1. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The chemical basis of life

2. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Essential Elements of Life Include carbon, hydrogen, oxygen, and nitrogen  96% of living matter

3. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Covalent bonds Figure 2.10 Between atoms within molecules Hydrogen atoms (2 H) Hydrogen molecule (H 2 ) + + + + ++ In each hydrogen atom, the single electron is held in its orbital by its attraction to the proton in the nucleus. 1 When two hydrogen atoms approach each other, the electron of each atom is also attracted to the proton in the other nucleus. 2 The two electrons become shared in a covalent bond, forming an H 2 molecule. 3

4. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Hydrogen Bonds  – –  + +  + + Water (H 2 O) Ammonia (NH 3 ) O H H  + +  – – N H H H A hydrogen bond results from the attraction between the partial positive charge on the hydrogen atom of water and the partial negative charge on the nitrogen atom of ammonia. ++ ++ Figure 2.15 Between molecules, e.g. water molecules

5. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Photosynthesis – example of a chemical reaction Figure 2.18