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Biology 102
Lecture 2: Atoms, molecules
and life
Lecture outline
[0. See notes for Lecture 1 for first part of
today’s lecture]
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Characteristics of living organisms
Basic atomic structure
Electron shells
The Periodic Table of the elements
The formation of chemical bonds
1. Characteristics of living
organisms
1.
2.
3.
4.
5.
6.
7.
Have complex, organized structure made
mostly of organic molecules.
Respond to stimuli from their environment
Actively maintain their complex structure/
internal environment (homeostatis)
Acquire and use materials and energy/
convert to different forms
Grow
Reproduce themselves (involves DNA)
Have the capacity to evolve.
2. Basic atomic structure

Atoms: The smallest unit of an element
which has characteristics of that element.
Example:
One atom of gold is still gold
Element: Cannot be broken down or
converted to other substances by ordinary
chemical means.
 Atoms are made of subatomic particles

Number
and configuration of these subatomic
particles give each element its unique
properties
Subatomic particles

Protons: positively charged particles (+1)
 Number


“Atomic number” on periodic table
Example: Ir (iridium) = 77; has 77 protons
 Located

of protons identifies an atom
in the nucleus of the atom
Neutrons: particles without charge
 The
number of neutrons is variable
 Different isotopes of the same element have
different numbers of neutrons
 Neutrons are also located in the nucleus
 Mass number: Sum of protons and neutrons
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Example: Carbon-12 isotope: 6 protons and 6 neutrons
Electrons: negatively charged particles (-1)
3. Electron shells
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Electrons: negativelycharged particles (-1)
 Orbit
around nucleus
 Shown as “ring” in 2-D,
but orbits can be
complex shapes
 Several levels of
“electron shells”
 In a neutral atom, the
number of electrons
equals the number of
protons
Audesirk, Fig. 2-1
Audesirk, Fig. 2-2
4. The Periodic Table
The Periodic Table is organized according to the number of
electron shells, and the number of electrons per shell
The first 18 elements
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
5. The formation of chemical bonds
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CHEMICAL BONDS: Attractive forces that
hold atoms together in molecules
The way atoms react with each other to form
bonds depends upon the emptiness or
fullness of their electron shells
The most stable state: a completely empty
or completely full outer shell.
Achieved in three ways:
 Lose
electrons and thus empty outer shell
 Gain electrons and thus fill outer shell
 Share electrons
Types of chemical bonds

Ionic bonds
One
atom completely loses an electron, while
the other gains an electron

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Ex: Na+Cl- (sodium chloride, or salt)
Note their locations on the Periodic Table! Once
the exchange occurs, + attracts – and the ions
(charged particles) stick together.
Covalent bonds
Two



atoms share electrons
Ex: Carbon-carbon bonds (plus other examples)
Note where carbon is in the Periodic Table!
Hydrogen bonds (Will introduce next time)
Can tell how atoms will react with each
other based on their position in the table!
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings