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substance consisting of 2 or more elements in a fixed ratio - has characteristics different from those of its elements


composed of individual atoms - cannot be broken down to other substances by chemical reactions

essential elements of life

25 - CHON make up 96% of living matter - most of remaining 4% are Ca, P, K, S, Na, Cl and Mg

Trace elements

required by an organism in minute quantities, <0.01% -ex: iodine, iron

Ionic bonds

result from electrical attractions between atoms that gain or lose valence electrons completely - strong bonds


positively charged ion - loses an electron


negatively charged ion - gains an electron

covalent bonds

form when atoms share a pair of valence electrons rather than gaining or losing - C, O, N and S readily form these - strong bonds


results when electrons are shared unequally - partial charges on the atoms


measure of an atom's attraction for the electrons it shared in a chemical bond with another atom

hydrogen bonds

involve unequal electron sharing - O, N and S share unequally with H - not strong individually, but collectively strong - often critical for maintaing shape of many biological molecules

van der waals forces

weak forces that develop over very short distances between non polar molecules as moving electrons accumulate by chance in one part of a molecule or another - temporary zones of + or - charge make molecule polar - collectively, can stabilize a molecule- gecko


structure of molecules allow them to interact with other molecules - leads to emergent properties - support & maintain living systems

hydrogen bond lattices

make ice float in water - less dense

emergent properties of water

1. expansion upon freezing
2. ability to moderate temp
3. cohesion and surface tension
4. versatility as a solvent

moderation of temperature

water can absorb or release large amounts of heat with only a slight change in its own temperature

evaporative cooling

as a liquid evaporate, its remaining surface cools - helps stabilize temperatures in organisms and bodies of water


hydrogen bonds hold water molecules together - helps the transport of water against gravity in plants


an attraction between different substances - ex: water and plant cell walls

surface tension

a measure of how hard it is to break the surface of a liquid - related to cohesion - causes water to form droplets


has an affinity for water


does not have an affinity for water - oil molecules, have relatively non polar bonds

hydration layer

water molecules are strong and slightly polar, so they ten to coat the surfaces of other polar and charged molecules and ion - reduce attraction between molecules or ions and promote their entry into a solution - prevents polar molecules or ions from reassociating - why water is called "the solvent of life"


any substance that increases the H+ concentration of a solution - proton donor


any substance that reduces the H+ concentration of a solution - proton acceptor


substances that minimize changes in concentrations of H+ and OH- in a solution - consist of an acid/base pair that reversibly combines with H+

acid precipitation

rain, snow or fog with a pH lower than 5.6 - cause by mixing of pollutants with water in the air - can damage life in lakes and streams - contributing to decline of some forests

acid in the oceans

CO₂ + H₂0 → H₂CO₃ (carbonic acid) - corrosive to shellfish and corals - causes "energy tax" where organisms spend more energy maintaining pH and less on reproduction

atomic number

number of protons in nucleus

mass number

number of protons + neutrons

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