Chemistry Notes BI

Chemistry Notes

I. Matter Is Composed of Elements

A. Matter

1. Matter refers to anything that takes up space and has mass.

2. All matter (living and nonliving) is composed of basic elements.

a. Elements cannot be broken down to substances with different chemical or physical properties.

b. Six elements (C, H, N, O, P, S) are commonly found in living things.

B. Elements Contain Atoms

1. Chemical and physical properties of atoms (e.g., weight) depend on the subatomic particles.

a. Different atoms contain specific numbers of protons, neutrons, and electrons.

b. Protons and neutrons are in nucleus of atoms; electrons move around nucleus.

c. Protons are positively charged particles; neutrons have no charge; both have about 1 atomic mass unit of weight.

d. Electrons are negatively charged particles; weight about 1/1800 atomic mass unit.

2. Isotopes have different weights.

a. Isotopes are atoms with the same number of protons but differ in number of neutrons; e.g., a carbon atom has six protons but may have more or less than usual six neutrons.

b. Isotopes have many uses:

1) Determine diet of ancient peoples by determining proportions of isotopes in mummified or fossilized human tissues

2) Tracers of biochemical pathways

3) Determine age of fossils using radioactive isotopes

4) Source of radiation used in medical diagnostic and treatment procedures

C. Atoms Have Chemical Properties

1. Protons are positively charged; electrons are negatively charged; Oppositely charged protons and electrons are attracted to each other.

2. Atom’s proton number determines atom’s number of electrons and its chemical properties.

3. Arrangement of atom’s electrons is determined by total number of electrons and electron shell they occupy.

a. Energy is the ability to do work.

b. Electrons with least amount of potential energy are located in K shell closest to nucleus; electrons having more potential energy are located in shells farther from nucleus.

c. Atomic Configurations

1) Bohr model helps determine number of electrons in outer shell.

2) Inner shell contains up to two electrons; additional shells contain eight electrons.

3) Elements are arranged in rows in periodic table according to number of electrons in outer shell.

d. How atoms react with one another is dependent upon number of electrons in outer shell.

1) Atoms with filled outer shells do not react with other atoms.

2) In atom with one shell, outer shell is filled when it contains two electrons.

3) For atoms with more than one shell, the octet rule applies; outer shell is stable when it contains eight electrons.

4) Atoms with unfilled outer shells react with other atoms so each has stable outer shell.

5) Atoms can give up, accept, or share electrons in order to have a stable outer shell.

e. Electrons Occupy Orbitals

1) Orbital is a volume of space where rapidly moving electrons are predicted to be found.

2) An orbital has a characteristic energy state and a characteristic shape.

3) At first energy level (K shell), there is only one spherically shaped orbital, where at most two electrons are found about the nucleus.

4) At second energy level (L shell), there is one spherically shaped orbital and three dumbbell shaped orbitals; the second energy level contains at most eight electrons.

5) Higher energy levels may contain more orbitals; however, outer shells have a maximum of four orbitals and eight electrons.

II. Atoms Form Compounds and Molecules

A. Molecules

1. Molecules are atoms held together by chemical bonds.

2. Molecules form when two or more atoms of same element react with one another (e.g., O2).

3. Two or more different elements react or bond together to form a compound (e.g., H2O).

4. Electrons possess energy; bonds that exist between atoms in molecules contain energy.

B. Opposite Charges in Ionic Bonding

1. Ionic bonds form when electrons are transferred from one atom to another.

2. Losing or gaining electrons, atoms participating in ionic reactions fill outer shells, and are more stable.

3. Example: sodium with one less electron has positive charge; chlorine has extra electron that has negative charge. Such charged particles are called ions.

4. Attraction of oppositely charged ions holds the two atoms together in an ionic bond.

C. Sharing in Covalent Bonding

1. Covalent bond results when two atoms share electrons so each atom has octet of electrons in outer shell.

2. Hydrogen can give up electron to become hydrogen ion (H+) or share with another atom to complete its outer shell of two electrons.

3. Structural formulas represent shared atom as a line between two atoms; e.g., single covalent bond (H H), double covalent bond (O O), and triple covalent bond (N N).

D. Oxidation Is the Opposite of Reduction

1. Oxidation merely means the loss of electrons (or loss of hydrogen atoms).

2. Reduction merely means the gain of electrons (or gain of hydrogen atoms).

3. In ionic reaction Na + Cl Na+Cl-, sodium has been oxidized, chlorine has been reduced.

E. Some Covalent Bonds Are Polar

1. In nonpolar covalent bonds, sharing of electrons is equal.

2. With polar covalent bonds, the sharing of electrons is unequal.

a. In water molecule (H2O), sharing of electrons by oxygen and hydrogen is not equal; the oxygen atom with more protons dominates the H2O association.

b. Attraction of an atom for electrons in a covalent bond is called electronegativity; oxygen atom is more electronegative than hydrogen atom.

c. Oxygen in water molecule, more attracted to electron pair, assumes small negative charge.

3. Hydrogen Bonding

a. Hydrogen bond is weak attractive force between slightly positive hydrogen atom of one molecule and slightly negative atom in another or the same molecule.

1) Many hydrogen bonds taken together are relatively strong.

2) Hydrogen bonds between complex molecules of cells help maintain structure and function.

III. Water Is Essential to Life

A. Life Evolved in Water

1. All living things are 70-90%.

2. Because water is a polar molecule, water molecules are hydrogen bonded to each other.

3. With hydrogen bonding, water is liquid between 0° C and 100° C which is critical for life.

B. Water Has Unique Properties

1. The temperature of liquid water rises and falls more slowly than that of most other liquids.

a. Calorie is amount of heat energy required to raise temperature of one gram of water 1° C.

b. Because water holds heat, its temperature falls more slowly than other liquids; this protects organisms from rapid temperature changes and helps them maintain normal temperatures.

2. Water has a high heat of vaporization.

a. Hydrogen bonds between water molecules require a large amount of heat to break.

b. This property moderates earth’s surface temperature; permits living systems to exist here.

c. When animals sweat, evaporation of the sweat takes away body heat, thus cooling the animal.

3. Water is universal solvent, facilitates chemical reactions both outside of and within living systems.

a. Water is a universal solvent because it dissolves a great number of solutes.

b. Ionized or polar molecules attracted to water are hydrophilic.

c. Nonionized and nonpolar molecules that cannot attract water are hydrophobic.

4. Water molecules are cohesive and adhesive.

a. Cohesion allows water to flow freely without molecules separating, due to hydrogen bonding.

b. Adhesion is ability to adhere to polar surfaces; water molecules have positive, negative poles.

c. Water rises up tree from roots to leaves through small tubes.

1) Adhesion of water to walls of vessels prevents water column from breaking apart.

2) Cohesion allows evaporation from leaves to pull water column from roots.

5. Water has a high surface tension measured by how difficult it is to break the surface of a liquid.

a. As with cohesion, hydrogen bonding causes water to have high surface tension.

b. Permits a rock to be skipped across pond surface; supports insect walking on water surface.

6. Unlike most substances, frozen water is less dense than liquid water.

a. Below 4° C, hydrogen bonding becomes more rigid but open, causing expansion.

b. Because ice is less dense, it floats; therefore, bodies of water freeze from the top down.

c. If ice was heavier than water, ice would sink and ponds would freeze solid.

C. Water and Acids and Bases

1. Covalently bonded water molecules ionize; the atoms dissociate into ions.

2. When water ionizes or dissociates, it releases a small (107 moles/liter) but equal number of H+ and OHB ions; thus, its pH is neutral.

3. Water dissociates into hydrogen and hydroxide ions: H O H H+ + OH-.

4. Acid molecules dissociate in water, releasing hydrogen ions (H+) ions: HCl Cl H+ + Cl-.

5. Bases are molecules that take up hydrogen ions or release hydroxide ions. NaOH Cl Na+ + OH-.

6. The pH scale indicates acidity and basicity (alkalinity) of a solution.

a. Measure of free hydrogen ions as a negative logarithm of the H+ concentration (-log [H+]).

b. pH values range from 0 (100 moles/liter; most acidic) to 14 (1014 moles/liter; most basic).

1) One mole of water has 107 moles/liter of hydrogen ions; therefore, has neutral pH of 7.

2) Acid is a substance with pH less than 7; base is a substance with pH greater than 7.

3) As logarithmic scale, each lower unit has 10× amount of hydrogen ions as next higher pH unit; as move up pH scale, each unit has 10× basicity of previous unit.

7. Buffers keep pH steady and within normal limits in living organisms.

a. Buffers stabilize pH of a solution by taking up excess hydrogen or hydroxide ions.

b. Carbonic acid helps keep blood pH within normal limits: H2CO3 H+ + HCO3-.