How do you tell how many electrons an element wants to gain or lose?

How does the law of conservation of mass apply  to this reaction: C2H4 + O2 → H2O + CO2?

the population of unionid mussels in the hudson river decreased after zebra mussels were introduced. zebra mussels, a nonnative species, competed with the unionid mussels for food. the zebra mussels were able to outcompete the unionid mussels, allowing the zebra mussel population to increase dramatically over six years. the unionid mussel population decreased due to a lack of food.

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