How to Use a Valence
- 1). Determine the molecular formula of the molecular compound or ion you want to draw. If you're working a problem on a chemistry homework assignment or exam, this information is usually given to you.
- 2). Locate each element on the periodic table, and count how many columns it is away from the left-hand side of the table. Include the column containing the element when you are counting. If the element is in the p-block, the part of the table from boron onwards, SKIP all the columns from scandium to zinc -- the so-called d-block elements. If oxygen is your element, for example, you would count the columns and skip the d-block to get 6, whereas if nitrogen is your element, you would get 5. This number is the number of valence electrons in your element. The valence for your element is usually 8 minus the number of valence electrons -- but not always, as you'll see in a moment.
- 3). Add up the valence electrons for each atom in the compound to get the total number of valence electrons available. If your compound is an ion like the carbonate ion, it will have a charge. If the charge is negative, add it to the number of valence electrons.
Example: Carbonate ion has one carbon atom, three oxygen atoms and a net charge of -2. Consequently, the total number of valence electrons is 4 + 6 + 6 + 6 + 2 for the net charge = 24. - 4). Create a "skeleton structure" for your compound, a map showing where each element is located with respect to the others. Depict each element using the symbol for it shown on the periodic table -- oxygen is O, for example, while lithium is Li. Put the least electronegative element (the one farthest to the left on the table) in the center of your skeleton structure and arrange the others around it. If you find out later that your skeleton structure was flawed, you can always come back to this step and try a new one.
- 5). Draw a bond connecting the central atom(s) to each of the outside atoms. Each bond takes up two electrons. Subtract the number of electrons you've used so far from your total valence electron count to see how many you have left.
- 6). Allocate the remaining electrons by drawing two dots next to an atom (these represent an unbonded pair of electrons, also called a lone pair) or by drawing a second line from an atom to the central atom to make a double bond. Make sure that each atom ends up with eight electrons -- no more, no less. There could be different combinations that will give an atom eight electrons; two double bonds = four single bonds = three single bonds and one lone pair = two single bonds and two lone pairs, for example.
There's an exception to this rule that involves atoms farther down in the periodic table. If the central atom is in period 3 or below, it can accommodate more than eight electrons by using its d-orbitals as well. Elements like sulfur and phosphorus, for example, can have more than eight electrons when they are part of a compound. - 7). Determine the formal charge for each atom. The formal charge is equal to the number of electrons in lone pairs plus one half of the number of electrons in bonds minus the number of valence electrons that atom had originally.
- 8). Rearrange your compound as needed to minimize the formal charges. The best structure will be the one with smaller formal charges. A structure with a formal charge of zero on all atoms, for example, is better than a structure that has a formal charge of -1 on one atom and +1 on another, which is better than a structure that has a formal charge of -2 on one atom and +2 on another. Some compounds will end up with formal charges on different atoms because that's just the nature of that particular compound; if you end up with a structure like this, that's fine, but make sure first that you've eliminated other possibilities that would reduce the formal charges from what you have. If the compound is an ion like the carbonate ion, some of the atoms will end up with formal charges; the sum of the formal charges should be equal to the net charge of the ion if you've done everything correctly.
- 9). Work with some examples. You can practice with some examples at the link under the References section below.
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