chemical combination which were in existence at that time. However, he made a mistake in assuming that the simplest compound of two elements must be binary, formed from atoms of each element in a 1:1 ratio, and his system of atomic weights was not very accurate - he gave oxygen an atomic weight of seven instead of eight.Despite these errors, Dalton's theory provided a logical explanation of concepts, and led the way into new fields of experimentation.<br />
The Plum Pudding Model/Chocolate Chip Cookie Model was developed by J.J. Thomson. This is how Thomson discovered the electron in 1897. Thomson suggested this theory in 1904. The theory said that atoms as a whole were neutrally charged. He also suggested that the atom was made up electrons embedded into a cloud of positive particles. This was known as the Chocolate Chip Cookie Model because the electrons are like the chocolate chips embedded into the cookie and the dough is like the cloud of positive particles surrounding the chocolate chips(electrons).<br />
Basic Ideas of the Theory<br /><ul><li>Atoms are a sphere with a positive charge distributed evenly through the atom containing negatively charged electrons separately embedded into the atom.
Atoms as a whole are neutrally charged because the amount of positive charge in it cancels out the amount of negatively charged electrons. This means as a whole atoms are neutrally charged.
The electrons are randomly scattered throughout the atom just like chocolate chips in a cookie. The rest of the atom is a positive charge which surrounds the electrons, like the dough in a cookie.
The electrons can be moved around or taken out of the atom, the positive charge can not. </li></li></ul><li>
Experiment that Proved the Theory <br />The Crooke's tube experiment showed J.J. Thomson that there were electrons in an atom. This experiment also showed him that electrons were negatively charged. This meant that if atoms are neutrally charged as a whole and elctrons are negatively charged then there must be a positive charge in the atom to cancel out the negative charge of the electrons.<br />Thomson experiment worked like this:<br />- The Crooke's tube showed Thomson that he could bend a beam of "Cathode Rays"( electrons) using a set of negative and positively charged plates. Because the beam of "Cathode Rays" bent towards the positively charged plate it meant that the beam was negatively charged because opposites attract.<br />- Thomson called the particles in the beam electrons.<br />-Now that Thomson knew there were electrons in the atom he knew that there had to be a positive charge in the atom to offset the negative charge of the electrons to make the atom neutrally charged.<br />
Before J.J Thomson’s atomic theory John Dalton had an atomic theory that said that atoms couldn't be destroyed or broken but J.J. Thomson proved that wrong when he extracted the electrons from the atom in his Crooke's tube experiment. Thomson extracted the electrons from the atom, hence, disproving John Daltons theory that atoms were one solid thing. <br />
Rutherford's model did not make any new headway in explaining the electron-structure of the atom; in this regard Rutherford merely mentioned earlier atomic models in which a number of tiny electrons circled the nucleus like planets around the sun, or a ring around a planet (such as Saturn). However, by implication, Rutherford's concentration of most of the atom's mass into a very small core made a planetary model an even more likely metaphor than before, as such a core would contain most of the atom's mass, in an analogous way to the Sun containing most of the solar systems' mass.<br />
Rutherford also directed the famous Geiger-Marsden experiment in 1909, which suggested on Rutherford's 1911 analysis that the so-called "plum pudding model" of J. J. Thomson of the atom was incorrect. Rutherford's new modelfor the atom, based on the experimental results, had the new features of a relatively high central charge concentrated into a very small volume in comparison to the rest of the atom and containing the bulk of the atomic mass (the nucleus of the atom).<br />
In 1909, Hans Geiger and Ernest Marsden carried out an experiment called the Hans/Geiger Experiment (also called the Gold foil experiment or the Rutherford experiment) which was to probe the structure of the atom, under the direction of Ernest Rutherford at the Physical Laboratories of the University of Manchester. The unexpected results of the experiment demonstrated for the first time the existence of the atomic nucleus, leading to the downfall of the plum- pudding modelof the atom, and the development of the Rutherford(planetary) model.<br />
The gold foil experiment consisted of a series of tests in which positively charged helium nuclei were directed at a very thin layer of gold foil. The expected result was that the positive particles would be moved just a few degrees from their path as they passed through the sea of positive charge proposed in the plum pudding model. The result, however, was that the positive particles were repelled from gold foil at very high angles, up to 180 degrees. However, most of the remaining particles were not deflected at all, but rather, passed through the foil.<br />In detail, a beam of alpha particles, generated by the radioactive decay of radium was directed normally onto a sheet of very thin gold foil. The gold foil was surrounded by a circular sheet of zinc sulphide (ZnS) which was used as a detector: the ZnS sheet would light up when hit with alpha particles. Under the prevailing plum pudding model, the alpha particles should all have been deflected by, at most, a few degrees; measuring the pattern of scattered particles was expected to provide information about the distribution of charge within the atom. However they observed that a very small percentage of particles were deflected through angles much larger than 90 degrees. According to Rutherford:<br />It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you. On consideration, I realized that this scattering backward must be the result of a single collision, and when I made calculations I saw that it was impossible to get anything of that order of magnitude unless you took a system in which the greater part of the mass of the atom was concentrated in a minute nucleus. It was then that I had the idea of an atom with a minute massive center, carrying a charge.<br />
In 1911, Niels Bohr went to England to study with J.J. Thomson, who had discovered the electron in 1897. Most physicists in the early years of the twentieth century were engrossed by the electron, such a new and fascinating discovery. Few concerned themselves much with the work of Max Planck or Albert Einstein. Thomson wasn't that interested in these new ideas, but Bohr had an open mind. Bohr soon went to visit Ernest Rutherford (a former student of Thomson's) in another part of England, where Rutherford had made a brand-new discovery about the atom.<br />
When Bohr joined Rutherford he realized that Rutherford's model wasn't quite right. By all rules of classical physics, it should be very unstable. For one thing, the orbiting electrons should give off energy and eventually spiral down into the nucleus, making the atom collapse. Or the electrons could be knocked out of position if a charged particle passed by. Bohr turned to Planck's quantum theory to explain the stability of most atoms. He found that the ratio of energy in electrons and the frequency of their orbits around the nucleus was equal to Planck's constant (the proportion of light's energy to its wave frequency, or approximately 6.626 x 10-23 ). Bohr suggested the revolutionary idea that electrons "jump" between energy levels (orbits) in a quantum fashion, that is, without ever existing in an in-between state. Thus when an atom absorbs or gives off energy (as in light or heat), the electron jumps to higher or lower orbits. Bohr published these ideas in 1913 to mixed reaction. Many people still hadn't accepted the idea of quanta, or they found other flaws in the theory because Bohr had based it on very simple atoms. But there was good evidence he was right: the electrons in his model lined up with the regular patterns (spectral series) of light emitted by real hydrogen atoms. <br />
Bohr's theory that electrons existed in set orbits around the nucleus was the key to the periodic repetition of properties of the elements. The shells in which electrons orbit have different quantum numbers and hold only certain numbers of electrons -- the first shell holds no more than 2, the second shell up to 8, the third 10, the fourth 14. Atoms with less than the maximum number in their outer shells are less stable than those with "full" outer shells. Elements that have the same number of electrons in their outermost shells appear in the same column in the periodic table of elements and tend to have similar chemical properties. <br />Over the years other investigators refined Bohr's theory, but his bold application of new ideas paved the way for the development of quantum mechanics. Bohr went on to make enormous contributions to physics and, like Rutherford, to train a new generation of physicists. But his atomic model remains the best known work of his very long career.<br />
HENRY g. j. MOSELEY<br />Five years before Rutherford announced the discovery of the proton, Henry Moseley, a scientist from Rutherford’s research team, carried out an experiment by bombarding different metal targets with cathode rays and measured the frequency of the X-rays that emerged from the anode. He discovered that the energy of x-rays emitted by the elements and frequency increased in a linear fashion with each successive element in the periodic table, with an increase in the mass of the metal atom. In 1913, he proposed that the relationship was a function of the positive charge on the nucleus( which Rutherford called the Atomic Number).This rearranged the periodic table by using the atomic number instead of atomic mass to represent the progression of the elements. This new table left additional holes for elements that would soon be discovered.<br />
The atomic number tells us the following:<br /><ul><li>What the element is(only atoms of the same element have the same atomic number
The element’s numbered position in the Periodic Table is arranged in atomic order from the smallest to largest.
The number of electrons in a neutral atom(the charge on an electron is equal, but opposite, to the charge on a proton.</li></li></ul><li> JAMES CHADWICK<br />
In 1932, English Physicist James Chadwick, another member of Ernest Rutherford’s research team, after a decade-long struggle to track down this tricky particle (all the methods available at the time were used only to detect charged particles), performed tests on a new type of radiation which had been baffling physicists for years, and which had previously been mistaken for “gamma rays” (a form of radiation consisting of high-energy photons) along<br />
The test, to simplify as much as possible, went like this: <br />A sample of Beryllium was bombarded with alpha particles (another type of naturally occurring radiation which are technically just ionized helium nuclei), which causes it to emit this mysterious radiation. It was then discovered by Irene Joliot-Curie (daughter of Marie and Pierre Curie) and her husband Frederic Joliot-Curie that this radiation, upon striking a proton-rich surface (paraffin was the preferred example), would discharge some of the protons, which could then be detected using a Geiger counter (a device that measures radiation). <br />This was the premise, and from here, Chadwick simply had to play detective and put all the pieces of the puzzle together. For instance, he could tell that the mysterious radiation in question was neutral due to the fact that it was not affected by proximity to a magnetic field, and, unlike standard gamma radiation, did not invoke the photoelectric effect (when photons, such as gamma rays, strike certain surfaces, they discharge electrons, which can be simply measured), but rather discharged protons, which meant that the particles had to be more massive than previously expected. <br />
Rutherford guessed that the protons were coming from the parrafin because some sort of radiation was hitting it. He said that this radiation was like the effect of an invisible man who cannot be seen directly, but who is known to be there because he collides with other people in a crowd. The invisible particle of Chadwick’s experiment (with no charge and with its mass equal to that of a proton), was named the neutron. Hence, this completed the long search by many scientists to come up with the perfect model and of the atom. An atom is now known to be, the smallest part of an element which contains a minute central nucelus of positively charged protons and neutral neutons surrounded by negatively charged elctrons orbiting in fixed shells of different energy levels around the nucleus.<br />