Download 09/09/03 lecture

Introduction to Atomic and
Molecular Structure
Chem 101 Lecture
9/9/03
After a few thousand years of
observing matter, folks learned:
•
Some substances cannot be broken down into other substances by
chemical means (elements).
•
Elements can be combined to form substances with new properties.
•
The total mass of materials present after a chemical reaction is the
same as the mass before the reaction (Law of Conservation of Mass).
John Dalton proposed four postulates
central to modern chemical thinking
•
Each element is composed of extremely small particles called atoms.
•
All atoms of a given element are identical; atoms of different elements are
different.
•
Atoms of an element are not changed into different types of atoms by
chemical reactions.
•
Compounds are formed when atoms of more than one element combine; a
given compound always has the same relative number and kinds of atoms.
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Differences in properties determined
by differences in atoms
• But, what are atoms? What makes them
different from each other?
– Through careful quantitative studies, chemists
determined that elements have unique atomic masses.
– But, what are atoms made of?
The electron was the first
subatomic particle discovered
• Discovered by Thomson in 1897 using a cathode ray tube.
Thomson and his Tube
• Thomson concluded that the “ray” consisted of small, negatively charged
particles (now called “electrons”)
•From the deflections, measured the mass to charge ratio of an electron (e/m = 1.76 x 108 C/g)
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Thomson’s “plum pudding”
model of the atom
“electrons” (raisins)
Positive charge (pudding)
Structure of atom revealed by the
Rutherford Foil Experiment
The Modern View of the Atom
• A positively charge nucleus
containing protons and neutrons.
• The nucleus is surrounded by a
cloud of rapidly moving
electrons.
• The total chare of the nucleus =
the total charge of the electrons
(i.e., # protons = # electrons).
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Atoms can be visualized using
atomic force microscopy
Carbon atoms on a layer of graphite
What’s different about atoms
from different elements ?
• Different elements have different numbers of protons in
their atoms.
• Atomic number: number of protons in the atom; different
elements have different atomic numbers.
• A neutral atom (i.e., one with no net charge) will have the
same number of electrons as protons).
• Most chemical properties are determined by the number
and arrangement of electrons (more on this later).
What about the neutrons?
• Two atoms may have the same number of protons in their
nucleus (same atomic number), but different numbers of
neutrons: these are called isotopes.
• The mass number of an atom is equal to the total number
of protons + neutrons.
Mass number
X
Atomic number
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Atoms can gain or lose electrons
to form ions
• If an atom loses an
electron, it is left with a
net positive charge (called
a cation).
• If an atom gains an
electron, it is left with a
net negative charge (called
an anion).
• Number of electrons =
atomic number - charge
Atoms can be combined by forming
chemical bonds
• Atoms interact by forming
chemical bonds.
• Covalent bonds are formed
when atoms share electrons;
the resulting collection of
atoms is called a molecule.
H2O -- Water
Methanol
Cisplatin: PtH6N2Cl2
Charged species interact via
electrostatic attraction/repulsion
Attraction
Repulsion
•
Positive and negative charges are attracted by the Coulomb
force(opposite charges attract, like charges repel).
•
Interactions given by Coulomb’s Law:
Fµ
-Q Q
r
1
2
2
1, 2
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Ionic bonds involve the forces of attraction
between oppositely charged ions
Sodium chloride : NaCl
We’ve raised a bunch of questions (which
we’ll answer in the next couple weeks):
• How is the structure of an atom (number of electrons and
protons, etc.) related to the properties of the element?
• Why do some atoms tend to form positive ions, others
negative ions, and some no ions at all?
• What does it really mean to say that atoms “share”
electrons?
Where we’ve been so far...
• Elements consist of atoms.
• Elements can be distinguished by differences in the
number of protons, electrons, and neutrons in their atoms.
• Protons are heavy, positively charged particles located in
the nucleus.
• Electrons are light, negatively charged particles
surrounding the nucleus.
• Questions: How are the electrons organized in the atom?
Why do some atoms lose electrons to form cations, while
others gain electrons to form anions?
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