Pre-Quiz
What is a valence electron?
Chapter 2: The Chemical Context of
Life
Objectives of Learning:
1.
2.
3.
4.
5.
6.
Atomic structure determines the behavior of an element
Life requires elements and compounds
Atoms combine by chemical bonding
Weak bonds play an important role in the chemistry of life
A molecule’s biological function is related to its shape
Chemical reactions make and break bonds.
Themes: Emergent Properties, Structure
and Function
CHAPTER 2 THE CHEMICAL CONTEXT OF
LIFE
What you do NOT need to know:
1. Exact atomic numbers, atomic mass, or Electron
orbitals.
2. Table 2.1; Fig. 2.9; & Methods Box pg. 30. Table
2-1 - you should become familiar with the major
elements in the human body not the atomic #’s & %
column. Also Fig. 2.15; 2.16 & 2:18.
The Test will give you atomic number & mass. You
will use them to explain atomic structure & bonding
properties.
Root Words – Index Cards
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AnCo-valent
ElectroIsoNeutrPro-
You should know:
1. Atomic number = # of protons & electrons.
Each has atomic mass of approx. 1 dalton (1.7 x
10-24 grams.). Thus all elements are electrically
neutral. (protons are +1 charge & electrons are -1
charge.)
2. Atomic mass (wt) = sum of protons & neutrons.
3. Energy levels (shells; clouds) outside the
nucleus (holds the protons & neutrons) have
maximum of 2 in the first shell, 8 in the second,
and 8 in the third).
4. Life made of about 25 elements, 4 major ones
(96%) are: O (1st); C (2nd); H (3rd); N (4th).
Example: Neon has Atomic # of 10; Atomic
weight of 20 so there are:
10 protons; 10 neutrons (20-10) and 10 electrons.
Shells = 2 in the first (fill first shell first); 8 in the
second.
Note: what
happens to PE
(potential
energy) as
electrons move
from lower to
outer shells &
vice versa..
Chemical properties of an atom - is determined
by the # of outer ( or valence) electrons (e-).
1. An atom with a completed valence shell (8 e-) is
un-reactive & will not combine with other elements
to make a compound. They are INERT. These are
atoms in the last column of the periodic table. The
“happy” atom has a full valence shell.
2. All other atoms are chemically reactive because
their valence shell does not have 8 e-.
3. Valence 1 electron in outer shell = +1 valence; 2
in outer shell = +2; 3 in outer shell = +3; 5 in outer
shell = -3; 6 in outer shell = -2; 7 in outer shell = -1.
Think of Valence as the charge the atom will have when it
gains or losses electrons in a chemical reaction.
Chemical Bonds – Atoms are held together to make
compounds either by sharing or transferring their
valence electrons.
1. Strongest bonds are Covalent (sharing valence
e-) & ionic (transfer of valence e- from one
element to another).
NOTE: a = single
bond; b = double bond;
c = 2 single bonds; d =
simplest organic
compound. Sharing is
also in number but not
force. These are
structural formulas.
Covalent bonding - shared
Molecular Formula - (like H20) shows only what the
compound consists of.
Carbon & Covalent Bonding - Always 4 bonds. Either
4 singles; 2 doubles; a double & 2 singles; or a triple &
one single.
Electronegativity - Attraction of an atom for e-.
The more electronegative an atom is the more it
pulls shared e- toward it.
Nonpolar covalent bond – e- are shared equally
(like H2 & 02). These are diatomic molecules,
because they are 2 of the same atom.
Polar covalent bonds - one atom is more
electronegative than another. This means electrons
in a bond are not shared equally.
Example: H20
NOTE: O (the most
electronegative of all
“life” elements) attracts
the shared e- more
strongly than H (low
electronegativity).
H-0 bonds are always very
polar.
Chart of Electronegativities in Elements: Note
that O is much more electronegative than any other
element except F (which is not found in most
biological systems).
Ionic & Covalent Bond Comparisons
NOTE: The difference between ionic & covalent
bonds is not always a clear cut line.
IONIC BONDS: Atoms are so unequal in their
attraction for valence e- that the more
electronegative atom strips an e- completely
away from its partner.
Atoms then become ions. If the atom gains
electrons it becomes a negative (anion) ion. If
the atom loses electrons it becomes a positive
(cation) ion.
Ionic Bond - is the attraction between atoms, or
ions due to the fact that they are unlike charged.
Ionic Compounds - are called salts. (ex. NaCl). They
have stronger bonds as solids & weaker in solution.
IMPORTANCE OF WEAK CHEMICAL BONDS IN
CHEMISTRY OF LIFE:
The connection between molecules always them to
come in contact, respond to each other & then
separate.
Why would this be important in the body or other
organisms?
EX. OF WEAK BONDS - H BONDS
1. Occurs when a hydrogen
atom covalently bonds to an
electronegative atom that
is attracted to another
electronegative atom.
VERY IMPORTANT IN BIOLOGY!!!!………
A molecules biological function is related to
its shape!!
The shape of molecules determine how most
molecules of life recognize & respond to each other.
Example of a natural
& man-made
substance that have
areas of the molecule
that have similar
shapes, thus they will
have similar reactions
in the body.
Chemical reactions - making & breaking bonds:
Reactants - go into a reaction. The ingredients
Products - come out of a reaction. The cake
Reactions - some go to completion (all reactants
converted to products.
Most Reactions - are reversible. Products become
reactants & vice versa.