ATOMS AND MOLECULESA Chemistry Seminar for the partial fulfillment ofFormative Assessment – 4
ATOM The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons (except in the case of hydrogen-1, which is the only stable nuclide with no neutrons). The electrons of an atom are bound to the nucleus by the electromagnetic force. An atom containing an equal number of protons and electrons is electrically neutral, otherwise it has a positive charge if there are fewer electrons (electron deficiency) or negative charge if there are more electrons (electron excess). A positively or negatively charged atom is known as an ion. An atom is classified according to the number of protons and neutrons in its nucleus: the number of protons determines the chemical element, and the number of neutrons determines the isotope of the element.
THE STRUCTURE OF AN ATOM We have learnt Dalton’s atomic theory which suggested that the atom was indivisible and indestructible. But the discovery of two fundamental particles (electrons and protons) inside the atom, led to the failure of this aspect of Dalton’s atomic theory. It was then considered necessary to know how electrons and protons are arranged within an atom. For explaining this, many scientists proposed various atomic models.
PLUM PUDDING MODEL The plum pudding model of the atom by J. J. Thomson, who discovered the electron in 1897, was proposed in 1904 before the discovery of the atomic nucleus in order to add the electron to the atomic model. In this model, the atom is composed of J. J. Thomson electrons surrounded by a soup of positive charge to balance the electrons negative charges, like negatively-charged "plums" surrounded by positively-charged "pudding". The electrons were thought to be positioned throughout the atom, but with many structures possible for positioning multiple electrons, particularly rotating rings of electrons. Instead of a soup, the atom was also sometimes said to have had a "cloud" of positive charge.
In this model, the electrons were free to rotate within the blob or cloud of positive substance. These orbits were stabilized in the model by the fact that when an electron moved farther from the center of the positive cloud, it felt a larger net positive inward force, because there was more material of opposite charge, inside its orbit. In Thomsons model, electrons were free to rotate in rings which were further stabilized by interactions between the electrons, and spectra were to be accounted for by energy differences of different ring orbits. Thomson attempted to make his model account for some of the major spectral lines known for some elements, but was not notably successful at this. Still, Thomsons model was an earlier harbinger of the later and more successful solar-system-like Bohr model of the atom.
RUTHERFORD MODEL The Rutherford model is a model of the atom devised by Ernest Rutherford. Rutherford directed the famous Geiger-Marsden experiment in 1909, which suggested on Rutherfords 1911 analysis that the so-called "plum pudding model" of J. J. Thomson of the atom was incorrect. Rutherfords new model for 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). Shield of the atomic planetary model Ernest Rutherford
RUTHERFORDS EXPERIMENT A stream of alpha particles (much like a stream of tiny bullets) was directed at a thin foil of gold atoms and a detector arranged to surround the sample completely except for a small hole for entry of the particles. The foil was several thousands of atoms thick. Conclusion: •Nucleus of an atom is positively charged •Nucleus is very dense and hard •Nucleus is very small compared to the size of the atom.Scattering of alpha particles Atomic model Rutherford: electronsby the atoms of a gold foil (green) and nucleus (red)
DRAWBACK OF RUTHERFORDS ATOMICMODEL Rutherford proposed that electrons revolve at high speed in circular orbits around the positively charged nucleus. But according to the electromagnetic theory, if a charged particle were accelerated around another charged particle then there would be a continuous radiation of energy. The loss of energy would slow down the speed of the electron and eventually the electron would fall into the nucleus. But such a collapse does not occur. Rutherfords model was unable to explain it. An energy losing electron could fall into the nucleus
BOHR MODEL In atomic physics, the Bohr model, introduced by Niels Bohr in 1913, depicts the atom as a small, positively charged nucleus surrounded by electrons that travel in circular orbits around the nucleus— similar in structure to the solar system, but with electrostatic forces providing attraction, rather than gravity. This was an improvement on the earlier cubic model (1902), the plum-pudding model (1904), the Saturnian model (1904), Niels Bohr and the Rutherford model (1911). Since the Bohr model is a quantum- physics–based modification of the Rutherford model, many sources combine the two, referring to the Rutherford–Bohr model.
In the early 20th century, experiments by Ernest Rutherford established that atoms consisted of a diffuse cloud of negatively charged electrons surrounding a small, dense, positively charged nucleus. However, the model has a technical difficulty. The laws of classical mechanics predict that the electron will release electromagnetic radiation while orbiting a nucleus. Because the electron would lose energy, it would gradually spiral inwards, collapsing into the nucleus. This atom model is disastrous, because it predicts that all atoms are unstable. To overcome this difficulty, Niels Bohr proposed, in 1913, what is now called the Bohr model of the atom. He suggested that electrons could only have certain classical motions:• The electrons can only travel in certain orbits at a certain discrete set of distances from the nucleus with specific energies.• The electrons of an atom revolve around the nucleus in orbits.• These orbits are associated with definite energies and are also called energy levels. Thus, the electrons do not continuously lose energy as they travel in a particular orbit. They can only gain and lose energy by jumping from one allowed orbit to another.• Like Einsteins theory of the Photoelectric effect, Bohrs formula assumes that during a quantum jump a discrete amount of energy is radiated.• The Bohr-Kramers-Slater theory (BKS theory) is a failed attempt to extend the Bohr model which violates the conservation of energy and momentum in quantum jumps. In 1925 a new kind of mechanics was proposed, quantum mechanics, in which Bohrs model of electrons traveling in quantized orbits was extended into a more accurate model of electron motion. The new theory was proposed by Werner Heisenberg. Another form of the same theory, wave mechanics, was discovered by the Austrian physicist Erwin Schrödinger independently, and by different reasoning. 12 C atom
LATER ATOMISM In the 1950s, the development of improved particle accelerators and particle detectors allowed scientists to study the impacts of atoms moving at high energies. Neutrons and protons were found to be hadrons, or composites of smaller particles called quarks. Standard models of nuclear physics were developed that successfully explained the properties of the nucleus in terms of these sub-atomic particles and the forces that govern their interactions.
ORIGIN AND CURRENT STATE OFATOM Atoms form about 4% of the total energy density of the observable universe, with an average density of about 0.25 atoms/m3. Within a galaxy such as the Milky Way, atoms have a much higher concentration, with the density of matter ranging from 105 to 109 atoms/m3. The Sun is believed to be inside the Local Bubble, a region of highly ionized gas, so the density in the solar neighborhood is only about 103 atoms/m3. Stars form from dense clouds in the ISM, and the evolutionary processes of stars result in the steady enrichment of the ISM with elements more massive than hydrogen and helium. Up to 95% of the Milky Ways atoms are concentrated inside stars and the total mass of atoms forms about 10% of the mass of the galaxy. (The remainder of the mass is an unknown dark matter.)