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Unit 2
Atoms, Molecules, and Ions
The Atomic Theory of Matter

Democritus is
generally credited
with putting forth
the first ideas about
atomic structure

Stated that all of
matter consisted of
indivisible particles
called “atoms”
Dalton’s Atomic Theory

Dalton proposed
the first serious
theories on the
structure of matter

Most of his
postulates still hold
true today and are
known as Dalton’s
atomic theory
Dalton’s Atomic Theory
Matter is composed of small indivisible particles called
atoms. The atom is the smallest unit of an element that
has all the properties of that element.
An element is composed entirely of one type of atom.
The chemical properties of all atoms of any element are
the same.
A compound contains atoms of two or more different
elements. The relative number of atoms of each
element in a particular compound is always the same.
Atoms do not change their identities in chemical
reactions. Chemical reactions rearrange only how
atoms are joined together.
1.
2.
3.
4.

Essentially a restatement of the Law of Conservation of Mass
Chemical Laws Associated with
Dalton’s Atomic Theory

Law of Constant Proportions

The relative numbers and kinds of atoms remain
constant for a given compound


Ex: Water (H2O) always has two hydrogens for every
one oxygen
Law of Multiple Proportions


If two elements are capable of combining to form
more than one type of product, they do so in a
ratio of small whole numbers
i.e. There is H2O and H2O2, but no H2O1.5
Atomic Composition and Structure

Dalton also said that the atom was
indestructable, which is untrue

Actually made up of subatomic particles




Protons
Neutrons
Electrons
The electron was the first subatomic particle
to be discovered as a result of experiments
performed with cathode ray tubes
Discovery of the Electron


J.J. Thomson is credited with discovery of the
electron through his work with cathode ray tubes
Referred to as “cathode rays” because they
originated from the positively charged cathode

Quantified a value of 1.76 x 108 C of charge per gram of
electrons
Millikan’s Oil Drop Experiment

Determined that
each electron
possessed
-1.60 x 10-19 C of
charge.
Mass of an Electron

With Thomson’s mass to charge ratio and
Millikan’s charge per electron, the mass
of a single electron could now be
calculated:
1.602  10-19 C
-28
Electron mass 

9.10

10
g
8
1.76  10 C/g
Plum Pudding Model

Positively charged
blob with individual
electrons randomly
distributed
throughout
Rutherford’s Gold Foil Experiment

Proved the existence of a positively
charged nucleus


According to Thomson’s model, a scattering
angle of 1 was expected; however scattering
at much larger angles was also observed
“It was almost as if you fired a fifteen inch
shell at a piece of tissue paper and it
bounced back and hit you.”
Experimental Design
Explanation of Scattering Data

Extremely dense,
positively charged
center of mass at
the core of each
atom

Later dubbed the
nucleus
The Neutron


Discovered by James Chadwick in 1932
Held together with protons by the strong
nuclear force inside the nucleus of atoms
Summary of subatomic particles:
Describing Atoms and Ions
Isotopes


Isotopes differ from other atoms only
by the number of neutrons contained
in the nucleus
For example:
Symbol
24Mg
25Mg
26Mg
# of Protons # of Electrons # of Neutrons
12
12
12
12
12
12
12
13
14
Example



Write the symbol that
describes each of the
following isotopes:
An atom that contains 7
protons and 8 neutrons
An atom that contains 31
protons and 39 neutrons
An atom that contains 14
protons and 14 neutrons
15
7N
70
31 Ga
28
14 Si
Ions

Formation of ions involves the gain or loss
of electrons from outside the nucleus



Never involves the gain or loss of protons
A cation is created from the loss of an
electron (positively charged)
An anion is created from the gain of an
electron (negatively charged)
Symbols of Ions
Give the chemical symbol,
including mass number,
for each of the following
ions:

The ion with 22 protons, 26
neutrons, and 19 electrons

The ion of sulfur that has
16 neutrons and 18
electrons

How many protons,
neutrons, and electrons
does the 79Se2- ion
possess?

48Ti3+

32S2-

34 protons, 45 neutrons, 36
electrons
Atomic Masses


Atoms have extremely small masses (largest
atom has m = 4 x 10-22 g)
Atomic Mass Unit (amu) was implemented to
avoid expression of such cumbersome numbers

Defined as 1/12 the mass of a 12C atom
Particle
Proton
Neutron
Electron
Charge
Positive (+1)
Neutral
Negative(-1)
Mass(amu)
1.0073
1.0087
5.486 x 10-4
Average Atomic Mass

Because most elements exist in nature as a mixture
of isotopes, we must express a large sample of
these elements in the form of an average atomic
mass (sometimes referred to as atomic weight
Schematic of a mass spectrometer
which is designed to measure
atomic mass distributions
Example
Data obtained with a
mass spectrometer show
that, in a sample of an
element, 60.11% of the
atoms have masses of
68.926 u, whereas the
remaining 39.89% of the
atoms have masses of
70.926 u. Calculate the
atomic mass of this
element and give its
name and symbol.

69.724 u

Gallium (Ga)