2. Carbon was selected as the key element for life due to its versatile bonding capacity 2.1 Carbon accounts for more than one-half the cell dry weight 2.2 Each carbon atom can form very stable single bonds with one, two, three, or four other carbon atoms, and double or triple bonds can also be formed between two carbon atoms 2.3 Covalently linked carbon atoms can form linear chains, branched chains, and cyclic and cagelike (E 的) structures 2.4 To these carbon skeletons are added functional groups conferring specific activities to the molecules
2. Carbon was selected as the key element for life due to its versatile bonding capacity 2.1 Carbon accounts for more than one-half the cell dry weight. 2.2 Each carbon atom can form very stable single bonds with one, two, three, or four other carbon atoms, and double or triple bonds can also be formed between two carbon atoms. 2.3 Covalently linked carbon atoms can form linear chains, branched chains, and cyclic and cagelike（笼形 的）structures. 2.4 To these carbon skeletons are added functional groups conferring specific activities to the molecules
2. 4 Molecules containing covalently bonding carbon backbones are called organic compounds(including mainly alcohols, amines, aldehydes and ketones, carboxylic acids, sulfhydryls,.. etc. Most biomolecules are organic compounds 2.5 Carbon atoms have a characteristic tetrahedral arrangement of their four single bonds. Carbon-carbon single bonds have freedom of rotation, but not double nor triple bonds 2.6 No other chemical element has the capacity to form molecules of such widely different sizes and shapes or with such a variety of functional groups
2.4 Molecules containing covalently bonding carbon backbones are called organic compounds (including mainly alcohols, amines, aldehydes and ketones, carboxylic acids, sulfhydryls, … etc. Most biomolecules are organic compounds. 2.5 Carbon atoms have a characteristic tetrahedral arrangement of their four single bonds. Carbon-carbon single bonds have freedom of rotation, but not double nor triple bonds. 2.6 No other chemical element has the capacity to form molecules of such widely different sizes and shapes or with such a variety of functional groups
H·+H H: H H-H Filled outer electron shells are Dihydrogen more stable: covalent bonds by :0·+2H :0: H sharing unpaired electrons between H H Water wo atoms H Number of Number of :N·+3H N: H H unpaired electrons in H electrons complete H tor (in red) outer shell Ammonia 2 H H H: C: H H-C-H 2 8 H H Methane 8 :S·+2H H H H ydrogen sulfide H : 0: OH 3H+:P+4·0::O:P:O:H=O=P-O :O: OH
Filled outer electron shells are more stable: covalent bonds by sharing unpaired electrons between two atoms
Versatility of carbon in forming covalent bonds +9:→c:0x0 →C::N .C: C C:::C
Versatility of carbon in forming covalent bonds
10959 109.5 (b) 120° B (c)