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Ch. 2: The Chemical Context of Life

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2.1 An element is a substance that cannot be broken down to other substances by chemical reactions. A compound is a substance consisting of two or more different elements combined in a fixed ratio. Essential elements are needed for a healthy life and to reproduce. Humans need 25, plants need 17 Oxygen (O), Carbon (C), Hydrogen (H) and Nitrogen (N)?make up 96% of living matter Calcium (Ca), Phosphorus (P), potassium (K), sulfur (S), and a few other elements accounting for most of the remaining 4% of an organism's mass. Trace elements are required by an organism in only minute quantities. ? 2.2 Protons and neutrons are electrically charged. Proton= positive, electron=negative, neutron=neutral
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