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Chapter 2
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The periodic table of the elements show all
elements, including man-made. Of all the
elements on the periodic table, only 92 are
found in nature and only 25 are necessary to
life.
There are 25 different elements necessary to
life can be classiffied into: SPONCH (98%)
and Trace elements (elements that the body
needs in small amounts).
Careful!
This
applies to
humans
only
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Repetition: There are 25 different elements
necessary to life can be classiffied into:
SPONCH (98%) and Trace elements (elements
that the body needs in small amounts).
The smallest particle of an element is an atom.
Different elements have different types of
atoms.
An atom consists of a nucleus (protons and
neutrons) and an outside cloud “electrons”.
Example: Carbon Atom
Electron
cloud
6e–
Atomic number
= the number
of protons.
Each element has
Its own unique number of
Protons. In this case,
Carbon’s atomic
number is 6 (6p)
“Nucleus”
6 Protons
6 Neutrons
6 Electrons
Mass number =
The sum of protons and
neutrons in the nucleus.
In this case, Carbon’s mass
number is 12 (6p +6n)
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Atoms of each element are distinguished by a
specific number of protons or Atomic number.
The sum of protons and neutrons produce the
Atomic mass (weight) of the element.
Atoms of the same element with different number
of neutrons are called isotopes and some
isotopes are radioactive.
Electron arrangement determines the
chemical properties of an atom
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Electrons are arranged in shells.
The outermost shell determines the
chemical properties of an atom.
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These shells are called “energy shells”.
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The following are examples of atoms.
4e2e-
6p+
6n
Carbon (C)
6e2e-
8p+
8n
Oxygen (O)
5e8e2e-
15p+
16n
Phosphorus (P)
2e8e8e2e-
20p+
20n
Calcium (Ca)
Atoms whose shells are not full tend to interact
with other atoms and gain, lose, or share
electrons
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When atoms gain or lose electrons, the atoms
gain or loose a charge and these charged atoms
called ions. The electrical attraction between ions
with opposite charges results in an ionic bond.
Some atoms share outer shell electrons with
other atoms, forming covalent bonds.
Ionic or covalent, atoms joined together by ionic
or covalent bonds form molecules.
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Here we have some examples
Outermost electron shell (can hold 8 electrons)
Electron
HYDROGEN (H)
Atomic number = 1
First electron shell (can hold 2 electrons)
CARBON (C)
Atomic number = 6
NITROGEN (N)
Atomic number = 7
OXYGEN (O)
Atomic number = 8
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Molecules can
be represented
in many ways
Polarity
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A polar molecule results from the unequal
distribution of electrical charges (polar
covalent bond)
A water molecule is a polar
molecule
(–)
(–)
O
H
(+)
H
(+)
The oxygen atom in water is highly electronegative; it attracts the shared
electrons much more than the Hydrogen atoms, so the shared electrons spend
more time near the Oxygen than the Hydrogen atoms
Water is a polar molecule
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The bonds that exist between 2 polar
molecules are called hydrogen bonds - very
weak (compared to ionic and covalent)
Hydrogen
bonds
Living Organisms are composed of two types of
molecules:
Inorganic (without carbon): Organisms are
composed of 70-90% H2O.
Organic (with carbon):
Carbohydrates
Lipids
Proteins
Nucleic Acids
The Molecule of Water
Like no other common substance, water
exists in nature in all three physical states:
as a solid
„ as a liquid
„ as a gas
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Why Is Water So Important to Life?
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1. Water Molecules Tend to Stick
together. This is called Cohesion among
water molecules.
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Cohesion due to
hydrogen bonding,
water molecules
can move from a
plant’s roots to its
leaves
2. Water’s hydrogen bonds moderate
temperature
Water is able to absorb a great deal of heat
energy without a large increase in temperature
and as it cools, a slight drop in temperature
releases a large amount of heat.
3. Evaporative cooling.
A water molecule takes a
large amount of energy
with it when it evaporates
This leads to evaporative
cooling.
Water is the solvent of life
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Because of water’s polar properties (and hence
its hydrogen bonds), water is an incredible
solvent
A solvent is the dissolving agent of a solution, a
liquid consisting of uniform mixture of 2 or more
substances
The substance that is being dissolved is called
a solute (think “u” goes into solution)
Ion in
solution
Salt
crystal