2.1 Chemical
Elements
A. Matter
1. Matter 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, and S) are commonly found in living things.
B. Atomic Structure
1. Chemical and physical properties of atoms (e.g., mass) depend on the
subatomic particles.
a. Different
atoms contain specific numbers of protons, neutrons,
and electrons.
b. Protons
and neutrons are in the nucleus of atoms; electrons move around the 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.
2. Isotopes have different mass.
a. Isotopes
are atoms with the same number of protons but differ in the number of neutrons;
e.g.,
a carbon atoms 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) Used as tracers of biochemical pathways.
3) Determine age of fossils using radioactive isotopes.
4) Radiation used in medical treatment.
5) Source of radiation used in medical diagnostic procedures including PET
scan.
C. Energy Levels
1. Protons are positively charged; electrons are negatively charged. Oppositely
charged protons and electrons are
attracted to
each other.
2. An atom's proton number
determines its number of electrons and its chemical properties.
3. Arrangement of an 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 the 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 depends upon the 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 give up, accept, or share electrons in order to have a stable outer
shell.
e. Electron 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.
4.Chemical
Formulas and Equations
a. A chemical
formula indicates the number of atoms in each substance; H2O has TWO
Hydrogen
(H) Atoms and ONE Oxygen (O) Atom.
b. The
formula also indicates the number of molecules; 6H2O is six molecules of water.
c. A chemical
equation is always balanced; the same number of each type of atom is on
both sides.
2.2
Compounds and Molecules
A. Molecules
1. Molecules are atoms held together by chemical bonds.
2. Molecules form when two or more
atoms 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. 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. 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 atoms as a line between two atoms; e.g., single covalent bond
(H-H),
double
covalent bond (O=O), and triple covalent bond (N three lines N).
4. Three
dimensional shape of molecules is not represented by structural formulas
but is critical in understanding
the
biological action of molecules: action of insulin, HIV receptors, etc.
D. Nonpolar and Polar Covalent Bonds
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; an 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.
b. Many
hydrogen bonds taken together are relatively strong.
c. Hydrogen
bonds between complex molecules of cells help maintain
structure and function.