Transcript Ch. 2 The Chemical Basis of Life

Chapter 2
Matter Consists of Elements
 Elements cannot be broken
down by ordinary chemical
means
 Atoms are unique building
blocks for each element
 Atomic structure
 Compound
 A substance consisting of
two or more different
elements combined in a
fixed ratio
 Ex. (C6H12O6)
Atomic Structure
 There are three important subatomic particles in atoms
 Neutrons
 No charge in the nucleus
 1 atomic mass unit (amu)
 Protons
 Positive charge, in the nucleus
 1 amu
 Electrons
 Negative charge , orbit nucleus
 Zero amu
 Equal in number to protons in atom
 Atoms of different elements contain different numbers of
subatomic particles
 Compare hydrogen, helium and lithium
Proton
Neutron
Electron
Hydrogen (H)
Helium (He)
Lithium (Li)
Figure 2.2
Atomic Number and Atomic Mass
 Atomic number = number of protons in
nucleus
 Mass number = mass of protons and
neutrons
 Mass numbers of atoms of an element:
are not all identical
 Isotopes: atoms of the same element that
differ in the # of neutrons they contain
Isotopes
Proton
Neutron
Electron
Hydrogen (1H)
Deuterium (2H)
Tritium (3H)
Radioactive Isotopes can Help or Harm Us
The Distribution of Electrons Determines an
Atoms Chemical Properties
 Electrons occupy up to seven electron shells (energy
levels) around nucleus
 Octet rule: Except for the first shell which is full with
two electrons, atoms interact in order to have eight
electrons in their outermost energy level (valence
shell)
 Stable elements have their valence shell fully occupied
 Reactive elements lack a full valence shell and tend to
gain, lose, or share electrons (form bonds) with other
atoms to achieve stability
(a)
Chemically inert elements
Valence shell complete
8e
2e
Helium (He)
2e
Neon (Ne)
(b)
Chemically reactive elements
Valence shell incomplete
1e
Hydrogen (H)
6e
2e
Oxygen (O)
4e
2e
Carbon ©
1e
8e
2e
Sodium (Na)
Covalent Bonds join atoms into molecules through electron
sharing
 Three Important Types of Chemical Bonds
 Covalent, Ionic and Hydrogen
 Covalent Bonds
 Formed when atoms share two or more
valence shell electrons

Molecule = two or more atoms held together by
covalent bonds
 Allows each atom to fill its valence shell at
least part of the time
 Sharing of electrons can be equal or unequal

Equal sharing produces electrically balanced,
nonpolar molecules
 Ex. CO2
 Unequal sharing by atoms with different
electron-attracting abilities produces polar
molecules

Ex. H2O
 Atoms with six or seven valence shell electrons
are electronegative, e.g., oxygen
Ionic bonds are attractions between ions of
opposite charge
 Ions are formed by: transfer of valence shell electrons between
atoms
 Anions (– charge): have gained one or more electrons
 Cations (+ charge): have lost one or more electrons
 Attraction of opposite charges: results in an ionic bond
+
Sodium atom (Na)
Chlorine atom (Cl)
Sodium ion (Na+)
–
Chloride ion (Cl–)
Sodium chloride (NaCl)
Attractive force

Hydrogen
Bonds
between
electropositive

hydrogen of one
Hydrogen bond
molecule and an
electronegative atom

of another molecule
+
–
+
+
–
–
–
+
+
+
–
(a) The slightly positive ends (+) of the water
molecules become aligned with the slightly
negative ends (–) of other water molecules.
Properties of Water
 Charged
 Exists as Gas, Liquid and
Solid
 Universal solvent
 Adhesion and Cohesion (via
H-bonds)
 Surface tension
 Ionizes into acid and base
Properties of Water
 Cohesion
 Water binding to water
 Adhesion
 Water binding to anything that has a
charge

Remember water has a charge and thus
will bind to anything with a charge
Capillary Action