Atoms, Molecules, and
Ions
Chemistry 11

The theory that atoms are the
fundamental building blocks of matter reemerged with John Dalton in the early
19th century.
Atomic Theory of Matter
Each element is composed of extremely
small particles called atoms.
 All atoms of a given element are identical
to one another in mass and other
properties, but the atoms of one element
are different from the atoms of other
elements.

Dalton’s Postulates
www.learner.org
The atoms of one element cannot be
changed into atoms of a different element
by chemical reactions; atoms are neither
created nor destroyed in chemical
reactions.
 Compounds are formed when atoms of
more than one element combine; a given
compound always has the same relative
number and kind of atoms.

Dalton’s Postulates Cont’d
In the late 1800s scientists began to look
for the various sub-atomic particles.
 Would use a partially evacuated tube that
when a high voltage was applied to the
electrodes in the tube, radiation was
produced. The radiation was called
cathode ray because they originated
from

The Electron
Through experiments it was noted that the
particles in the cathode ray tube were deflected
by electric or magnetic forces.
 Thomson’s experiment showed that no matter
what material is in the cathode ray tube, the
particle deflect the same.
 He is widely regarded as “discovering” the
electron (1897).
 Also found the charge-to-mass ratio
 His idea of the atom consisted of a
uniform positive sphere of matter in
which the electrons were embedded.

J.J. Thomson
www.chemheritage.com

In 1909, Robert
Millikan succeeded
in determining the
charge of an
electron by using
Thomson’s chargeto-mass ratio and
his oil-drop
experiment.
http://cwx.prenhall.com/bookbind/pubbooks/hillchem3/medialib/media_portfolio/text_im
ages/CH07/FG07_04.JPG
Millikan’s Oil-Drop Experiment
In 1896 Henri Bequerel was studying a
uranium compound when he discovered
that it spontaneously emits high-energy
radiation (radioactivity).
 The study of radioactivity was continued
by Marie Curie (and hubby Pierre) who
isolated the radioactive components of the
compound.
 Ernest Rutherford found that three types
of radiation were emitted (alpha, beta,
gamma).

Radioactivity
Designed an experiment to study the
angles at which the particles were
scattered/deflected as they passed
through a thin gold-foil
 A few particles were deflected at small
angles, others at large angles (Marsden),
some were reflected back in their original
direction.
 This did not match Thomson’s “plum
pudding”model”

Rutherford’s Gold Foil Experiment

Simple view of the atom shows only three
subatomic particles: proton, neutron, and
electron.
Comparison of the Proton, Neutron, and Electron
Particle
Charge
Mass (amu)
Proton
Positive (1+)
1.0073
Neutron
None (neutral)
1.0087
Electron
Negative (1-)
5.486 x 10-4
Modern View of Atomic Structure

Atoms with identical atomic numbers but
difference mass numbers (translation:
they have the same umber of protons but
different numbers of neutrons.
Mass Number (number of
protons + neutrons)
Atomic Number (number of
protons OR electrons)
Atomic Numbers, Mass Numbers,
and Isotopes
Element
Symbol
Atomic
Number
Mass
Number
Number
of
Protons
Number
of
Electrons
Number
of
Neutrons
Lead
Sn
41
88
112
75
51
Try these!
120
Isotopes are atoms of the same element
with different masses.
 Isotopes have different numbers of
neutrons

http://www.green-planet-solar-energy.com/images/helium-isotopes-5-to-8.gif
Isotopes

Atomic and molecular masses can be
measured very accurately with a mass
spectrometer
Isotopes and Mass Spectrometry
http://teachertube.com/viewVideo.php?video_id=48160&title=Average_at
omic_mass
Video Lecture

To find the AVERAGE ATOMIC MASS of an
atom, we take into account all of the
isotopes that exist and the percentage of
each type. The calculation of the average
atomic mass is a WEIGHTED AVERAGE.
Average atomic mass   (mass of isotope  relative abundance)
How to Calculate Average Atomic
Mass
There are two isotopes of silver. Silver-107 with an
atomic mass of 106.90509, and silver-109 that has
an atomic mass of 108.90470. Silver-107 makes
up 51.86% of all silver isotopes while silver-109
makes up the rest (48.14%). What is the average
atomic mass of silver?
Average atomic mass = (106.90509 x 0.5186) + (108.90470 x 0.4814)
= 107.8677 amu
Example - Silver
Copper has two isotopes – 63Cu
(62.929amu) and 65Cu (64.928amu). The
63Cu is 69.17% abundant, and the 65Cu is
30.83% abundant. Calculate the average
atomic mass of copper.
Average atomic mass = (62.929 x 0.6917) + (64.928 x 0.3083)
= 63.545 amu
Example #2
Worksheet