Transcript The Chemical Context of Life

The Chemical Context of Life
How Did Life Originate?
Understanding what life is and how it
originated from non-living matter
requires some understanding of
basic chemistry
In Terms of How They Function,
Living Things Are Literally
Biochemical Machines
So why do atoms interact?
How do they interact?
Chemical behavior results from
two tendencies of atoms:
• Atoms tend to fill their outer
electron shells …the Octet Rule
• Atoms interact with other atoms in
ways that cancel electrostatic
charges (+ and - charges)
There are 92 kinds of atoms in nature,
called elements. Each element has a
characteristic set of protons, neutrons,
and electrons (indicated by the atomic
number in each block).
Electrons are distributed in concentric regions outside the nucleus called
electron shells. Each electron shell has a limited capacity, as shown by
the numbers below:
8
8
8
2
nucleus
An atom’s inner shells fill first because negatively charged electrons are
attracted to the positively charged nucleus
Atoms whose outer shells are filled because they have just the right
number of electrons will be stable and not interact with other atoms.
These are called the inert elements, or noble gases.
8
8
8
2
nucleus
He
2
Ne
Ar
2+8 = 10
2+8+8 = 18 Kr
2+8+8+8 = 26
Inert
elements
Non-inert elements will interact with one
another in ways that fill their outer
electron shells or cancel charges
For example, consider the two smallest elements: hydrogen (1) and helium (2).
(a) Hydrogen atoms have an unfilled shell and will interact with other atoms in
ways that fill the shell.
(b) Helium atoms are non-reactive (and therefore solitary) because their only
(and therefore outer) electron shell is filled to capacity.
Kinds of Attractions Among Atoms
•
Ionic Bonds (attraction between oppositely charged ions)
•
Covalent Bonds (sharing electrons to fill outer shells), two kinds:
 Non-polar Covalent Bonds (electrons shared equally by identical or very
similar atoms, so no partial charges are produced)
 Polar Covalent Bonds (electrons shared unequally by different kinds of
atoms, producing + and - partial charges in participating atoms)
•
Hydrogen Bonds (attraction between partially charged atoms that are also
involved in a polar covalent bonds with additional atoms)
•
Van der Waals attractions (attraction between atoms that induce transient,
opposite, partial charges in one another)
•
Hydrophyllic (water loving) and Hydrophobic (water hating) Influences (charged
regions of molecules are attracted to water molecules, whereas uncharged
regions are repelled from watery environments)
Ionic Bonds
Ionic Bonds, e.g., sodium (Na, 11) and chlorine (Cl, 17)
lost an electron
has a filled outer shell
charged ion
unfilled
outer shell,
almost empty
Na
Na+
uncharged
atom
SALT:
outer shells filled
charges cancelled
stable now
11 p+
11 n
10 e-
11 p+
11 n
11 e-
Ionization: transfer of
electron creates ions
stole an electron
has a filled outer shell
charged ion
17 p+
17 n
17 e-
unfilled
outer shell,
almost full
uncharged
atom
Ionic
Cl
Cl-
Bond
17 p+
17 n
18 e-
NaCl
outer shells
almost empty
cations (+) form
outer shells
almost full
anions (-) form
Covalent Bonds
A hydrogen atom (H) with 1 electron
An unfilled outer electron shell; capacity of 2 electrons
A hydrogen Molecule (H2)
A non-polar covalent bond: 2 equally shared electrons
The electrons are shared equally because the two atoms are identical and
therefore have the same ability to attract electrons (electronegativity)
A hydrogen Molecule (H2)
Because the electrons are shared equally, the positive charges of each nucleus
are cancelled out by the electrons. Therefore hydrogen molecules are uncharged
or non-polar.
The Hindenberg Disaster
Reactive versus Invert Gasses
Oxygen Molecule:
O2
Shared electrons in
molecular orbitals
nucleus
nucleus
Unshared
electrons in
atomic orbitals
Shared electrons orbit both atoms,
although they are sometimes
diagrammed as if they were
between
Atoms that are 2 or more electrons short of a filled outer
shell are likely to fill the shell by sharing electrons from
other atoms, forming covalent bonds.
Carbon has 4 electrons in its
outer shell, needing 4 more
Oxygen has 6 electrons in its
outer shell, needing 2 more
Numbers of Covalent Bonds an Atom Can Form
Atomic Capacity of Electrons in
Number of
Atom number outer shell outer shell covalent bonds
Hydrogen
1
2
1
1
Carbon
6
8
4
4
Nitrogen
7
8
5
3
Oxygen
8
8
6
2
Phosphorus
15
8
5
3
Sulfur
16
8
6
2
P
P
P
Polar Covalent Bonds
Polar Covalent Bonds: Water
Atoms with different electronegativities share electrons unequally. The more
electronegative atom gets more than its fair share of the electron’s time and
therefore has a slightly negative charge. The other atom will have a partial +
charge because it gets less of the electron’s time than is required to cancel the
positive charges in its nucleus.
O
H2O
H
H
Hydrogen Bonds
Hydrogen Bonds:
Attraction between
oppositely charged
atoms engaged in
polar covalent
bonding with
other atoms
water tension: numerous
hydrogen bonds can be
very strong
Van der Waals Interactions
Van der Waals Interactions
Electrons are constantly on the move, mostly in random directions
around the atom’s nucleus.
Even when the number of electrons equals the number of protons,
there will be brief events where an atom’s electrons tend to be
bunched together a little.
This leaves one side of
those atoms + charged
and the other - charged
until the electrons
spread out again.
+
+
-
-
+
3 charged atoms (“dipoles”)
Van der Waals Interactions
(a) as two dipoles approach one another...
-
+
+
-
(b) their electron clouds interact (opposite
charges repel) and the two atoms face
each other with opposite charges.
+
-
+
(c) now they attract each other
-
+ -
+
Van der Waals Interactions
The two atoms will attract
one another until their
electron clouds begin to
overlap and repel the two
atoms …which remain
suspended at a distance
where attraction and
repulsion are balanced
Van der Waals Interactions
Van der Walls interactions are very
weak. The movement of atoms at
room temperature breaks them
apart almost immediately, except...
Biological molecules that interact
often have complementary shapes,
which brings large numbers of
atoms into position for Van der
Waals interaction …the cumulative
effect of which is very strong
Hydrophyllic/Hydrophobic Interactions
Many molecules have polar (charges) and non-polar (uncharged)
regions. The polar regions tend to orient toward the water that the
molecule is suspended in, whereas the no-polar parts fold up within
the interior of the molecules, hidden from water.