Atomic Structure

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Atomic Structure

Atomic theory, first put in a quantitative conceptual framework by John Dalton, and quantum theory, which emerged in the 1920s as a result of the work of Werner Heisenberg, Erwin Schrödinger, 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. Atoms are very small, their diameter lies between 0.7 and 6 Ångstroms (1 Ångstrom = 10-10 m), as revealed by x-ray crystallography, the method of choice to determine the size of atoms from a pattern of x rays diffracted on a crystal. At the turn of the century, the work of J.J. Thomson, Robert Millikan and Ernest Rutherford showed that atoms are electrically neutral and consist of a central positively charged nucleus surrounded by a cloud of negatively charged electrons.

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 Marden. 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 scattered 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.

Atomic nuclei are now considered to consist of two different types of particles of almost equal mass: protons, which have a positive charge and a mass of 1.6726 x 10-24 g and neutrons, which have no charge and a mass of 1.6750 x 10-24 g. The nucleus of the lightest element, hydrogen, consists of a single proton. Atoms with the same number of protons and neutrons are called nuclides and atoms with a unequal number of protons and neutrons are called isotopes.

mass number, A. The mass of the nucleus, in atomic mass units (amu), is nearly equal to its mass number. One amu is one twelfth of the mass of one isotope.. (1 amu = 1.660244 x 10-24 g).

The mass of an electron is 9.1094 x 10-28 g. The electron has the same charge as a proton: 1.6022 x 10-19 coulombs, but is oppositely charged, i.e. negative. Overall, atoms are neutral and the number of protons in their nuclei equals the number of electrons. This number (Z) is called the atomic number. For example, the atomic number of Na is 11; this means that sodium has 11 electrons occupying concentric energy levels around the nucleus, which consists of 11 protons.

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 to the orbital model of the atom, in which the discrete energy levels accomodating the electrons are ordered by increasing energy. Each level can only have one s (spherical) orbital containing a maximum of 2 electrons. In every energy level, these orbitals are always filled first and the others (p, d, f, g,...) afterwards, following the rules of the aufbau principle.

When the lowest energy levels are thus successively occupied by the electrons of an atom, it is said to be in the ground state. The addition of energy, for example heat or light, can promote electrons to higher energy levels. The atom is then said to be in an excited state. The study of the interaction of electromagnetic energy and matter is the subject of spectroscopy which is used to gain insight into the properties of matter and atomic structure.