Chem Essay

Chem Essay Chem Essay All matter is made up of atoms. An atom is like a tiny solar system. In the center of the atom is the nucleus which is a cluster of protons and neutrons. The protons have a positive electric charge while the neutrons are electrically neutral. The nucleus makes up almost all of an atom's mass or weight. Whirling at fantastic speeds around the nucleus are smaller and lighter particles called electrons which have a negative electric charge. . The protons have a positive electric charge while the neutrons are electrically neutral. The nucleus makes up almost all of an atom's mass or weight. Whirling at fantastic speeds around the nucleus are smaller and lighter particles called electrons which have a negative electric charge. An atom has the same number of electrons with positive charge and negative charge. An extremely powerful force, called the nuclear force, holds the protons together in the nucleus as they naturally repelled one another electrically. The atoms of each chemica l element have a different nucleus. An atom of hydrogen has one proton and no neutrons. An atom of nitrogen has 7 protons and 7 neutrons. Heavy elements have a large number of protons and neutrons. For example, the most common isotope of uranium, uranium-238 has 92 protons and 146 neutrons in its nucleus. Protons are positively charged and so would be deflected on a curving path towards the negative plate. Electrons are negatively charged and so would be deflected on a curving path towards the positive plate. Neutrons don't have a charge, and so would continue on in a straight line. Isotopes are atoms which have the same atomic number but different mass numbers. They have the same number of protons but different numbers of neutrons. The electrons are found at considerable distances from the nucleus in a series of levels called energy levels. Each energy level can only hold a certain number of electrons. Atomic theory, first put in a quantitative conceptual framework by Joh n Dalton, and quantum theory, which emerged in the 1920s as a result of the work of Werner Heisenberg, Erwin Schrà ¶dinger, and Max Planck, are the cornerstones of our present-day view on atomic structure. Atomic theory holds that matter consists of vast numbers of small particles called atoms which combine together to form molecules existing in the three main states of matter as gases, liquids or solids. Thomson's experiments led him to propose a "plum-pudding" view of the atom in which a continuous distribution of positive mass extends over the size of the atom with negative "plums" of much smaller mass (i.e. electrons) inserted into it. This model was overthrown by a series of alpha particle scattering experiments carried out by Rutherford, Hans Geiger and Ernest Madden. They were able to observe back-scattering of alpha particles emitted by a piece of radium as they were being shot through a thin gold foil. The fact that alpha particles are positive and that some of them were s cattered back could only be explained by proposing that the positive charge and mass in the gold atoms making up the foil could not be continuous and had to be concentrated in a very small region and that the negative region had to be large enough to let some alpha particles through. This led to a view of the atom in which the positive nucleus is central, very dense and significantly smaller than the size of the overall atom. Modern neutron scattering experiments have shown that the radius of a nucleus is proportional to the cubic root of its mass number and that atomic radii, including electron clouds, are about twenty thousand times bigger with a spherical shape or elongated like a football. The view that electrons can be thought of as being arranged in successive shells of increasing energy around the nucleus is called the Bohr model of the atom and one of the most significant contributions of quantum theory has been to show that these energy levels are quantized. This led t o the orbital