Slide5WHO DİSCOVERED THE ATOM Democritus was a Greek philosopher who was the first person to use the term atom ( atomos: meaning indivisible). He thought that if you take a piece of matter and divide it and continue to divide it you will eventually come to a point where you could not divide it any more. This fundamental or basic unit was what Democritus called an atom. He called this the theory of the universe: •All matter consists of atoms, which are bits of matter too small to be seen. •There is an empty space between atoms. •Atoms are completely solid . •Atoms have no internal structure . •Each atom (of a different substance) is different in size, weight and shape. 1.Timeline: 400 BC Scientist: Democritus (Greek Philosopher)
Slide7WHO DİSCOVERED THE ATOM AND WHEN John Dalton was the first to adapt Democritus’ theory into the first modern atomic model. In the early 1800s, John Dalton used the concept of atoms to explain why elements always react in ratios of small whole numbers (the law of multiple proportions). For instance, there are two types of tin oxide: one is 88.1% tin and 11.9% oxygen and the other is 78.7% tin and 21.3% oxygen (tin(II) oxide and tin dioxide respectively). This means that 100g of tin will combine either with 13.5g or 27g of oxygen. 13.5 and 27 form a ratio of 1:2, a ratio of small whole numbers. This common pattern in chemistry suggested to Dalton that elements react in whole number multiples of discrete units in other words, atoms. In the case of tin oxides, one tin atom will combine with either one or two oxygen atoms. 2.Timeline: 1800’s Scientist: John Dalton
Slide3Discovering the Atom The idea that matter is made from tiny particles goes back well over two thousand years. It is thought that the concept of the atom was originally conceived by the Greek philosopher Leucippusduring the early part of the fifth century BCE, although it is also sometimes accredited to his pupil Democritus. It would be hundreds of years, however, before these ideas could be backed up by experimental data.
Slide19Discovering the Atom The Anglo-Irish scientist Robert William Boyle (1627 - 1691), considered by some to be one of the founders of modern chemistry and probably best known for his ideas about the relationship between volume, pressure and temperature in gases, firmly endorsed the view that chemical elements were the fundamental constituents of all physical materials, and recognised the distinction between chemical elements and compounds (collectively classed as substances) and mixtures thereof. He also supported the view that elemental substances were composed of particles of various kinds, although he seems to have been reluctant to speculate on the exact nature of these particles.
Slide20Discovering the Atom The beginning of the nineteenth century saw real advances in our understanding of matter. In 1805, the English scientist John Dalton (1766-1844) published one of several papers outlining his theories on the behaviour of gases, at the end of which he puts forward a number of ideas about the nature of chemical elements, and their role in chemical reactions, in what is considered by many to be the first coherent atomic theory of the modern scientific age.
Slide8Discovering the Atom By the year 1864, scientists had identified over sixty elements, with new elements being discovered at the rate of about one per year. Some of these elements (metals such as tin and zinc, for example) had been known about since ancient times, probably because of the ease with which they could be extracted from the earth. Other elements were discovered as the result of painstaking experiment - and occasionally by accident. One approach to classifying the known elements is to order them according to their atomic weight, which we refer to today as relative atomic mass. The British chemist John Alexander Reina Newlands (1837-1898) was probably the first person to create a table of chemical elements ordered according to their relative atomic mass. Newlands had noticed that when he listed the elements in this way, elements occurring seven places apart in the list seemed to behave in a similar way. He therefore arranged the elements into seven rows of eight columns. We have reproduced Newland's table below.
Slide9Discovering the Atom n 1865, Newlands published his Law of Octaves, in which he stated that "any given element will exhibit analogous behaviour to the eighth element following it in the table". The title reflects the way in which Newlands likened each grouping of eight elements to an octave of music. Newlands seemed to have discovered a periodic pattern in the characteristics of the elements, but his ideas were initially rejected by his contemporaries. It was not until many years later that the significance of his work was formally acknowledged. The next major contribution came from the Russian chemist and inventor Dmitri Ivanovich Mendeleev (1834-1907). Mendelev was attempting to classify the elements according to their chemical properties. Despite the fact that he appears to have been unaware of the work done by Newlands, he too began to notice that elements with similar characteristics seemed to occur at regular intervals when ordered according to their atomic weight.
Slide11Discovering the Atom Mendelev claimed to have had a dream in which he had envisioned a table containing all of the elements in their natural order, and that upon waking he had immediately committed what he had seen to paper. Mendelev formally presented his findings to the Russian Chemical society in 1869. Mendelev had discovered that the atomic weights of elements with similar properties were either very close (as, for example, in the case of osmium, iridium and platinum) or increased by regular intervals (as in the case of calcium, rubidium and caesium). Mendelev suggested that the atomic weight of an element could be used to predict its chemical properties. He also suggested that there were a number of as yet unknown elements waiting to be discovered. These claims were later justified when he accurately predicted both the existence and the chemical properties of germanium, gallium and scandium Mendelev's periodic table was published in 1871, and is reproduced below.
Slide10Discovering the Atom Since Mendelev published his periodic table, many new elements have been discovered, and the periodic table has evolved to allow for new groupings of chemical elements. Nevertheless, Mendelev's work forms the basis for the periodic table we use today. In the modern periodic table, the elements are organised according to their relative atomic mass, electron configuration and chemical properties. The rows in the table are called periods. The columns are called groups. John Dalton's contention that atoms could not be created, destroyed, or broken down into smaller parts was proved false in 1897, when English physicist Joseph John Thompson (1856-1940) demonstrated that cathode rays were composed of negatively charged particles with a mass many hundreds of times smaller than that of a hydrogen atom. Thompson called this newly discovered particle a "corpuscle". It was the first sub-atomic particle to be discovered. We know it today as the electron.
Slide12Discovering the Atom Thompson soon realised that these particles were identical to those emitted by radioactive materials, and that they were also the particles that made it possible for electric current to flow in a conductor. He correctly theorised that atoms must be made up of a number of smaller (sub-atomic) particles that included electrons. However, his assumption that the negatively charged electrons were scattered throughout the atom in a kind of "sea" of evenly distributed positive charge (a concept known as the plum pudding model) was later shown to be false.
Slide13Discovering the Atom In a 1909 experiment, the German physicist Johannes Wilhelm Geiger (1882-1945) and the English-born New Zealand phycisist Ernest Marsden (1889-1970), working under the direction of New Zealand physicist Ernest Rutherford (1871-1937), bombarded a thin sheet of gold foil with a narrow beam of alpha particles emitted by radium bromide. A screen coated with zinc sulphide (which emits a detectable flash when an alpha particle collides with it) was used to detect the alpha particles as they passed through the foil.
Slide21Discovering the Atom If Thompson's plum pudding model was correct, the dispersed positive charge in the atoms of the gold foil should have been too weak to significantly deflect the positively charged alpha particles. In fact, Geiger and Marsden observed alpha particles being deflected at angles of more than ninety degrees (90°). Rutherford deduced that, rather than being evenly distributed, all of the positive charge must be concentrated in a tiny nucleus at the centre of the atom.
Slide22Discovering the Atom In 1913, the Danish physicist Niels Henrik David Bohr (1885-1962), together with Ernest Rutherford, proposed a model of the atom in which the electrons in an atom were in orbit around its nucleus, in much the same way that the planets in our solar system are in orbit around the Sun. One obvious difference is that, whereas the force holding a planet in its orbit around the Sun is gravity, the force holding an electron in orbit around the nucleus of an atom is electrostatic in nature.
Slide14Discovering the Atom Rutherford had already established a model of the atom in which a diffuse cloud of negatively charged electrons surrounded a small, dense and positively charged nucleus. The Rutherford- Bohrmodel refined this idea. Bohr suggested that the electrons occupied fixed, circular orbits around the nucleus of the atom.
Slide23Discovering the Atom This idea brought with it a new set of problems for physicists. According to the laws of classical physics, an electron orbiting a nucleus should experience acceleration. When a charged particle (such as an electron) accelerates, it emits energy in the form of electromagnetic waves. An electron orbiting the nucleus of an atom could therefore be expected to lose energy, and its orbit to decay as a consequence. If that were to happen, the atom would quickly become unstable.
Slide24Discovering the Atom There was no evidence, however, that this was happening. The experimental data seemed to indicate that electron orbits were stable. Bohr explained this by suggesting the electrons occupied "stationary orbits", each of which lay at a specific distance from the nucleus. Electrons could jump from a lower orbit to a higher orbit by absorbing electromagnetic radiation, thus gaining a discrete amount (or quantum) of energy. When electrons jumped from a higher orbit to a lower orbit, they emitted electromagnetic radiation, thereby relinquishing a quantum of energy.