CHAPTER 12 CHEMICAL BONDING Core electrons are found close to the nucleus, whereas valence electrons are found in the most distant s and p energy subshells. The valence electrons are responsible for holding two or more atoms together in a chemical bond. Octet rule states that an atom tends to bond in such a way that it acquires eight electrons in its outer shell. In order to obey this rule, atoms either transfer electrons or share one or more pair of electrons. Ionic bond is formed when a metal cation is attracted to a nonmetal anion and are held together by electrostatic attraction, example, NaCl, MgO, AlN. Covalent bond is formed when two nonmetal atoms share valence electrons. Molecules are held together by covalent bond, example, NH 3, H 2 O, CO 2. IONIC BOND Ionic bond results from the attraction between a positively charged cation and negatively charged anion. In the bonding process, energy is released. Electrostatic attraction is similar to the attraction between opposite ends of two magnets. When ionic bond created is strong, they create rigid crystalline structure, for example NaCl. FORMATION OF CATION AND ANION When metal loses its valence electrons, it becomes positively charged, example, Na (11 electrons) and Na + (10 electrons). Main group metals usually achieve a noble gas electron configuration after losing their valence electrons. When a nonmetal atom gains electrons, it becomes negatively charged. Example, chlorine atom (17 electrons) gains 1 electron to become Cl - (17 + 1 =18 electrons) IONIC RADII The atomic radius of sodium atom (0.186nm) reduces once it becomes sodium ion (0.095nm). The reason for this decrease in radius is because sodium atom has lost its 3S energy sublevel. In the case of chlorine atom, the atomic radius (0.099nm) increase when it changes to chloride ion (0.181nm). The reason for this increase in radius is that the additional electron repels the electron already present. In general, the atomic radius of cations is smaller than anions.
COVALENT BOND A covalent bond results from the sharing of electrons by nonmetal atoms. The electrons being shared belong to both nonmetals. Each atom uses these bonding electrons to complete its valence. Since a filled valence shell is very stable, the resulting bond is also stable. Let us consider the formation of hydrogen chloride, during bond formation; hydrogen atom shares its one valence electron with the chlorine atom. This additional electron gives chlorine eight electrons (stable octet Ar) in its valence shell while this additional electron gives hydrogen two electrons (stable octet of He) in its valence shell. The bonding electrons are distributed over each atom, the electrons are free to move about the entire molecule, thus the electrons are said to be DELOCALIZED. BOND LENGTH The distance between two nuclei is called bond length. From the above example, there is overlap of 1S energy sublevel of the hydrogen atom mixing with 3P sublevel of chlorine atom. The mixing of the sublevels draws the two nuclei closer together. The radius of hydrogen atom is 0.037nm while that of chlorine atom is 0.099nm, the formed bond s radius should have been ( 0.037 + 0.099 = 0.136nm) but the experiment showed that the bond length of HCl is 0.127. This explains the overlap of the orbital during bond formation. BOND ENERGY Energy is released when two ions are attracted to one another and form an ionic bond. For example, the attraction of Na + and Cl - to form NaCl released heat energy. In the same way, energy is released when two atoms are attracted and form covalent bond. Bond energy is the amount of energy required to break a covalent bond between two atoms. H (g) + Cl (g) HCl (g) + heat HCl (g) + heat H (g) + Cl (g) ELECTRON DOT FORMULAS OF MOLECULES Guidelines for drawing Electron Dot: Calculate the number of valence electrons from all the atoms in the molecule.
Divide the valence electron total by two to find the number of the electron pairs. The electron pairs (4 pairs) are placed around the central atom and then the remaining atoms in the molecule so as to provide octet. To complete the octet sometimes, the nonbonding electrons are utilized, in this case a double bond is formed. One pair of electrons shared by two atoms forms a SINGLE bond. A molecule can also contain two or three electron pairs between two atoms; these are referred to as DOUBLE or TRIPLE bonds. Double and triple bonds result from an insufficient number of valence electrons around each atom in a molecule. To provide an octet, it may be necessary to move nonbonding electrons between two atoms; these electrons then become bonding electrons. In the structural formula of a molecule, a single bond is shown as dash, a double bond is shown as two dashes, and a triple bond as three dashes. Electron dot formula for ammonium ion, 1. total number of valence electrons in this positive ion is 5 (valence electrons from nitrogen) + 4( valence electrons from hydrogen) 1 ( electron from the charge) = 8 valence electrons. 2. number of paired electrons = 8/2 = 4 paired electrons. 3. the central atom is nitrogen, 4 paired electrons are placed around the central atom. Since there no more paired electrons, the other atoms have no paired electrons on them. 4. the diagram is in the textbook. POLAR COVALENT BOND. Covalent bonds result from sharing of valence electrons. As we discussed earlier, electrons are shared equally, in many instances, one of the two atoms do not share electrons equally. When the electrons are drawn more closely to one of the atoms, the bond is said to be POLARIZED, also known as POLAR COVALENT BOND. ELECTRONEGATIVITY TRENDS Each element has an innate ability to attract valence electrons. This ability is related to Nearness of the valence shell to the nucleus. Magnitude of the positive charge in the nucleus. Linus Pauling, an American chemist, devised a method for measuring the electronegativity values for each of the elements. He assigned
carbon a value of 2.5 and the determined the electronegativity of other elements in relation to carbon. He found that fluorine is the most electronegative element (4.0). Other highly electronegative elements are oxygen (3.5), nitrogen (3.0) and chlorine (3.0). The most electronegative elements are nonmetals. Electronegativity increases from bottom to top also it increases from left to right on the periodic table. DELTA NOTATION FOR POLAR BONDS Lets us consider the molecule H-Cl, the electro negativity of H is 2.1 while Cl is 3.0. Since there is a difference in electro negativity between these two elements (3.0 2.1 = 0.9), the bond in an H-Cl is Polar. Since Cl is more electro negative, the bonding electrons are attracted toward Cl atom and away from H atom. The Cl atom thus becomes slightly negatively charged, whereas, the H atom becomes slightly positively charged. The Greek letter delta ( δ) is used to denote polar bond. δ - is used to indicate atom having partially negative charge. δ + is used to indicate atom having partially positive charge. These symbols are referred to as delta notation. δ+ H- Cl δ- In practice, a bond between two atoms having an electro negativity difference of 0.5 or less is usually considered a non polar bond. NONPOLAR COVALENT BOND Polar bonds result when there is unequal sharing of bonding electrons. In studying Pauling s electro negativity value, we find out that some elements have the same values, for example, N and Cl both have 3.0 while C, S, and I all have 2.5. When the electro negativity of each atom is the same, the bond is not polarized. A covalent bond between two atoms with the same electro negativity is referred to as nonpolar covalent bond or nonpolar bond. DIATOMIC NONPOLAR MOLECULES A diatomic molecule consist of 2 nonmetal atoms joined by a covalent bond. We recall the seven diatomic molecules, H 2, N 2,O 2, F 2, Cl 2, Br 2, and I 2. These molecules exhibit nonpolar covalent bond.
Electro negativity can also be said to occur when there is no difference between the two bonded atoms. COORDINATE COVALENT BOND Covalent bond is formed when an electron pair is shared by 2 atoms. Each nonmetal is surrounded by non-bonding electron pairs to complete its octet. A covalent bond resulting from one atom donating an electron pair to another atom is called a coordinate covalent bond. A good example is ozone molecule (O 3 ) where an oxygen molecule (O 2 ) donates a nonbonding electron pair to an oxygen atom to produce ozone. SHAPE OF MOLECULES The theory to explain the shapes of molecules was an extension of electron dot formulas in which pairs of electron surround a central atom. The theory states that the electron pairs surrounding an atom tend to repel each other and the shape of the molecule is the result of this electron pair repulsion. This model is referred to valence shell electron pair repulsion (VSEPR) VSEPR theory uses the term molecular geometry, or molecular shape, to indicate the arrangement of atoms around the central atom as a result of electron pair repulsion. Bond angle is the angle formed by any two atoms bonded to the central atom. TETRAHEDRAL MOLECULES According to VSEPR, any element with four electron pairs around the central atom has tetrahedral electron pair geometry. The result of a molecule of CH 4, the central C atom is surrounded by four electron pairs that are repelled to the four corners of s 3- dimensional figure called tetrahedron. The molecular shape of CH 4 is tetrahedral, the bond angle is 109.5 0 TRIAGONAL PYRAMIDAL MOLECULES In a molecule of ammonia, the central N atom is surrounded by three pairs of bonding electrons and one pair of nonbonding electron pair. The electron pair geometry is tetrahedral and the bond angle should have been 109.5 0 but experimentally, it was found to be 107 0. VSEPR explains the smaller bond angle by suggesting that the nonbonding electron pair exerts a stronger repelling force than the
bonding pairs, thus, the H atoms are pushed closer together ( from 109.50 to 1070) The molecular shape formed is trigonal pyramidal. BENT MOLECULES Water molecule has the central O atom is surrounded by two bonding electron pairs and two nonbonding electron pairs. The predicted shape should be tetrahedral with 109.5 0, but experimentally, it was found to be 104.5 0. The reason for this is again because the nonbonding electron pairs exert a greater repelling force than the bonding pairs. The resulting bond angle is smaller, allowing more space for the unshared electron pairs. The molecular shape is said to be bent of v-shaped. LINEAR MOLECULES In the molecule carbon dioxide, the central atom is bonded to each oxygen atom by two bonding electron pairs; there is a double bond to each O atom (O=C=O). Since the four electron pairs are on opposite sides of the C, the three atoms lie in a straight line. The electron pair geometry is linear and predicted bond angle is 180 0. SUMMARY OF VSEPR THEORY Bonding/nonbonding Electron pair Electron pair Geometry Molecular shape Bond Angle Example Molecule 4/0 Tetrahedral Tetrahedral 109.5 0 CH 4 3/1 Tetrahedral Trigonal 107 0 NH 3 planar 2/2 Tetrahedral Bent 104.5 0 H 2 O 4/0 Linear Linear 180 0 CO 2 3/0 Trigonal planar Trigonal planar 120 0 CH 2 O NONPOLAR MOLECULES WITH POLAR BONDS Carbon tetrachloride is a nonpolar molecule even though it has polar bonds because the attractive forces exerted by the four polar bonds cancel each other. C-Cl is a polar bond, the four C-Cl bonds formed molecular shape tetrahedral, indicating the four Cl atoms are located at the corners of a tetrahedron.
Consequently, a molecule may contain polar bonds and yet be nonpolar. In CO 2, there are two polar bonds, while O=C=O is nonpolar because the more electronegative oxygen atom pull equally in opposite directions. On the other hand, formaldehyde is a polar molecule because HC=O molecule has only one O atom. Oxygen atom pulls electrons away from the central carbon atom and the two hydrogen atoms have little effect on it. The molecular shape of formaldehyde is trigonal planar.