Unit 2: The Chemical Context of Life

I. Elements and Compounds A.Matter – anything that takes up space and has mass B.Element – can’t be broken down to other substances by chemical reactions

1)Essential elements – 20-25% of the 92 elements needed by organisms to stay healthy and reproduce 2)Of these essential elements, 96% of living matter is carbon, hydrogen, oxygen, nitrogen. 3)Ca, P, K, S, and few other elements account for most of the remaining 4% 4)Trace elements are only required in minute amounts (Iodine) C.Compound – 2 or more elements combined in a fixed ratio

II. Atoms and Molecules A.Atom – smallest unit of matter that still retains the properties of an element 1)Nucleus contains positive protons and neutral neutrons. 2) Negative electrons form a cloud around the nucleus.

3)The neutron and proton have almost identical masses Atomic # = # of protons ( 2 He); also # of electrons Mass # = protons + neutrons ( 4 He) Atomic Mass = mass of atom (in Daltons) = mass # 4)Isotopes – atom with different # of neutrons than protons, & therefore different mass 5)Radioactive Isotopes – nucleus decays spontaneously, giving off particles & energy.

B.Chemical Bonds – sharing or transferring valence electrons, creating attractions that hold atoms together 1)Covalent – sharing pairs of valence electrons; results in molecule Single, double, and triple bonds are covalent Valence = bonding capacity = # unpaired electrons Electronegativity = attraction for the electrons in a covalent bond

Nonpolar C. B. = electrons of covalent bonds are equally shared by all atoms Polar C. B. = an atom has higher electronegativity, so does not share electrons equally in covalent bonds.

2)Ionic Bonds – two atoms are so unequal in electronegativity, the more electronegative atom strips an electron completely from the other; transfer of electrons Produces ions = charged atom or molecule Cations are positive ions and anions are negative ions. Ionic compounds are also called salts.

3)Hydrogen Bonds – a hydrogen atom that is covalently bonded to an electromagnetic atom is attracted to another electronegative atom; H bonds are noncovalent. 4)Van der Waals Interactions – in nonpolar covalent bonds, there can by instances when a region of an atom is slightly charged. These ever-changing “hot spots” allow atoms and molecules to stick to each other.

III. Chemical Reactions A.Energy and Reactions 1.Energy must be added to break bonds between atoms 2.Forms of energy used Transferred as heat Electricity, Sound, and Light are other forms of energy

B.Conservation of Energy – the total energy before a reaction is equal to the total energy of the products and their surroundings. 1.Chemical energy – energy stored within atoms and molecules that can be released when a substance reacts

C.Exothermic reactions – a chemical reaction that releases energy, also called exergonic D.Endothermic reactions – a chemical reaction requiring energy in order to break bonds in the reactants, also called endergonic

IV. Balancing Chemical Reactions A.Reactants vs. Products 1.Reactants: substances before reaction 2.Products: substances formed after reaction Ex: Na + Cl  NaCl

B.Conservation of Mass – matter can be neither created nor destroyed C.Balancing Chemical Equations 1.Identify the reactants and products 2.Begin balancing one element at a time 3.Add numbers (coefficients) in front of chemical formulas (Don’t change chemical formulas) 4.Continue going back and forth balancing each individual element 5.Make a final count of each individual element before going on