Water

• Is the most common biological molecule, a reactant for photosynthesis and an important habitat for many organisms (it covers most of the Earth’s surface)
• Around 70% of body mass for most organisms/cells (mammals~65%); aqueous media in us: blood plasma, tears, and urine (also cell sap)
• Very small, with 2 covalent bonds; formula is H_2O, the oxygen is electronegative, slight negative charge (δ-) and the hydrogen is slightly positively charged (δ+)
• The uneven distribution of charge makes the water molecule polar, called dipolar as one end is slightly positive and the other slightly negative
• In water the electron sharing is unequal; oxygen has a larger nucleus and more protons - so has a greater attraction for the electrons compared to hydrogen
• Covalent Bonding is formed when 2 electrons in the outer shells of 2 atoms are shared
• Hydrogen bonds: Water is polar and attracts itself (positive end of one is attracted to the negative end of another), so they form a hydrogen bond ( H-bond) with weak intermolecular forces; this gives water many special properties