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Electron Pair Repulsion Theory The electron pair repulsion theory states that the electron pairs in the valence energy level of an atom repel each other and therefore are arranged as far apart as possible For example H2O Due to this theory different molecules with different amounts of pairs of electrons have different shapes Shapes of Molecules Some common shapes of molecules include linear trigonal planar tetrahedral trigonal pyramidal and v-shaped bent molecules Examples are drawn below- a linear CO2 b tetrahedral CH4 c v-shapedbent H2O d trigonal planar BF3 e trigonal pyramidal NH3 The electron dot diagrams must be drawn first in order to work out the shape of the molecule Molecular Polarity The polarity can be determined from the shape of the molecule In essence a molecule is polar if there is an overall electronegativity difference in the molecule This can be determined using some basic rules which state that If a molecule is of the form AB2 and is linear it is non-polar If a molecule is of the form AB3 and is trigonal planar it is non-polar If a molecule is of the form AB4 and is tetrahedral it is non-polar All other molecules are polar Using these rules we can determine the polarity of the earlier used examples a AB2 b AB4 c AB2 d AB3
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