1. The Chemical Context of Life
Unit 2:
The Chemical Context of Life

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

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

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

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

6. Chemical Bonds – sharing or transferring valence electrons, creating attractions that hold atoms together
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

7. 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.

8. 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.

10. Hydrogen Bonds – a hydrogen atom that is covalently bonded to an electromagnetic atom is attracted to another electronegative atom; H bonds are noncovalent.
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.

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

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

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

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

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