So it has four pairs altogether, giving it a tetrahedral arrangement and bent geometry (2-2 combination of 2 BP and 2 NBP), which corresponds to a 105 degree bond angle. Therefore, it has two bonding pairs of electrons (to each H) and also has 2 pairs of lone pairs (pair of dots). When drawing the Lewis structure of water, oxygen is the central atom. Double and triple bonds still count as 1 bonding domain (BD)!Įxample: H 2O.So count only the # pairs around the central atom not in the entire compound! NOTE: the # of BD and NBD is from the perspective of the CENTRAL ATOM. Now within that arrangement, what is the specific combination and geometry.Count the TOTAL number of electron domains (both bonding and non-bonding) and classify the ARRANGMENT.Draw the Lewis structure of the molecule.Each particular combination will determine a geometry.ĭetermining Electron Arrangement/Geometry Rules: Explain how -bonding affects the ability of atoms to rotate about a bond. Sketch molecular geometries on paper with wedges and dashes to represent their 3D shape. 9: (a) H 2 O has four regions of electron density around the central atom, so it has a tetrahedral electron-pair geometry. Then we look to see exactly how the total electron domains are distributed-meaning how many are bonding and how many are non-bonding (or lone pairs). After completing this experiment, you should be able to: Determine the Lewis structure, molecular geometry and polarity for molecules using Lewis structures. Thus, the electron-pair geometry is tetrahedral and the molecular structure is bent with an angle slightly less than 109.5°. The total count will determine the arrangement. Electron domains are pairs of electrons so we count both bonding pairs and non-bonding pairs to get the TOTAL. Electron configurations describe where electrons are located around the nucleus of an atom. We count the TOTAL # of electron domains. Arrangement is the general classification of “shape” but geometry is the specific “shape” of the molecule in question.įor example: 3 sided figures: triangles (general shape) BUT 3 sided figure with equal sides: equilateral triangle (a specific kind of triangle). Note: The difference between an arrangement and geometry is as follows. It allows us to predict the shape of a molecule 3-dimensionally. VSEPR: Valence Shell Electron Pair Repulsion model uses the repulsion of electron pairs to determine strain on various bonds and the resulting electron ARRANGEMENT and GEOMETRY.
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