Shapes of Molecules
|
VSEPR (pronounced "vesper") is short for valence shell electron pair repulsion. This theory in chemistry is a model used to predict the geometry of molecules. It is based on the concept that negatively charged orbitals try to stay as far away from one another as is possible. Bond pairs and lone pairs are all simultaneously repelling one another, thereby determining the shape of the molecules.
|
|
A molecule consisting of two atoms is actually diatomic. By definition, two points identify a line. To describe shapes, there should actually be at least three atoms present in the molecule. That is when multiple possibilities come into existence.
|
Linear molecules have the atoms arranged in a straight line. For example, carbon dioxide (CO2) has a central carbon atom bonded by double bonds to two oxygen atoms on opposite sides. Cyanic acid (HCN) has a carbon atom between a hydrogen atom and a nitrogen atom, bonded to the nitrogen by a triple bond. Acetylene (C2H2) has two carbon atoms bonded together by a triple bond with two hydrogen atoms joined on the opposite ends of the molecule. All of these are linear molecules.
|
If three or more atoms are bonded in a nonlinear arrangement, the molecule is said to be bent or angular. This geometry is caused by the existence of filled orbitals applying repelling forces on other atoms. Group VA atoms (such as nitrogen) have one filled orbital. Group VIA atoms (such as oxygen) have two filled orbitals. These atoms can create bent molecules. Examples of bent molecules include water (H2O), hydrogen peroxide (H2O2) and azanone, also known as nitroxyl (HNO). In the first two, the oxygen atom has two filled orbitals that are repelling the hydrogen atoms. In the third, the nitrogen has one filled orbital, and there is a double bond between the nitrogen and oxygen atoms.
|
If three atoms are joined to a central atom with no filled pairs influencing the central atom, the atoms are each 120o from the central atom. Such molecules are called trigonal planar or triangular molecules. All four atoms are on a single plane. Examples of triangular molecules are aluminum hydride, also called aluminum trihydride (AlH3) and formaldehyde (CH2O).
|
If the central atom is not on the same plane as the three other atoms, the shape is referred to as trigonal pyramidal or simply as pyramidal. A filled orbital on the central atom repels the other atoms, thus creating the three-dimensional shape. This commonly occurs when an atom from group VA (the nitrogen family) is joined to three other atoms. Examples of pyramidal molecules are nitrogen trichloride (NCl3), phosphane (PH3) and ammonia (NH3).
|
Tetrahedral molecules have four atoms joined symmetrically to a central atom. This can by the case of a carbon atom joined to other atoms with single bonds. Such molecules have the shape of a d4 die. Examples of tetrahedral molecules are methane (CH4) and carbon tetrachloride (CCl4).
|
Trigonal bipyramidal molecules have five atoms joined to the central atom. Such molecules can be pictured as being similar to an atlas, with one atom joined at each of the opposing poles and the other three atoms evenly distributed 120o apart around the nucleus. If the pair of electrons in the single filled orbital in a phosphorus atom are divided to create separate bonding sites, such a molecule can result. An example is phosphorus pentachloride (PCl5).
|
Octahedral molecules have six atoms evenly distributed to a central atom. Such molecules have the shape of a d8 die. If the two filled orbitals in a sulfur atom are divided to create separate bonding sites, it can result in an octahedral molecule. Examples include sulfur hexafluoride (SF6) and persulfurane (SH6).
|
The polarity of molecules is due to the distribution of polar or nonpolar bonds. If the bonds are nonpolar, such as oxygen (O2) or carbon tetraiodide (CI4), the molecule is nonpolar. If the molecule is symmetrical, the molecule is likewise nonpolar, even if the bonds are polar, such as carbon tetrachloride or carbon dioxide. However, if the molecules are not symmetrical and the bonds are polar, then the molecules are polar. Examples include formaldehyde (also known as methanal), water and ammonia. Polar substance, such as water and ethanol (CH3COOH), will mix in one another. However, polar and nonpolar liquid substances, such as water and vegetable oil, are immiscible. That is, they will separate from one another, with the less dense substance floating atop the denser one.
|