COVALENT BONDS IN COMPOUNDS

4.5 COVALENT BONDS IN COMPOUNDS

Consider next some example covalent bonds between atoms of some of the lighter elements. These are best understood in reference to Figure 4.2 , the abbreviated version of the periodic table showing the Lewis symbols (outer shell valence electrons) of the first 20 elements. As is the case with ions, atoms that are covalently bonded in molecules often have an arrangement of outer shell electrons Consider next some example covalent bonds between atoms of some of the lighter elements. These are best understood in reference to Figure 4.2 , the abbreviated version of the periodic table showing the Lewis symbols (outer shell valence electrons) of the first 20 elements. As is the case with ions, atoms that are covalently bonded in molecules often have an arrangement of outer shell electrons

Higher

Distance between hydrogen atom nuclei

Bond energy, 435 kJ/mole Lower

Bond length, 75 pm

Figure 4.9 Relationship between energy and distance between hydrogen nuclei (energy diagram of H 2 ). The nuclei are shown at the most energetically favored distance apart, that is, the bond length of the H 2 atom.

As shown in Figure 4.10 , when each of the 4 H atoms shares an electron with 1

C atom to form a molecule of methane, CH 4 , each C atom attains an 8-electron outer shell (octet) like neon. Every hydrogen atom attains an outer shell of 2 electrons through the arrangement of shared electrons. There is a total of 4 covalent bonds, 1 per H attached to the C atom, and each composed of a shared pair of electrons. Each of the hydrogen atoms has 2 electrons like the noble gas helium and carbon has an octet of outer shell electrons like the noble gas neon.

Also shown in Figure 4.10 , nitrogen bonds with hydrogen such that 1 N atom shares 2 electrons with each of 3 H atoms to form a molecule of ammonia, NH 3 . This arrangement gives the N atom 8 outer shell electrons, of which 6 are shared with H atoms and 2 constitute an unshared pair of electrons. In both CH 4 and NH 3 , each H atom has 2 shared electrons, which provides a shell of 2 electrons like that in the noble gas helium.

Figure 4.10 Formation of stable outer electron shells by covalent bonding in compounds. In discussing covalent bonds and molecules it is sometimes convenient to use the

term central atom in reference to an atom to which several other atoms are bonded. In the case of CH 4 which was just discussed, C is the central atom. For NH 3 nitrogen, N, is the central atom. The use of Lewis symbols and formulas in the preceding examples readily shows how many atoms are bound together in each of the compounds and the types of bonds in each. The products of each reaction are shown in two ways, one in which all the valence electrons are represented as dots, and the other in which each pair of valence electrons in a chemical bond is shown as a dash, and the unbonded valence electrons as dots. In cases where all that is needed is to show which atoms are bonded together and the types of bonds (one dash for a single covalent bond, two for

a double bond of 2 shared electron pairs, and three for a triple bond of 3 shared electron pairs), the dots representing unshared electrons may be omitted. Figure 4.11 shows covalent bonding in the formation of hydrogen compounds of O and F. In the case of oxygen, 2 H atoms are combined with 1 O atom having 6

valence electrons, sharing electrons such that each H atom has 2 electrons and the

H :: O Also represented as H O

H . . F H : F : Also represented as HF

Figure 4.11 Covalent bonding of H to O and F to produce H 2 O and HF, respectively.

O atom has 8 outer shell electrons, 4 of which are in 2 shared pairs. To form HF, only 1 H atom is required to share its electron with an atom of F having 7 outer shell electrons, leading to a compound in which the F atom has 8 outer shell electrons, 2 of which are shared with H.

Of the compounds whose formation is shown above, CH 4 is methane, the simplest of the hydrocarbon compounds and the major component of natural gas. The compound formed with nitrogen and hydrogen is ammonia, NH 3 . It is the second most widely produced synthetic chemical, a pungent-smelling gas with many uses in the manufacture of fertilizer, explosives, and synthetic chemicals. The

product of the reaction between hydrogen and oxygen is, of course, water, H 2 O. (Another compound composed only of hydrogen and oxygen is hydrogen peroxide,

H 2 O 2 , a reactive compound widely used as a bleaching agent). Hydrogen fluoride, HF, is the product of the chemical combination of H and F. It is a toxic corrosive gas (boiling point 19.5°C) used as a raw material to manufacture organofluorine com- pounds and Teflon plastics.

Having seen how hydrogen combines with carbon, nitrogen, oxygen, and fluorine, it is easy to predict the hydrogen compounds with the elements directly below C, N, O, and F in the third period of the periodic table. These elements are silicon, phosphorus, sulfur, and chlorine, atomic numbers 14 through 17. As shown

in Figure 4.12 , these elements combined with hydrogen have covalent bonds such that every hydrogen atom has 2 outer shell electrons and each of the other elements has an octet of outer shell electrons.

.. H .. H .. Si :

H : Si : H (SiH 4 ) H : P : (PH 3 )