When you are assessing the properties of a compound, such as its ability to take part in a reaction, you need to know whether it is likely to have single or multiple bonds. Write the Lewis structures of

(a) Water, H₂O;

(b) Methanal, H₂CO; and

(c) The chlorite ion, ClO₂¯. Use the rules in Toolbox 2B.1; note that you must add one electron for the negative charge of ClO₂¯.

ANTICIPATE It is hard to anticipate Lewis structures when you are starting out in your studies, but as you gain experience, you will be able to write them down without going through the systematic procedure used here.

PLAN Go through the steps set out in Toolbox 2B.1.

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Toolbox 2B.1 HOW TO WRITE THE LEWIS STRUCTURE OF A POLYATOMIC SPECIES CONCEPTUAL BASIS A valid Lewis structure accommodates all the valence electrons to give each atom, if possible, an octet (or duplet). PROCEDURE Step 1 Count the number of valence electrons on each atom; for ions, adjust the number of electrons to account for the charge. Divide the total number of valence electrons in the molecule by 2 to obtain the number of electron pairs. Step 2 Write down the most likely arrangements of atoms by using common patterns and the clues indicated in the text. Step 3 Place one electron pair between each pair of bonded atoms. Step 4 Complete the octet (or duplet, in the case of H) of each atom by placing any remaining electron pairs around the atoms. If there are not enough electron pairs, form multiple bonds in place of one or more single bonds. Step 5 Represent each bonding electron pair by a line. To check on the validity of a Lewis structure, verify that each atom has an octet or (for hydrogen) a duplet. As explained in Topic 2C, a common exception to this rule arises when the central atom is an atom of an element in Period 3 or later. Such an atom can accommodate more than eight electrons in its valence shell. Consequently, the lowest energy Lewis struc- ture may be one in which the central atom has more than eight electrons. This procedure is illustrated in Examples 2B.1 and 2B.2.