Hybrid Orbitals - Molecular Shape PDF

Hybrid Orbitals Its Influence on Molecular Shape Hybridization  Hybridization is the mixing of a set of atomic orbitals to form a new set of atomic orbitals with the same total electron capacity and with properties and energies intermediate between those of the original unhybridi...

Hybrid Orbitals Its Influence on Molecular Shape Hybridization  Hybridization is the mixing of a set of atomic orbitals to form a new set of atomic orbitals with the same total electron capacity and with properties and energies intermediate between those of the original unhybridized orbitals.  Hybridization is the mixing of two or more atomic orbitals of different energies to form new orbitals of the same or equivalent energy and shape. Hybridization  Hybridization is the mixing of two or atomic orbitals of different but compare energies to produce a new set of orbitals (hybrid orbitals)which are equivalent in energy and shape. Characteristics of Hybridization 1. The hybridized orbitals are always equivalent energy and shape. 2. The number of hybridized orbitals formed equal to the number of orbitals that undergo hybridization. 3. Hybridized orbitals form more stable bonds 4. Hybridized orbitals orient themselves preferred directions in space and so give a fix geometry or shape to the molecules Conditions for Hybridization 1. Only the valence shell orbitals of the at0m are hybridized. 2. Orbitals undergoing hybridization should be have only a small difference in their energies. 3. It is not necessary that only half filled orbitals participate in hybridization. Even filled orbitals of the valence shell can take part hybridization. 4. Rearrangement by way of promotion to different orbitals is not an essential condition for hybridization Procedures involved in the Hybridization of Atomic Orbitals Whenever atomic orbitals are to be hybridized or mixed, always note whether the ground state electronic configuration of the atom under discussion should be used or not. Then promote an electron from one orbital to a higher orbital; depending on the type of hybridization involved. The next procedure is to mix the orbitals so that they will be more stable. The number of orbitals which are mixed will be equivalent to the number of orbitals which are formed after the hybridization. Procedures involved in the Hybridization of Atomic Orbitals Observe that the orbitals formed after the hybridization are called hybrid orbitals and they have similar shapes and sizes. Hybridization of Orbitals  How it works for many elements is what is called “hybridization of orbitals”.  In this process, an element creates free electrons by forming a hybrid orbital.  This occurs by combining orbitals of We move one electron the same quantum number. from the pair to the available space in  For BeCl2, we see this: the next orbital type Type of Hybridization- sp hybridization  This is a process in which one s orbital and one p- orbital are mixed to produce two similar/equivalent hybrid sp orbitals directed along a straight line.  The angle between the sp orbitals is 180°.  This is also known as the diagonal hybridization  Each sp hybridized orbital contains the same amount of s and p character.  All beryllium compounds such as BeF , BeH and 2 2 BeCl2 Type of Hybridization- sp hybridization  This is a process in which one s orbital and one p- orbital are mixed to produce two similar/equivalent hybrid sp orbitals directed along a straight line.  The angle between the sp orbitals is 180°.  This is also known as the diagonal hybridization  Each sp hybridized orbital contains the same amount of s and p character.  All beryllium compounds such as BeF , BeH and 2 2 BeCl2 he beryllium atom contains all paired electrons and so must also undergo hybridization. One of the 2s2s electrons is first promoted to the empty 2px2px orbital Type of Hybridization- sp hybridization  The beryllium atom contains all paired electrons and so must also undergo hybridization. One of the 2s electrons is first promoted to the empty 2px orbital Now the hybridization takes place only with occupied orbitals, and the result is a pair of sp hybrid orbitals. The two remaining p -orbitals (py and pz) do not hybridize and remain unoccupied  Type of Hybridization- sp hybridization  The set of sp orbitals appears similar in shape to the original p orbital, but there is an important difference. Hybridization of an s orbital (blue) and a p orbital (red) of the same atom produces two sp hybrid orbitals (yellow). Each hybrid orbital is oriented primarily in just one direction. Note that each sp orbital contains one lobe that is significantly larger than the other. The set of two sp orbitals are oriented at 180°, which is consistent with the  Type of Hybridization- sp hybridization  The set of sp orbitals appears similar in Other molecules whose electron domain geometry is linear and for whom hybridization is necessary also form sp hybrid orbitals. Examples include CO2 and C2H2. Type of Hybridization- sp hybridization Using these sp-orbitals, BeCl2 can now be constructed.  For maximum overlap between the sp orbitals of BeCl2 and p-orbitals of chlorines, the two chlorines atom must lie along the axes of the p-orbitals Type of Hybridization- sp hybridization Using ethyne (C2H2)  In the formation of ethyne, both the carbon atoms undergo 'sp' hybridization leaving two unhybridized orbitals on each (2py,and 2pz).  One 'sp' hybrid orbital of one carbon atom overlaps axially with the 'sp‘ hybrid orbital of the other carbon atom to form C-C sigma bond.  The remaining hybridized orbital of each carbon atom overlaps axially with half filled orbitals of hydrogen forming sigma bonds.  Each of the two unhybridized orbitals of one carbon atom overlaps sidewise with the similar orbitals of the other carbon atom to form two pi bonds. Type of Hybridization- sp hybridization Using ethyne (C2H2)  First, a paired 2s2s electron is promoted to the empty 2py2py orbital Type of Hybridization- sp2 hybridization  This is a process in which one s orbital and two p- orbitals are combined to produce three similar/equivalent hybrid sp2 orbitals.  The angle between the sp2 orbitals is 120°.  All three lie in one plane.  First, a paired 2s2s electron is promoted to the empty 2py2py orbital Type of Hybridization- sp2 hybridization  These three sp2 hybrids lie in a plane and are oriented toward the corners of an equilateral triangle at angles of 120° to one another. One p orbital remains unchanged and is oriented at a 90° angle to the plane of the sp2 hybrids  First, a paired 2s2s electron is promoted to the empty 2py2py orbital Boron trifluoride (BF3)(BF3) is predicted Type of Hybridization- sp2 hybridization  Using Boron trichloride (BCl3)  First, a paired 2s electron is promoted to the empty 2py orbital  This is followed by hybridization of the three occupied orbitals to form a set of three sp2 hybrids, leaving the 2pz orbital unhybridized  First, a paired 2s2s electron is promoted to the empty 2py2py orbital Boron trifluoride (BF3)(BF3) is predicted Type of Hybridization- sp2 hybridization  Using Boron trichloride (BF3)  The geometry of the sp2 hybrid orbitals is trigonal planar, with the lobes of the orbitals pointing towards the corners of a triangle.  Each can bond with a 2p orbital from a fluorine atom to form the trigonal planar BCl3 molecule.  First, a paired 2s2s electron is promoted to the empty 2py2py orbital Boron trifluoride (BF3)(BF3) is predicted Type of Hybridization- sp2 hybridization  Using Boron trichloride (BCl3)  First, a paired 2s2s electron is promoted to the empty 2py2py orbital Boron trifluoride (BF3)(BF3) is predicted Type of Hybridization- sp2 hybridization  Using ethene (C2H4)  C2H4 has an sp2 Hybridization process.  In this Hybridization one ‘s’ and two ‘p’ orbitals are mixed to give three new sp2 hybrid orbitals which all are in the same shape and equivalent energies.  These three sp2 hybrid orbitals are at an angle of 120 degrees and give a trigonal planar shape.  First, a paired 2s2s electron is promoted to the empty 2py2py orbital Boron trifluoride (BF3)(BF3) is predicted Type of Hybridization- sp2 hybridization  Using ethene (C2H4)  Ethene has two 2CH molecules and 4H molecules.  Carbon consists of 6 electrons and hydrogen has 1 electron.  During the formation of CH2=CH2, the electronic configuration of carbon in its ground state (1s2 2s2 2px1 2py1 2pz0) will change to an excited state and change to 1s2 2s1 2px1 2py1 2pz1.  In an excited state, carbon needs an electron to form bonds; one of the electrons from the 2s2 orbital will move to 2pz orbital to give four unpaired electrons.  First, a paired 2s2s electron is promoted to the empty 2py2py orbital Boron trifluoride (BF3)(BF3) is predicted Type of Hybridization- sp2 hybridization  Using ethene (C2H4)  First, a paired 2s2s electron is promoted to the empty 2py2py orbital Boron trifluoride (BF3)(BF3) is predicted Type of Hybridization- sp3 hybridization  This is a process in which one s orbital and three p- orbitals are combined to produce four similar/equivalent hybrid sp3 orbitals.  The angle between the sp3 orbitals is 109.5°.  Each of the four equivalent sp3 hybrid orbitals has two lobes of different phase like an atomic p orbital.  One of the lobes is larger than the other, giving the orbital a directionality. The four large lobes are oriented toward the four corners of a tetrahedron at angles of 109.5°  First, a paired 2s2s electron is promoted to the empty 2py2py orbital Boron trifluoride (BF3)(BF3) is predicted Type of Hybridization- sp3 hybridization  First, a paired 2s2s electron is promoted to the empty 2py2py orbital Boron trifluoride (BF3)(BF3) is predicted Type of Hybridization- sp3 hybridization  Using methane (CH4)  First, a paired 2s2s electron is promoted to the empty 2py2py orbital Boron trifluoride (BF3)(BF3) is predicted Type of Hybridization- sp3 hybridization  The same kind of sp3 hybridization that describes the bonds to carbon in the tetrahedral methane molecule can also be used to describe bonds to nitrogen in the trigonal pyramidal ammonia molecule and to oxygen in the bent water molecule.  First, a paired 2s2s electron is promoted to the empty 2py2py orbital Boron trifluoride (BF3)(BF3) is predicted Type of Hybridization- sp3 hybridization Using ammonia (NH3)  Justlike the carbon atom in methane, the central nitrogen in ammonia is sp3–hybridized. With nitrogen, however, there are five rather than four valence electrons to account for, meaning that three of the four hybrid orbitals are half-filled and available for bonding, while the fourth is fully occupied by a nonbonding pair (lone pair) of electrons.  First, a paired 2s2s electron is promoted to the empty 2py2py orbital Boron trifluoride (BF3)(BF3) is predicted Type of Hybridization- sp3 hybridization Using ammonia (NH3)  The bonding arrangement here is tetrahedral: the three N-H bonds of ammonia can be pictured as forming the base of a trigonal pyramid, with the fourth orbital, containing the lone pair, forming the top of the pyramid.  The lone pair, with its slightly greater repulsive effect, ‘pushes’ the three N-H s bonds away from the top of the pyramid, meaning that the H-N-H bond angles are slightly less than tetrahedral, at 107.3˚ rather than 109.5˚.  First, a paired 2s2s electron is promoted to the empty 2py2py orbital Boron trifluoride (BF3)(BF3) is predicted Type of Hybridization- sp3 hybridization Using water (H2O)  Justlike the carbon atom in methane and nitrogen in ammonia, the oxygen in the atom is sp3– hybridized. With oxygen, however, there are six rather than four/five valence electrons to account for, meaning that two of the four hybrid orbitals are half-filled and available for bonding, while other two are fully occupied by a nonbonding pair (lone pair) of electrons.  First, a paired 2s2s electron is promoted to the empty 2py2py orbital Boron trifluoride (BF3)(BF3) is predicted Type of Hybridization- sp3 hybridization Using water (H2O) A water molecule is ‘bent’ at an angle of approximately 104.5˚. The bonding in water results from overlap of two of the four sp3 hybrid orbitals on oxygen with 1s orbitals on the two hydrogen atoms. The two nonbonding electron pairs on oxygen are located in the two remaining sp3 orbitals.  Boron trifluoride (BF3)(BF3) is predicted Sigma (s) bond and Pi (p) Bond A covalent bond is of two types depending on the type of overlapping between the two atoms:  Sigma (s) bond  Pi (p) Bond  When there is end to end overlapping of atomic orbitals along the internuclear axis, the bond resulted is called Sigma (s) bond.  This type of overlapping between the atomic orbitals is also called “head-on” overlapping or “axial” overlapping. It results when one of the following types of overlapping takes place :  First, a paired 2s2s electron is promoted to the empty 2py2py orbital Boron trifluoride (BF3)(BF3) is predicted Sigma (s) bond and Pi (p) Bond (a) s – s overlapping  Here s-orbital of one atom overlaps with the s- orbital of other atom. An example of this type of overlapping is the formation of hydrogen molecule from two H-atoms. s-orbital s-orbital s-s Overlap H2 Molecule  First, a paired 2s2s electron is promoted to the empty 2py2py orbital Boron trifluoride (BF3)(BF3) is predicted Sigma (s) bond and Pi (p) Bond (b) s – p overlapping  In this type of overlap s-orbital of one atom overlaps with the half filled p-orbital of the other atom as shown below : s-Orbital p-Orbital sp-overlap Examples : HF, HCl etc  First, a paired 2s2s electron is promoted to the empty 2py2py orbital Boron trifluoride (BF3)(BF3) is predicted Sigma (s) bond and Pi (p) Bond (c) p – p overlapping Here p-orbital of one atom overlaps with the p-orbital of the other atom on internuclear axis. It is shown below : p-orbital p-orbital p-p overlap Examples : F2, Cl2, Br2 etc.  First, a paired 2s2s electron is promoted to the empty 2py2py orbital Boron trifluoride (BF3)(BF3) is predicted Sigma (s) bond and Pi (p) Bond Pi (p) Bond  This type of covalent bond is formed by the sidewise overlap of the half filled atomic orbitals. It is also called lateral or sidewise overlap. This type of overlapping takes place perpendicular to the internuclear axis as shown below : p-orbital p-orbital p-p overlap (sideways)  First, a paired 2s2s electron is promoted to the empty 2py2py orbital Boron trifluoride (BF3)(BF3) is predicted Sigma (s) bond and Pi (p) Bond Pi (p) Bond  Although we cannot experimentally observe a pi bond directly (all we can observe are the positions of the atoms), the structure of ethylene provides strong support for its presence.  First, the C C bond length in ethylene is much shorter than in compounds with C C single bonds consistent with the presence of a stronger C C double bond.  Second, all six atoms in C2H4 lie in the same plane.  First, a paired 2s2s electron is promoted to the empty 2py2py orbital Boron trifluoride (BF3)(BF3) is predicted Sigma (s) bond and Pi (p) Bond Pi (p) Bond  First, a paired 2s2s electron is promoted to the empty 2py2py orbital Boron trifluoride (BF3)(BF3) is predicted Sigma (s) bond and Pi (p) Bond  Thestrength of a covalent bond depends upon the extent of overlapping between the atomic orbitals of participating atoms. During the formation of s bond the extent of overlapping is more and hence a Sigma bond is stronger than Pi bond.  First, a paired 2s2s electron is promoted to the empty 2py2py orbital Boron trifluoride (BF3)(BF3) is predicted Sigma (s) bond and Pi (p) Bond Differences between Sigma (s) bond and Pi (p) Bond Sigma (s) bond Pi (p) Bond 1. It is formed by end to end overlapping of 1. It is formed by the sidewise overlapping half filled atomic orbitals. of half filled p-orbitals only. 2. Overlapping takes place along internuclear axis. 2. Overlapping takes place perpendicular to internuclear 3. The extent of overlapping is large and bond formed is axis. stronger. 3. The extent of overlapping is small and bond formed is 4. The molecular orbital formed as a result of overlapping is weaker. symmetrical about the internuclear axis. 4. The molecular orbital formed as a result of overlapping 5. There is free rotation about s bond and no geometrical consists of two lobes above and below the internuclear isomers are possible. axis. 6. The bond can be present alone. 5. There is no free rotation about p bond and geometrical isomers are possible. 7. s and p orbitals can participate in the formation of s bond. 6. The bond is always formed in addition to sigma (s) bond. 7. Only p-orbitals participate in the formation of p bond.  First, a paired 2s2s electron is promoted to the empty 2py2py orbital Boron trifluoride (BF3)(BF3) is predicted Multiple Bonds  The overlap of atomic orbitals to form molecular orbitals and hence covalent bonds which involves the sharing of one electron from each atom.  This sharing of an electron pair results in the formation of single covalent bonds. The majority of chemical bonds are single bonds.  Double and triple bonds are formed when more than one electron pair are shared. The double and triple bonds are described together as multiple bonds.  First, a paired 2s2s electron is promoted to the empty 2py2py orbital Boron trifluoride (BF3)(BF3) is predicted Multiple Bonds  Double bonds are two bonds between two atoms arising from the sharing of two electron pairs or the overlap of two pairs of orbitals from each atom.  Usually one of the two bonds is a strong Sigma (s) bond. The other is a weak Pi (p) Bond which is broken first during a chemical reaction.  Triple bonds are three bonds between two atoms arising from the sharing of three electron pairs or the overlap of two pairs of orbitals from each atom.  As in double bonds, one of the bonds is a strong Sigma (s) bond. The remaining two are weaker Pi (p) Bond that are broken first during a chemical reaction.  First, a paired 2s2s electron is promoted to the empty 2py2py orbital Boron trifluoride (BF3)(BF3) is predicted VSEPR Theory  The Lewis structure of a molecule tells us the number of pairs of electrons in the valence shell of the central atom.  These electron pairs are subject to electrostatic attractions between them. On this basis, R.G.Gillespie (1970) proposed a theory called the Valence-Shell Electron Pair Repulsion or VSEPR (pronounced as ‘Vesper’) theory  It states that : The electron pairs (both lone pairs and shared pairs, surrounding the central atom will be arranged in space as far apart as possible to minimise the electrostatic repulsion between them.  First, a paired 2s2s electron is promoted to the empty 2py2py orbital Boron trifluoride (BF3)(BF3) is predicted VSEPR Theory  The VSEPR theory is simple but remarkably powerful model for predicting molecular geometries and bond angles. While working out the shapes of molecules from this theory, it must be remembered :  Multiple bonds behave as a single electron-pair bond for the purpose of VSEPR. They represent a single group of electrons.  Order of repulsions between lone pair and lone pair (lp-lp), lone pair and bonding pair (lp-bp), and bonding pair and bonding pair (bp- bp) is lp – lp > > lp – bp > bp – bp  When a molecule has lone pairs of electrons, the bonding electron pairs are pushed closer and thus the bond angle is decreased.  First, a paired 2s2s electron is promoted to the empty 2py2py orbital Boron trifluoride (BF3)(BF3) is predicted Molecular Shapes  The shape of a particular molecule is determined by the specific arrangement of atoms in it and the bond angles.  Molecular shapes are important because they are helpful in the investigation of molecular polarity, molecular symmetry or asymmetry. Physical and chemical properties of compounds depend on these factors.  VSEPR theory throws light on the three dimensional shapes of molecules.  So far we have depicted molecules by Lewis structures in the flat plane of paper but all molecules containing three or more atoms are three-dimensional.  First, a paired 2s2s electron is promoted to the empty 2py2py orbital Boron trifluoride (BF3)(BF3) is predicted How To Work Out The Shape Of A Molecule  It is possible to work out the shape of a small molecule that has a formula XYn by applying a few simple rules. We will use ammonia as an example to illustrate the idea.  First, a paired 2s2s electron is promoted to the empty 2py2py orbital Boron trifluoride (BF3)(BF3) is predicted How To Work Out The Shape Of A Molecule  Rule 1 First find the number of bonding pairs of electrons in the molecule. The number of bonding pairs of electrons in the molecule NH3 can be seen in the formula. There must be three bonding pairs of electrons holding the three hydrogens onto the nitrogen.  Rule 2 Find the number of valence electrons (electrons in the outer energy level) on an atom of the central atom (The one of which there is only one.) Nitrogen is in group V, so the nitrogen has five electrons in the outer energy level.  Rule 3 Find the number of lone pairs on the central atom by subtracting the number of bonding pairs (3) from the valence electrons (5) to find the number of electrons (2) that will make up lone pairs of electrons. Divide this number by 2 to find the number of lone pairs, 2/2 = 1  First, a paired 2s2s electron is promoted to the empty 2py2py orbital Boron trifluoride (BF3)(BF3) is predicted How To Work Out The Shape Of A Molecule  Rule 4 Distribute all the electron pairs around the central atom and learn the angles they will make from molecules with no lone pairs.  Rule 5 Learn that the repulsion between lone pairs of electrons is greater than the repulsion between bonding pairs, and subtract 2° from the bond angles for every lone pair.  Rule 6 Learn the names of the shapes. The shapes are named from the position of the atoms and not the position of the orbitals.  First, a paired 2s2s electron is promoted to the empty 2py2py orbital Boron trifluoride (BF3)(BF3) is predicted How To Work Out The Shape Of A Molecule