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Transcript
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
Karen Hattenhauer (Fall 2007)
2
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.
Karen Hattenhauer (Fall 2007)
3
1
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
Karen Hattenhauer (Fall 2007)
4
Representations of Molecules
Karen Hattenhauer (Fall 2007)
5
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
Karen Hattenhauer (Fall 2007)
6
2
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
Karen Hattenhauer (Fall 2007)
7
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
Karen Hattenhauer (Fall 2007)
8
A Cathode-ray Tube
Karen Hattenhauer (Fall 2007)
9
3
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
Karen Hattenhauer (Fall 2007)
10
Millikan s Oil-drop Experiment
Karen Hattenhauer (Fall 2007)
11
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
Karen Hattenhauer (Fall 2007)
12
4
The Rutherford Scattering Experiment
Karen Hattenhauer (Fall 2007)
13
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)
Karen Hattenhauer (Fall 2007)
14
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
Karen Hattenhauer (Fall 2007)
15
5
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)
Karen Hattenhauer (Fall 2007)
16
6
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