Download Ch. 2 The Chemical Basis of Life

Basic Chemistry
CH. 2
Organisms are Composed of Elements
Elements cannot be broken down by ordinary
chemical means
◦ Elements are composed of atoms or unique building
blocks for each element
Compounds
◦ Substances consisting of two or more different
elements combined in a fixed ratio
◦ (C6H12O6 and NaCl)
Molecules
oSubstances consisting of two or more elements;
elements can be the same or different
Atoms and Atomic Structure
 Three important subatomic particles exist:
 Neutrons
 No charge in the nucleus, 1 atomic mass unit (amu)
 Protons
 Positive charge, in the nucleus, 1 amu
 Electrons
 Negative charge, orbit nucleus, 0 amu
 Equal to the number of protons in an atom
 Atoms of different elements contain different numbers of
subatomic particles
 Compare hydrogen, helium and lithium
Atoms Have a Unique Number of Protons
Proton
Neutron
Electron
Hydrogen (H)
Helium (He)
Lithium (Li)
Figure 2.2
Identifying Atoms Using Atomic Number and
Atomic Mass
Atomic number = number of protons in nucleus
Mass number = mass of protons and neutrons in
the atoms nucleus
◦Atoms of the same element can have different
mass numbers
◦Isotopes are atoms of the same element that
differ in their number of neutrons
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 (unreactive) elements have a full valence shell
 Unstable (reactive) elements lack a full valence shell and will
gain, lose, or share electrons with other atoms to achieve
stability, & thereby forming bonds
(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
 Covalent Bonds
 Electrons are shared
 Allows each atom to fill its valence
shell part of the time
 Sharing may be equal or unequal
 Equal sharing produces nonpolar
molecules (Ex. CO2)
 Unequal sharing by atoms with
different electron-attracting abilities
produces polar molecules (H2O)
 Atoms with six or seven valence shell
electrons are electronegative, e.g., oxygen
Reacting atoms
Resulting molecules
+
Hydrogen
atoms
or
Carbon
atom
Molecule of
methane gas (CH4)
(a) Formation of four single covalent bonds:
Reacting atoms
Resulting molecules
+
Oxygen
atom
or
Oxygen
atom
Molecule of
oxygen gas (O2)
(b) Formation of a double covalent bond:
Figure 2.7b
Reacting atoms
Resulting molecules
+
Nitrogen
atom
or
Nitrogen
atom
Molecule of
nitrogen gas (N2)
(c) Formation of a triple covalent bond:.
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)
Hydrogen Bonds
+
An electrostatic
attraction between a
positive hydrogen and
an electronegative
atom
–
–
Hydrogen bond
+
+
–
–
+
+
+
–
(a) The slightly positive ends (+) of the water
molecules become aligned with the slightly
negative ends (–) of other water molecules.
Properties of Water
Charged
Ionizes into an acid and base
Great solvent
Exists as Solid, Liquid,
Gas
Adhesion and Cohesion (via H-bonds)
Surface tension
Properties of Water
Cohesion
◦ Water binding to water
Adhesion
◦ Water binding to anything that has a
charge
Capillary Action