![]() Molecular Geometry is the arrangement of atoms in a molecule, normally relative to a single central atom.Įlectron Geometry is the arrangement of electron pairs around a central atom. Valence Shell Electron Pair Repulsion (VSEPR) Theory.Lewis Structure (Representation of Valence Electrons in a molecule).Its molecular geometry is trigonal pyramidal while its electron geometry is tetrahedral. With the central atom nitrogen having 5 valence electrons, it possesses 3 bond pairs and a lone pair of electrons. Its molecular, as well as electronic geometry, is tetrahedral.Īn example with differing molecular and electron geometries is that of ammonia, NH 3. An example is a methane molecule, CH 4 with 4 bond pairs and no lone pairs, all 4 of carbon’s valence electrons are bonded with hydrogen atoms. If all of the electron groups are bond pairs (no lone pairs), the molecular geometry and electron geometry are the same. This causes a slight decrease in bond angles (angles between bonds or bond pairs). Under the influence of a single nucleus, a lone pair offers more repulsion than a bond pair which is influenced by two nuclei. This repulsion causes the electron pairs around the central atom to arrange as far apart from each other as possible. It is well known that the electron pairs, being negatively charged, repel each other. Whereas, electron geometry is the 3D arrangement of electron pairs around a central atom, whether bonding or non-bonding.Ī lone (non-bonding) pair refers to a pair of valence electrons that are not shared with another atom in a covalent bond, while a bond pair is a pair of electrons present in a bond. Molecular geometry is described as the 3D arrangement of atoms in a molecule, normally relative to a single central atom. 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.Geometry in chemistry refers to the shape of molecules in 3-Dimensional space. ![]() 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: 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). 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. 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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