Chapter 2 - Chemistry

Chapter 2
(Essentials of General Chemistry, 2nd Edition)
(Ebbing and Gammon)
Atoms, Molecules
and Ions
Atomic Theory of Matter
John Dalton (British Chemist)
- basic theory of modern chemistry
- all matter whether element, compound or
mixture is composed of small particles called
atoms
- purpose of atomic theory:
to provide explanation of the structure of
matter in terms of different combinations
of very small particles
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Postulates of Dalton s Atomic Theory
1.) All matter is composed of indivisible atoms. An atom is
an extremely small particle of matter that retains its
identity during chemical reactions.
2.) An element is a type of matter composed of only one
kind of atom, each atom of a given kind having the same
properties.
3.) A compound is a type of matter composed of atoms of
two or more elements chemically combined in fixed
proportions.
4.) A chemical reaction consists of the rearrangement of
the atoms present in the reacting substances to give new
chemical combinations present in the substances formed
by the reaction.
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Atomic Symbols and Models
Atomic symbol
- a one- or two-letter notation used to represent an
atom corresponding to a particular element
- convention: first letter capital
second letter lowercase
- origin: from the name of the element
from a name in another language (Latin)
Atomic Models
- Dalton used spheres to represent atoms and used
combinations of these spheres to represent
compounds
- still used today but more refined
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Representations of Molecules
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Deductions from Dalton s Atomic Theory
Explains:
1.) difference between element and compound
2.) Law of Mass Conservation
- states that total mass remains constant
during a chemical reaction
3.) Law of Definite Proportions
- compound is type of matter containing atoms of two
or more elements is definite proportions
4.) Law of Multiple Proportions
- when two elements form more than one compound,
the masses of one element in these compounds for a
fixed mass of the other element are in ratios of
small whole numbers
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The Structure of the Atom
Two Particles:
1.) nucleus (atom s central core)
- positively charged
- contains most of atoms mass
2.) electron (outside atom s central core)
- negatively charged
- very light
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Discovery of the Electron
Cathode-ray Tube (J. J. Thomson- 1897)
- used experimental apparatus to conclude that
cathode ray consists of beam of negatively charged
particles (or electrons)
charge-to-mass ratio calculation (e/m)
- ratio of e- electric charge to its mass
modern (excepted value):
e/m = 1.758820 x 108 C/g
e = magnitude of charge of e- in coulombs (C)
m = mass of e- in grams
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A Cathode-ray Tube
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Measuring the Mass of an Electron
Oil-drop Experiment (Robert A. Millikan 1909)
- experiment used to obtain the charge on an
electron (1.602 x 10-19 coulombs)
- using charge-to-mass ratio (by Thomson)
e = 1.758820 x 108 C/g
m
then
m=
e
= 1.602176 x 10-19 C
8
1.758820 x 10 g/C
1.758820 x 108 C/g
= 9.109382 x 10-28 g
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Millikan s Oil-drop Experiment
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Nuclear Model of Atom
Ernest Rutherford (British Physicist - 1911)
- proposed that metal atom he was studying
must be almost entirely empty space with
mass concentrated in tiny central core
- showed that atoms consisted of a positively
charged nucleus at the center of the atom,
around which negatively charged electrons
move
- nucleus occupies only very small portion of
space of atom
nuclei diameters
10-3 picometers
atomic diameters
100 picometers
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The Rutherford Scattering Experiment
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Nuclear Structure: Isotopes
Structure of Nucleus:
1.) proton
-a nuclear particle
having a positive
charge equal to that
of the electron and
has a mass more than
1800 times that of the
electron
2.) neutron
- a nuclear particle having a mass almost
identical to that of the proton but no electric
charge
Note: characterize nucleus by atomic number (Z) and
mass number (A)
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Atomic Composition
atomic number (Z)
- number of protons in nucleus of an atom
- charge of nucleus of particular atom is equal to number
of protons
element
substance whose atoms all have the same atomic
number
atomic number (Z) = number of protons
mass number (A)
- total number of protons and neutrons in a nucleus
A = number of protons + number of neutrons
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Atomic Composition: Nuclear Symbol
nuclide
atom characterized by a definite atomic number
and mass number
atomic no. - subscript at lower left of the element
symbol
mass no. - superscript at upper left of the element
symbol
A X
Z
where: X = element symbol
A = mass number
Z = atomic number
isotopes atoms whose nuclei have the same atomic
number but different mass numbers (ie. nuclei
have the same number of protons but different
numbers of neutrons)
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