LS1a Fall 2008 9-18 lecture notes

LS1a Fall 2008 9-18 lecture notes - page 17 Covalent...

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Unformatted text preview: page 17 Covalent bonding Water (H 2 O) = = O H H Carbon dioxide (CO 2 ) = = Nitrogen (N 2 ) = = Valence bond theory: Each atom in a molecule forms as many bonds as it needs to Fll its outermost shell If the electronegativity difference between two atoms in a bond is small, the bond is called a “covalent” bond. When two atoms have similar electronegativities, they share their valence electrons when they interact with each other rather than one giving up an electron and the other acquiring an electron. Most of the chemical bonds between atoms in the molecules of life are covalent bonds . There are different types of covalent bonds: single bonds, double bonds, and triple bonds. In a single covalent bond, two valence electrons are shared by the two adjacent atoms. ¡or example, a water molecule is made of one oxygen atom connected to each of two hydrogen atoms through single covalent bonds. We have different ways of understanding bonding -- that is, different intellectual frameworks for picturing and talking about what chemical bonding is. We use more than one kind of simpliFcation, or model, in talking about bonding because each one highlights different properties of atoms and molecules. The simplest way of understanding why the SPONCH atoms form different numbers of single bonds to other atoms is called valence bond theory. Valence bond theory simply states that each atom forms as many bonds as it needs to in order to completely Fll its outermost shell. Oxygen is in Group VI and it has six valence electrons (red), but there is room for eight electrons in its outermost shell. Each hydrogen atom has one valence electron (blue), but there is room for two in its outermost shell. When two hydrogen atoms and one oxygen atom come together to form water, oxygen shares a valence electron with each of the two hydrogen atoms to form two covalent bonds, as depicted. Each hydrogen atom now has two valence electrons and so their outermost shells are complete. The outermost shell of the oxygen atom is now also complete, because it contains the two additional electrons from the hydrogens. page 18 Covalent bonding Water (H 2 O) = = O H H Carbon dioxide (CO 2 ) = = Nitrogen (N 2 ) = = Atoms can fll their outermost shells by Forming single bonds to several atoms or by Forming double or triple bonds to the same atom We can represent the sharing oF valence electrons in a variety oF ways. One way is with the Lewis “dot” structure in which covalent bonds are represented by pairs oF dots (electrons) located between adjacent atoms. This is a good way oF keeping track oF where the valence electrons in a molecule are, but it is not the best or easiest representation to draw. In the more commonly used Lewis structure, we depict a single covalent bond by a single short line connecting two atoms, and we understand that single bond to be composed oF two electrons shared between the atoms....
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This note was uploaded on 09/23/2008 for the course LIFESCI 1a taught by Professor Kahne during the Fall '08 term at Harvard.

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LS1a Fall 2008 9-18 lecture notes - page 17 Covalent...

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