Nivaldo J. Tro
http://academic.cengage.com/chemistry/tro
Chapter 3
Atoms and Elements
Deborah Koeck • Texas State University, San Marcos
Understanding the Macroscopic World
• The connection between the microscopic atom and the
macroscopic element is the key to understanding the
chemical world.
Atoms
• An atom is the smallest
identifiable unit of an element.
• Similarities and differences on
the atomic scale correlate
with similarities and
differences on the
macroscopic scale.
Atoms
Protons Determine the Element
• The atomic number, Z, represents the number of protons in
the nucleus of an atom.
• The charge of a proton is assigned numerical value of +1.
• Protons constitute a significant part of the mass of an atom.
– Mass of a proton is 1.0 amu
Chemical Symbols
• Symbols are an abbreviation of the
chemical name which can be
based on:
– The English name for the
element: hydrogen  H
– The Greek or Latin name:
ferrum (iron)  Fe
– Their place of discovery:
europium  Eu
– The scientist honored by the
naming: curium  Cm
Periodic Table of Elements
Electrons
• A neutral atom has as many
electrons outside its nucleus
as protons within its
nucleus.
• The opposing charges of
protons and electrons hold
electrons within a spherical
region surrounding the
nucleus.
• Atoms can lose and gain
one or more electrons.
Determining Protons and Electrons
Neutrons
• Neutrons are almost as
massive as protons but carry no
electrical charge.
• The number of neutrons in the
atoms of an element can vary
resulting in ISOTOPES.
– Can be naturally occurring or
man-made
• The sum of neutrons and
protons in an atom is called the
mass number of the atom.
Atomic Mass
• Atomic masses listed on the periodic table are weighted
averages of the masses of each naturally occurring isotope
for that element.
• Silver has two naturally occurring isotopes. One has a
mass of about 107 amu and the other has a mass of about
109 amu.
• Silver -107 has an abundance in nature of 51.84%.
• Silver -109 has an abundance in nature of 48.16%.
Atomic Mass
Calculate the average atomic mass of silver.
Silver has two naturally occurring isotopes.
• One has a mass of 106.90509 amu (abundance in nature of
51.84%.)
• The other has a mass of 108.90476 amu (abundance in
nature of 48.16%.)
106.90509 amu ( 0.5184) = 55.42 amu
108.90476 amu (0.4816) = 52.45 amu
107.87 amu
Molar Mass
Calculate the Molar Mass of Calcium carbonate
The mass of one mole of a compound.
• Calcium carbonate
• Chemical Formula  CaCO3
Calcium
40.08 grams X 1 mole = 40.08 grams
Carbon
12.01 grams X 1 mole = 12.01 grams
Oxygen
16.00 grams X 3 moles = 48.00 grams
100.09 grams
Periodic Law
• Mendeleev listed the
known elements in order of
increasing atomic mass,
grouping those with similar
properties.
• He noticed that certain
similar properties would
recur in a periodic fashion.
• His tabulation is the
precursor to our modern
periodic table.
Mendeleev and the Periodic Table
• Proposed that elements would be discovered to fill in gaps in
the table.
• Proposed that some measured atomic masses were in error.
• Summarized a large number of observations
– The underlying reasons for periodic behavior were as yet
unknown
– The Bohr model and the quantum mechanical model were
needed to explain the observations.
The Bohr Model
• This is a MODEL of the atom that links electron behavior
(microscopic) to the periodic law (macroscopic).
Bohr Diagrams/Electron Configurations
• Based on the behavior of
electrons as particles
• Electron orbits
– Specified with n, the orbit’s
quantum number
• Fixed energies
• Fixed radii
• Maximum number of electrons
based on n
• This model is an
oversimplification (as are most
models).
• Atoms with full outer orbits
are extremely stable.
• Atoms with outer orbits
that are not full are
unstable and will undergo
chemical reactions
attempting to fill the outer
orbit.
The Quantum Mechanical Model
• Based on the behavior of electrons as waves
• Replace Bohr’s orbits with orbitals, a representation of
electron location as the probability of finding it in a certain
region of space.
• Orbitals are grouped into shells and fill similarly to Bohr’s
orbits.
Determinism and Quantum Mechanics
• Specifics of orbital filling are beyond the scope of this
text…BUT
– The statistical nature of the quantum mechanical model
rocked the scientific establishment.
• An indeterminate universe
• Both models are useful even though Bohr’s, by experiment,
has been shown to be invalid.
Families of Elements
• Also called groups
• Based on outer electron configurations
• Vertical columns
– Alkali metals
– Alkaline earth metals
– Chalcogens
– Halogens
– Noble gases
Element Families
Metals, Nonmetals, and Metalloids
Molecular Elements
• Some elements occur as diatomic molecules.
The Mole Concept
• Counting particles as small as atoms is impractical.
• The mole concept allows us to relate the mass of a sample
of an element to the number of atoms within it.
• The unit of comparison is called a mole and corresponds to
6.022 x 1023.
Avogadro’s Number
• Determined such that the
numerical value of the atomic
mass of an element in amu is
equal to the molar mass of
that element in grams per
mole.
• These relationships of
grams, moles, and atoms
can be used as conversion
factors.
Chapter Summary
Molecular Concept
• Atomic Number and Mass
Number
• Isotopes
• Molar Mass
• Bohr Model
• Quantum Mechanical
Model
• Orbitals
Societal Impact
• Chemical processes that
cause change are caused by
changes in atoms or
molecules.
• Pollution results from
“misplaced” atoms. Atoms
that are not where they are
supposed to be located.
• Models are used to describe
nature.