Transcript chemistry

CHAPTER 2
CHEMISTRY & LIFE
1) Matter consists of chemicals in
pure form (elements) and in
combinations (compounds)
• Organisms are composed of matter.
• Matter takes up space and has mass.
• An element is a substance that cannot be
broken down into other substances by
chemical reactions.
• 92 naturally-occurring elements.
• Each element has a unique symbol.
• A compound is a substance consisting of
two or more elements.
• Table salt (sodium chloride or NaCl) is a
compound.
• Pure sodium is a metal and chlorine is a gas,
their combination forms an edible compound.
2) Life requires about 25 chemical elements
• Four elements - carbon (C), oxygen (O),
hydrogen (H), and nitrogen (N) - make
up 96% of living matter (HONC!!!!)
• Most remaining (4%) of an organism’s
mass consists of phosphorus (P), sulfur
(S), calcium (Ca), and potassium (K).
• Trace elements, e.g., iodine
3) Atomic structure determines the
behavior of an element
• An atom is the smallest unit of matter.
• Atoms are small… teensy weensy
• Atoms are composed of even smaller
parts, called subatomic particles.
What are they??
Subatomic Particles of an Atom
Particle
Charge
Neutrons-Protons
No charge
(+)
-Electrons
(-)
(+)
Protons
Electrons
(-)
Mass
1.009 Dalton*
1.007 Dalton
1/2000 Dalton
* Dalton: unit to express mass (1.66 x 10-24g)
Helium (2He)
Neon Atom ( Ne)
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Atoms and Molecules
Atomic Number – Number of protons in an
atom.
This number is unique for each element.
• The atomic number is written as a subscript before the
symbol for the element (e.g. Helium = 2He).
In a neutral atom,
the # of _________= the # of _________
• Atomic number = number of protons = number of
electrons
Atoms and Molecules
Mass Number – # protons + # neutrons in an
atom.
The number of neutrons in an element can vary
(Isotopes).
• The mass number is written as a
superscript before the symbol for the
element (e.g. Helium = 4He).
How does one calculate the number of Neutrons?
= the Mass Number - Atomic Number
Atoms and Molecules
• Examples::
The element sodium (Na) has a Mass
Number of 23 (23Na) and an Atomic Number
of 11 (11Na) .
How many Protons does Sodium have?
How many Neutrons?
How many Electrons?
What about Carbon:
Or
The Isotope of Carbon:
12
6
14
6
C
C
Atoms and Molecules
-Elements occur as mixtures of isotopes.
e.g., carbon
-Different isotopes of the same element react in
the
same way.
-Some Isotopes are unstable and thus are
radioactive.
Radioactive Isotopes
-Elements occur as mixtures of isotopes.
e.g., carbon
98.89% of carbon atoms have 6 neutrons (12C).
1.11% of carbon atoms have 7 neutrons (13C).
0.0000000001% of carbon atoms have 8 neutrons (14C).
12C
14C
and 13C are stable.
is radioactive.
Electron Energy Levels:
• Electrons = Negatively (-) charged particles that
orbit around the nucleus.
• Electrons have orbitals.
• Each orbital is a certain distance from nucleus
and can only contain 2 electrons
• Orbitals the same distance from the nucleus are
grouped into SHELLS (energy levels)
Electron Orbitals and Electron-shell
Configurations for Neon
( Ne)
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The chemical behavior of an atom is
determined by its electron
configuration
Elements in the periodic table are
grouped together based on their
valence shell electrons
Atoms combine to form molecules
• Atoms with incomplete valence shells
interact by either sharing or transferring
valence electrons.
• These interactions hold atoms together by
attractions called chemical bonds.
A Molecule is two or more atoms held
together by covalent bonds
Atoms combine by chemical bonding
to form molecules
• Chemical bond types:
• Covalent bonds = strong bond
• Ionic bonds = strong bond (but…)
• Weak bonds:
• Hydrogen bonds
• Van der Waal forces
Covalent Bonds
• A covalent bond is the sharing of pairs of
electrons by two atoms.
• If two atoms come close enough that their
unshared orbitals overlap, each atom can
count both electrons toward its goal of filling
the valence shell.
• Every atom has a characteristic total number of
covalent bonds that it can form - an atom’s valence.
(HONC?)
• Hydrogen is 1.
• Oxygen is 2.
• Nitrogen is 3.
• Carbon is 4.
Covalent Bond Polarity
- The attraction of an atom for the electrons of
another atom is called its electronegativity.
- Strongly electronegative atoms attempt to pull
the shared electrons toward themselves.
Electronegativity Scale (FYI):
F = 4.0
O = 3.5
N = 3.0
S and C = 2.5
P and H = 2.1
Li = 1.0
Nonpolar Covalent Bonds
•If electrons in a covalent bond are shared
equally, it’s a nonpolar covalent bond.
• Covalent bond between two atoms of the same
element is always nonpolar.
•A covalent bond between atoms that have similar
electronegativities is also nonpolar.
•Because carbon and hydrogen do not differ
greatly in electronegativities, the bonds of CH4
are nonpolar.
Electrons are shared EQUALLY.
Polar Covalent Bonds
• If the electrons in a covalent bond are not
shared equally by the two atoms, it’s a
polar covalent bond.
• O (3.5) is more electronegative than H (2.1)
• Shared electrons spend more time around
the more electronegative O atom.
( -)
( -)
(+)
(+)
Covalent bond summary:
• Atoms share electrons to fill valence shells
• Nonpolar covalent bond
• Atoms have same or similar electronegativity
• Polar covalent bond
• Atoms have dissimilar electronegativity
• Gives molecule unique properties
Ionic Bonds
• If two atoms are VERY unequal in their
electronegativity one atom strips an electron
completely from the other.
• E.g., sodium with one valence electron in its outer shell
transfers this electron to chlorine with 7 valence
electrons in its outer shell.
• Now, sodium has a full valence shell (the second) and
chlorine has a full valence shell (the third).
• Compounds formed by ionic bonds are called salts.
Biologically Important Weak Bonds
Examples:
1) Hydrogen 2) Ionic (weak in water) 3)Van der Waals
Because these bonds are transient and easily broken,
they can be used for:
1) Cell Signaling
2) Linking Molecules Together
3) 3D shape
Hydrogen Bonds
H-bonds: formed by a charge attraction between a hydrogen
atom that is covalently bonded to an electronegative atom and
another electronegative atom.
Weak Bond: only about 5%
of the strength of a covalent bond
-Allows water to remain as a liquid
over a wide range of temperatures.
-Causes surface
tension
Van der Waal forces
Van der Waals: attractions between small differences in
polarity in a molecule.
A very WEAK BOND and transient, highly affected by
temp etc.
Chemical Reactions
Process of making and breaking chemical bonds
• The starting molecules are called reactants and the
end molecules are called products.
• In a chemical reaction, all of the atoms in the
reactants must be accounted for in the products.
• The reactions
must be
“balanced.”