Atoms Molecules and Ions

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Chapter 2

• Atoms Molecules and Ions

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Fundamental Chemical Laws
Conservation of Mass Lavoisier First
analytical chemist
1787, Irenee du Pont was apprenticed to
Lavoisier. Later founded the company as a
provider of gunpowder.
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1793 October Marie Antoinette executed.
November France legislates against belief in
God.
1794 Thomas Paine publishes The Age of Reason.
Lavoisier executed.
August Robespierre is executed, ending the Reign
of Terror in France.
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Law of Definite Proportion
A given compound always contains the same
proportion of elements, by mass.
Dalton, John, 1766 -1844 Lithograph
Dalton
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Law of Definite Proportion
A given compound always contains the same
proportion of elements, by mass.
Law of Multiple Proportions
When two elements form a series of compounds, the
mass of the second element is a small whole
number relationship to the mass of the first
element.
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Mass of nitrogen that combines with 1 gram of
oxygen
Compound A 1.750g Compound B 0.8750g Compoun
d C 0.4375g
Divide by smallest number to get ratios
1.750/0.4375 0.8750/0.4375
0.4376/0.4375
4 2 1
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We really do not know the real formula
NO N2O2 NO2 N2O4 NO4 N2O8
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Daltons Atomic Theory
Each element is made up of atoms
Atoms of a given element are identical while
atoms of different elements differ.
Chemical compounds are made up of specific whole
number ratios of atoms.
Reactions involve reorganization of atoms .
.atoms do not change.
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Atomic Masses
A relative comparison of the average masses of
atoms.
Avogadros Hypothesis
Equal volumes of a gas contain the same number of
atoms under the same conditions of T and P.
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Essay on a Manner of Determining the Relative
Masses of the Elementary Molecules of Bodies,
and the Proportions in Which They Enter Into
These Compound Lorenzo Romano Amadeo Carlo
Avogadro Journal de physique, 73 58-76 (1811)
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Atomic Structure
Until the end of the 19th century, not much was
known about the atom. People knew
There were different elements
Each had a unique mass and reacted in a special
way.
BUT, WHAT WAS THE ATOM?
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J.J. Thomson
Thomson, J. J. (Joseph John), Sir, 1856 - 1940
Thomson was awarded the 1906 Nobel Prize.
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in recognition of the great merits of his
theoretical and experimental investigations on
the conduction of electricity by gases.
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LVIII. On the Masses of the Ions in Gases at Low
Pressures. By J.J. Thomson, M.A., F.R.S.,
Cavendish Professor of Experimental Physics,
Cambridge Philosophical Magazine
December 1899 Series 5, Vol.
48, No. 295 p. 547-567
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J.J.Thomson
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The cathode ray is deflected in A magnetic field.
So?
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With electric plates the cathode ray Is attracted
towards the positive plate. So??
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The gas molecule ( - the electron) is Left. It
is called the canal ray.
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J. J. Thomson
Cathode ray
Electron.
The cathode ray a fundamental particle of all
atoms.
When an electron is booted out of an atom, the
piece left over (CanalRay) must have a positive
charge.
Thomson found e/m ratio
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Introduction to Atomic Theory
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Introduction to atomic theory
The atom was a black box. People Knew there were
different substances (different boxes), each with
unique Properties, but why was a question.
The early experiments of J.J. Thompson cast light
on the question. An evacuated tube, containing a
small amount of a gas was attached to a power
supply
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If a positive plate was up here, the cathode ray
was attracted to it.

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If a negative plate was down here, the cathode
ray was repelled from it.
These and other experiments showed the cathode
ray to be a negatively charged material.
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This same cathode ray was seen no matter what
the tube was filled with.

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Conclusion
The cathode ray was a fundamental unit of All
materials.
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Through a number of experiments, J.J was able to
determine the charge (e) to mass (m) ratio of the
cathode ray.

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e/m some number.
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By similar analysis he was also able to look
at the canal Ray . . .the positive piece
left over after the Cathode ray had been
ejected.

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e/m was different for each gas that was in the
evacuated tube..
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Starting with our same two atomic Boxes
The positive pieces left over are different
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At this point J.J was ready to talk about the
constitution of the atom.
A large sea of positive charge, imbedded
with electrons.
The Plum Pudding Model
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The e/m ratio. What does it tell us?
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The e/m ratio. What does it tell us?
Millikans Oil Drop Experiment
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Small spheres of oil
Zap with radiation
What does this experiment tell you?
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Rutherford supports his friend, JJ.
A scientist working in his lab Geiger and a
student doing research with him Marsden
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Back to Plum Pudding
A sea of positive charge containing imbedded
electrons
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The gold foil experiment
They would shoot a-particles through the foil.
This should be equivalent to shooting a howitzer
shell through paper tissue.
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A solar system model of the atom.
Electrons circle the nucleus.
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On a Diffuse Reflection of the a-Particles Proc.
Roy. Soc. 1909 A vol. 82, p. 495-500 By H.
GEIGER, Ph.D., John Harling Fellow, And E.
MARSDEN, Hatfield Scholar, University of
Manchester. (Communicated by Prof. E.
Rutherford, F.R.S. Received May 19, -- Read June
17, 1909.)
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Rutherford supports his friend, JJ.
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Composition of the Atom
But, the nucleus is made of protons and neutrons
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Electron
Essentially no mass
Charge of -1
Proton
Mass of 1
Charge of 1
Neutron
Mass of 1
No charge
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The Concept of Atomic Number
Z number of protons
You will notice that most of the Periodic Table
is well-arranged according to Atomic Mass.
X
Z
atomic number protons
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Concept of Isotopes
X
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Compounds and Molecules
Pure substances composed of two or more different
elements are called compounds.
Compounds have definite percent composition of
their combining elements.
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Compounds and Molecules
Some compounds are composed of ions.
Some compounds are composed of molecules
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Molecular Structure
The arrangement of atoms in a molecule
Isomers
Same number of constituent atoms, But different
arrangements.
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Isomers
Structural
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Isomers
Structural
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Isomers
Structural
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Ions Charges
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When an atom gains electrons it becomes an
anion. Anion Are larger than the neutral Atom.
When an atom loses electrons It becomes a
cation. Cations are smaller Than the
neutral Atom.
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The attractive force of and - is called a
Coulombic Attraction
Q1Q2 Force of
attraction -------
d2
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Predict the ionic bond strength of the following
ionic bonds.
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Consider a cation and an anion
NaCl
Na Cl
MgCl2
Mg Cl Cl
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How to write formulas for ionic compounds
What you Have really done is Conserve
charge. The - !
1. Identify ions
2.Cross charges
3. Write formula
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Naming Ionic Compounds
Cations are named as the metal ion
Na sodium ion
Multicharged ions
Stock System Metal (Charge in Roman Numerals)
ion Cu 1 Cu2
Copper(I) ion
Copper(II) ion
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Naming Ionic Compounds
MgBr2
Magnesium bromide
Fe2O3
Iron(III) oxide
Ca3(PO4)2
Calcium phosphate
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Learn complex ions on Page 65 Table 2.5
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More Examples
Mg(NO3)2
FeO
CuBr2
NaHCO3
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Naming Binary Compounds of the Non-metals
Named in order of increasing group number
Dinitrogen tetroxide
N2O4
SF6
Sulfur hexafluoride
Dinitrogen oxide
N2O