5: Building Bonds in .NET framework

Encoding EAN-13 Supplement 5 in .NET framework 5: Building Bonds

(a) Figure 5-4: Three representations of the formation of a (b) covalent bond in dihydrogen (c)

Atoms can share more than a single pair of electrons When atoms share two pairs of electrons, they are said to form a double bond, and when they share three pairs of electrons they are said to form a triple bond Examples of double and triple bonds are shown with electron dot and line structures in Figure 5-5

A few guidelines can help you figure out the correct Lewis structure for a molecule if you know the molecule s formula As an example, we work out the Lewis structure of formaldehyde, CH2O (Figure 5-6 can help you follow along):

These are the electrons you can use to build the structure Account for any extra or missing electrons in the case of ions For example, if you know your molecule has +2 charge, remember to subtract two from the total number of valence electrons In the case of formaldehyde, C has four valence electrons, each H has one valence electron, and O has six valence electrons The total number of valence electrons is 12

The central atom is usually one that can form the most bonds, which is often the atom with the most empty valence orbital slots to fill In larger molecules, some trial-and-error may be involved in this step, but in smaller molecules, some choices are obviously better than others For example, carbon is a better choice than hydrogen to be the central atom because carbon tends to form four bonds, whereas hydrogen tends to form only one bond In the case of formaldehyde, carbon is the obvious first choice because it can form four bonds, while oxygen can form only two, and each hydrogen can form only one

Each single bond counts for two electrons In the case of formaldehyde, attach the single oxygen and each of the two hydrogen atoms to the central carbon atom

central atom In our example, carbon and oxygen should each have eight electrons in their valence shells; each hydrogen atom should have two However, by the time we fill the valence shells of our outer atoms (oxygen and the two hydrogens), we have used up our allotment of 12 electrons

If the central atom has a full valence shell, then your Lewis structure is drawn properly it s formally correct even though it may not correspond to a real structure If the central atom still has an incompletely filled valence shell, then use electron dots (nonbonding electrons) from outer atoms to create double and/or triple bonds to the central atom until the central atom s valence shell is filled Remember, each added bond requires two electrons In the case of our formaldehyde molecule, we must create a double bond between carbon and one of the outer atoms Oxygen is the only choice for a double-bond partner, because each hydrogen can accommodate only two electrons in its shell So, we use two of the electrons assigned to oxygen to create a second bond with carbon Sometimes a covalent bond is formed in which one atom donates both electrons to the bond, with the other atom contributing no electrons This kind of bond is called a coordinate covalent bond Atoms with lone pairs are capable of donating both electrons to a coordinate covalent bond A lone pair consists of two electrons paired within the same orbital that aren t used in bonding Even though covalent bonding usually occurs between nonmetals, metals can engage in coordinate covalent bonding Usually, the metal receives electrons from an electron donor called a ligand

1 C(4 e-) + H(1 e-) + H(1 e-) + O(6 e-) = 12 e2 Carbon is central atom; it can form more bonds (4) than O, H O