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1 2.1 THE PLANETARY MODEL OF AN ATOM We know that atoms consist of a small, positively charged nucleus that is surrounded by negatively charged electrons. The nucleus is composed of protons and neutrons. Each proton carries an electric charge of + 1.6x10 -19 C, and the neutron has no charge. The atomic number refers to the number of protons in a given element, and this number is fixed. Each electron carries a negative charge of - 1.6x10 -19 C. A neutral atom has the same number of protons and electrons. If this number is not equal, then the atom carries a net positive or negative charge, and it is called an ion. Protons and neutrons are both about 1840 times heavier than an electron, so the electron mass is negligible in comparison. The number of protons and neutrons a given element possesses thus determines its atomic mass . All atoms of a given element have the same number of protons but not all have the same number of neutrons. Atoms of a particular element which have different numbers of neutrons are referred to as isotopes, and, because different isotopes have slightly different atomic masses, the atomic mass characterizing a given element is not necessarilty an integer. Iron, for instance, has atomic number 26 and its atomic mass is 55.85. A mole of iron is the number of atoms that weighs 55.85 grams. This is Avogadro’s number, N, which equals 6.023x10 23 . A simple way to picture an atom employs the so-called planetary model. According to the planetary model, electrons circle the nucleus in well-defined orbits in close analogy with how the planets orbit the sun. A schematic illustration of the planetary model of a sodium (Na) atom is presented in figure 2-1. At its center is the nucleus consisting of 11 protons and 11 neutrons with 11 electrons orbiting this nucleus. The radius of each orbit is determined by Coulombic interaction between the electrons and the positively charged nucleus. In the simplest atom, hydrogen which consists of a single electron orbiting a single proton, the radius of the electron orbit is referred to as the Bohr radius and is equal to 0.059 nm. This radius is of order 10 4 times greater than the nuclear radius, and we see that the size of the hydrogen is determined almost entirely by the size of the electron orbital. This is true for all atoms. Appendix A lists atomic radii for most elements, and we see that these range between 0.06 nm for oxygen (O) to 0.263 nm for cesium (Cs). Most elements have atomic radii between about 0.1 and 0.15 nm despite the fact that different elements
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2 contain very different numbers of electrons. The fact that many-electron atoms are all roughly the same size because the electrons close to the nucleus feel the entire Coulombic potential of the nucleus and the radii of their orbitals are small whereas the outermost electrons feel only a fraction of the nuclear charge due to the partial shielding of the nuclear by the inner electrons.
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