Atoms
The Basics…
or Part I
CHM 1010
PGCC
Barbara A. Gage
The Atom
• ~400 BCE Democritus: small
particle (“atomos” is Greek for
indivisible)
• 1803
John Dalton:
atomic model with several
postulates
• 1897-now Modern
Investigations…
CHM 1010
PGCC
Barbara A. Gage
Dalton’s Atomic Theory
The Postulates
1. All matter consists of atoms which are indivisible
and indestructible.
2. Atoms of one element cannot be converted into
atoms of another element.
3. Atoms of an element are identical in mass and
other properties and are different from
atoms of any other element.
4. Compounds result from the chemical combination of
a specific ratio of atoms of different elements.
From Silberberg, Principles of Chemistry
CHM 1010
PGCC
Barbara A. Gage
And so…
• Law of Conservation of Matter – matter
cannot be created or destroyed during a
physical or chemical change.
reactant 1
+
reactant 2
product
total mass
calcium oxide
+
=
carbon dioxide
total mass
calcium carbonate
CaO
+
CO2
CaCO3
56.08g
+
44.00g
100.08g
CHM 1010
PGCC
Barbara A. Gage
And so…
• Law of Definite Proportions
A given
compound always contains the same
proportion of elements by mass
– CaCO3
10 g Ca + 3 g C + 12 g O
40 g Ca + 12 g C + 48 g O
or
– H2O
1gH+8gO
7 g H + 56 g O
or
CHM 1010
PGCC
Barbara A. Gage
And so…
• Law of Multiple Proportions - When two
elements form more than one compound
the ratio of the masses of the second
element with 1 g of the first can always
be reduced to small whole numbers
•
H2O (water)
H2O2 (hydrogen peroxide)
1gH
1gH
8gO
16 g O
16/8 = 2
CHM 1010
PGCC
Barbara A. Gage
Atomic Structure
• ~1870-1935 saw experimentation that
showed that Dalton was right and wrong.
• 1874 – Stoney: electricity is made of
individual particles with negative charge
called electrons
• 1879 – Crookes: discovered “cathode
rays” have distinct properties like
electrons
CHM 1010
PGCC
Barbara A. Gage
Figure 2.4 Silberberg, Principles of Chemistry
Experiments to determine the properties of cathode rays.
CHM 1010
PGCC
Barbara A. Gage
Atomic Structure
• 1896 – Becquerel discovers emissions
from materials (radioactivity)
• 1898 – Rutherford uncovers properties
of the some Becquerel emissions and
names them alpha and beta
www.lbl.gov/abc/graphics/magnet.gif
CHM 1010
PGCC
Barbara A. Gage
Atomic Structure
• 1897 – Thomson discovers “canal rays”
are the same as positive hydrogen
atoms
• Model of atom is “plum
pudding” with + and
– particles mixed together
http://commons.wikimedia.org/wiki/File:Plum_pudding_atom.svg
• 1911 – Rutherford conducts “gold foil
experiment”
CHM 1010
PGCC
Barbara A. Gage
CHM 1010
PGCC
Barbara A. Gage
CHM 1010
PGCC
Barbara A. Gage
Atomic Structure
• After gold foil experiment, atomic
model changes to one with + charges
(protons) in dense center with – charges
(electrons) surrounding the center
• 1932 – Chadwick: discovers the missing
mass in the atom comes from neutral
particles named neutrons
CHM 1010
PGCC
Barbara A. Gage
The coulomb (C) is the SI unit of charge.
The atomic mass unit (amu) equals 1.66054x10-24 g.
CHM 1010
PGCC
Barbara A. Gage
Atomic Symbols, Isotopes, Numbers
A
X
Z
The Symbol of the Atom or Isotope
X = Atomic symbol of the element
A = mass number; A = Z + N
Z = atomic number
(the number of protons in the nucleus)
N = number of neutrons in the nucleus
Isotope = atoms of an element with the same
number of protons, but a different number
of neutrons
Figure 2.8 Silberberg, Principles of Chemistry
CHM 1010
PGCC
Barbara A. Gage
CHM 1010
PGCC
Barbara A. Gage
The Modern Reassessment of Dalton’s Atomic Theory
1. All matter is composed of atoms that are indivisible and
indestructible. The atom is the smallest body that retains the unique
identity of the element. However, it can, under unusual
circumstances, be destroyed (converted to energy) and it can be
divided into smaller parts.
2. Atoms of one element cannot be converted into atoms of another
element in a chemical reaction. Elements can only be converted into
other elements in nuclear reactions.
3. All atoms of an element have the same number of protons and
electrons, which determines the chemical behavior of the element.
Isotopes of an element differ in the number of neutrons, and thus in
mass number. A sample of the element is treated as though its
atoms have an average mass.
4. Compounds are formed by the chemical combination of two or more
elements in specific ratios.
CHM 1010
PGCC
Barbara A. Gage