Chapter 2
Atoms, Molecules, and Ions
Early Atomic Theory History
Democritus – “Atomos” (has idea of atoms)
 John Dalton & His Laws
 J.J. Thompson – Discovered electron
 Rutherford – Discovered Nucleus in atom

Dalton’s Atomic Theory
1) Elements are made up of atoms
2) Atoms of each element are identical.
Atoms of different elements are different.
3) Compounds are formed when atoms
combine. Each compound has a specific
number and kinds of atom.
4) Chemical reactions are rearrangement of
atoms. Atoms are not created or destroyed.
Dalton’s Laws
Conservation of Mass – Matter cannot be
created or destroyed
 Law of Definite Proportion- compounds
have a constant composition.
 The react in specific ratios by mass.
 Water always has 2 hydrogen atoms and 1
oxygen atom.

Law of Multiple Proportions
When two elements form more than one
compound, the ratios of the masses of the
second element that combine with one gram
of the first can be reduced to small whole
numbers.
What?!
Water (H2O)has 8 g of oxygen per 1.0 g of
hydrogen.
 Hydrogen peroxide (H2O2) has 16 g of
oxygen per 1.0 g of hydrogen.
 16/8 = 2/1
 The ratios of the masses of oxygen is small
whole number.

Example 2:
Sulfur and oxygen react to form both SO2
and SO3. In SO2 there are 32.02g of sulfur
and 32.00g of oxygen. In SO3, 32.06g of
sulfur are combined with 48.00grams of
oxygen.
 What is the ratio of the weights of oxygen
that combine with 32.06g of sulfur?
 b) How does this illustrate the law of
multiple proportions?


SO2
32.00 g Oxygen = 0.999g O
32.02 g Sulfur

SO3
48.00 g Oxygen
32.06 g Sulfur
= 1.497 g
Shows law of multiple proportions because
ratio of 1.497g/0.999g = 1.5 ratio
Which is 3/2 ratio!
Thomson’s Experiment
J.
J. Thomson- used Cathode ray tubes

Voltage source
+
Passing an electric current makes a beam
appear to move from the negative to the
positive end.
Thomson’s Experiment
Voltage source
+
 By adding an electric field, he found that
the moving pieces were negative
Thomsom’s Model
Found the electron.
 Couldn’t find positive
(for a while).
 Said the atom was like
plum pudding.
 A bunch of positive
stuff, with the
electrons able to be
removed.

Millikan’s Experiment
Atomizer
Oil droplets
+
-
Oil
Telescope
Millikan’s Experiment
X-rays
X-rays give some electrons a charge.
Millikan’s Experiment
Some drops would hover
From the mass of the drop and the charge on
the plates, he calculated the mass of an electron
Rutherford’s Experiment
Used uranium to produce alpha particles.
 Aimed alpha particles at gold foil by
drilling hole in lead block.
 Since the mass is evenly distributed in
gold atoms alpha particles should go
straight through.
 Used gold foil because it could be made
atoms thin.

Lead
block
Uranium
Florescent
Screen
Gold Foil
What he expected
Because
Because, he thought the mass was
evenly distributed in the atom.
What he got
How he explained it
Atom is mostly empty
 Small dense,
positive piece
at center.
 Alpha particles
are deflected by
it if they get close
enough.

+
+
Modern View
The atom is mostly
empty space.
 Two regions
 Nucleus- protons
and neutrons.
 Electron cloudregion where you
might find an
electron.

Sub-atomic Particles
Z - atomic number = number of protons
determines type of atom.
 A - mass number = number of protons +
neutrons.
 Number of protons = number of electrons if
neutral.

Symbols
A
X
Z
23
Na
11
Using Atomic Number and Mass Numbers to determine the
number of protons, neutrons and electrons
Mass #
39
19
K
Element Symbol
Atomic #
Number of Protons = Atomic number = 19 protons
Number of Neutrons = Mass # - Atomic #
= 39 – 19 = 20 neutrons
Number of electrons = # protons = 19 electrons
(In a neutral atom!)
How Can Atoms of the Same Element be Different?
Isotopes — Atoms of the same element that have
different number of neutrons
Since neutrons do not have a charge, adding
more does not affect the atom.
This only adds to the mass of the atom
(increases mass number).

Each particle in the nucleus has a mass of one.
That means . . . The number of particles in the
nucleus = Mass Number

The number of electrons will still be the same as
the protons.
Ions — Atoms of the same element that have
a different number of electrons than protons.
•Having more or less electrons creates an imbalance
in the charge of the atom.
•The number in the charge is how many more or less
electrons there are.
•If there are more protons than electrons,
then the charge is positive. (Loss of electrons)
•If there are more electrons than protons,
the charge is negative. (Gain of electrons)
Ions
16
8
Protons
O
-2
Charge
8
Neutrons 8
Electrons 10
(-2 )charge means that there
was a gain of 2 electrons,
so you add 2 to the number
of protons.
Practice – Finding the number of
no and e1.)
3.)
142
56
Ba
56 no___
86 e-___
56
p+ ___
2.)
27
13
+3
Al
13 no___
14 e-___
10
p+ ___
* (+) charge means
subtract from p+
38
17
-1
Cl
* (-) charge
means add to p+
17 no_____
21 e-___
18
p+ ___
4.)
60
27
Co
27 no___
33
p+ ___
p +,
e-___
27