
Bruce P. answered 08/30/17
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20+ year college biology/genetics teacher; I want you to understand.
When you see the lines in a molecular drawing (such as H-O-H) those represent a covalent bond, or shared pair of electrons. Each atom is contributing ONE of the pair, but each atom 'counts' both electrons towards fulfilling the 'octet rule' (eight electrons in the outer shell)*** [note that hydrogen, helium have outer shells of only TWO). In a double bond, both atoms contribute TWO electrons, and it's written like: O=C=O (carbon dioxide).
Note there is also unequal sharing; look up 'electronegativity' to learn more. Briefly, oxygen 'pulls harder' on the shared electrons than hydrogen does, so oxygen gets 'more time' with them... and therefore ends up a little bit negative in charge overall, while hydrogen ends up a little bit positively charged. This is the origin of 'hydrogen bonding', a NON-covalent interaction between TWO different water molecules, where the oxygen (partial negative) on one has a weak interaction with a hydrogen on the other (partial positive)