The Atomic and Molecular Perspective of Chemistry

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The Study of Chemistry

• The Atomic and Molecular Perspective of Chemistry

Matter is the physical material of the
universe. Matter is made up of relatively few
(ca. 100) elements. Elements are the
building blocks of matter. On the nano
(ultramicroscopic) level, matter consists of
atoms. An atom is a nano-basketball -- nano
10 -9. Atoms usually are found in the combined
state, either molecules, salts, or alloys.
Molecules may consist of the same type of
atoms or different types of atoms.
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Classification of Matter
Elements
The next five elements are Na 2, K 2, Mg 2,
H 1, Ti 0.5.
The next six elements are N 3, Ca 1.5, P 1,
K,S,Na 0.75
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Elements in the Human Body including trace
elements
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The Periodic Table
Bring your Periodic Table to each class!
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Properties of Matter
Physical and Chemical Changes

• When a substance undergoes a physical change, its
  physical appearance changes, but its chemical
  nature does not.

Example the melting of ice (physical change)
results in a solid being converted into a liquid,
but it is still water. Physical changes do not
result in a change of composition.
When a substance changes its composition, it
undergoes a chemical change
Example when pure hydrogen and pure oxygen
react completely, they form pure water. In the
flask containing water, there is no oxygen or
hydrogen left over.
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Units of Measurement
Powers of ten are used for convenience with
smaller or larger units in the SI system.
What is a GigaByte?
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Units of Measurement - Temperature
There are three temperature scales Kelvin Scale
(used in science) Same temperature increment as
Celsius scale. Lowest temperature possible
(absolute zero) is zero Kelvin. Absolute zero 0
K -273.15oC.
Celsius Scale (used in science) Also used in
science. Water freezes at 0oC and boils at
100oC. To convert K oC 273.15.
Fahrenheit Scale (used in US engineering and
commerce) Water freezes at 32oF and boils at
212oF. To convert
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Units of Measurement - Temperature
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Units of Measurement - Temperature
A user-friendly way to view the Celsius Scale 0
- Cold! (coat) 10 - Cool (sweat shirt) 20 -
Pleasant (long sleeves) 25 - Room temperature
(short sleeves) 30 - Very warm (T-shirt) 40 -
Hot! (swimming pool!)
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Units of Measurement - Density

• Used to characterize substances.
• Defined as density mass /volume.
• Units g/cm3, also known as specific gravity.
• Originally based on mass -- the density was
  defined as the mass of 1.00 g of pure water.

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The Atomic Theory of Matter

• John Dalton Elements are composed of atoms.
• All atoms of an element are identical
  (chemically). (Dalton stressed identical in
  weight but he didnt know about isotopes)
• In chemical reactions, the atoms are not changed.
• Compounds are formed when atoms of more than one
  element combine.

(e.g., H2O, C6H6, C12H22O11 but not H2, Cl2)
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The Discovery of Atomic Structure
The ancient Greeks were the first to postulate
that matter consists of indivisible
constituents. Later scientists realized that the
atom consisted of charged ( or -) entities.
A charged particle will have its path bend in
either an electric or magnetic field.
Cathode Rays and Electrons A cathode ray tube
(CRT) is a hollow vessel with an electrode at
either end. A high voltage is applied across
the electrodes.
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The Discovery of Atomic Structure
The ancient Greeks were the first to postulate
that matter consists of indivisible
constituents. Later scientists realized that the
atom consisted of charged ( or -) entities.
A charged particle will have its path bend in
either an electric or magnetic field.
Cathode Rays and Electrons A cathode ray tube
(CRT) is a hollow vessel with an electrode at
either end. A high voltage is applied across
the electrodes.
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The Discovery of Atomic Structure
Three spots are noted on the detector

• a spot which is not affected by the electric
  field,

• a spot in the direction of the positive ()
  plate,

• a spot in the direction of the negative (-) plate.

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The Discovery of Atomic Structure
The Nuclear Atom From the separation of radiation
we conclude that the atom consists of neutral,
positively, and negatively charged entities. J.
J. Thomson assumed all these charged species were
found in a sphere.
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The Discovery of Atomic Structure
The Nuclear Atom Rutherfords a-particle
experiment
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The Discovery of Atomic Structure
The Nuclear Atom In order to get the majority of
?-particles through a piece of foil to be
undeflected, the majority of the atom must
consist of a low mass, diffuse negative charge -
the electron.
To account for the small number of high
deflections of the ?-particles, the center or
nucleus of the atom must consist of a dense
positive charge.
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The Discovery of Atomic Structure
The Nuclear Atom Rutherford modified Thomsons
model as follows assume the atom is spherical
but a massive positive charge must be located at
the center, with a diffuse light negative charge
surrounding it.
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The Modern View of Atomic Structure
The atom consists of positive, negative, and
neutral entities (protons, electrons, and
neutrons).
Protons and neutrons are located in the
nucleus of the atom, which is small. Most of the
mass of the atom is due to the nucleus.
Electrons are located outside of the nucleus.
Most of the volume of the atom is due to
electrons.
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The Nucleus
(1Å 10-8cm 10-10 m)
Ångstrom unit
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The Periodic Table
Columns in the periodic table are called groups
(numbered from 1A to 8A or 1 to 18).
Rows in the periodic table are called periods.
Metals are located on the left hand side of the
periodic table (most of the elements are
metals). Non-metals are located in the top right
hand side of the periodic table. Elements with
properties similar to both metals and non-metals
are called metalloids and are located at the
interface between the metals and non-metals.
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The Periodic Table
Non-Metals
Metalloids semiconductors
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Different Kinds of Compounds
A molecule, formed by covalent bonding, is formed
between a nonmetal and a nonmetal, (e.g., CO2,
PBr3, H2O).
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Different Kinds of Compounds
An alloy, formed by metallic bonding, is formed
between a metal and a metal, (e.g., brass or
nickel-steel)
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The Mole
The mole connects the visible with the
invisible. A fluorine molecule (F2) weighs 38.000
amu. A mole of fluorine molecules weighs 38.000
grams. The number of fluorine molecules in a mole
is an incredibly large number, called Avogadros
Number, N, which is 6.022 x 1023. We will be
using the mole concept very often.
Amedeo Avogadro 1776-1856
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The Mole
Examples A mole of H is 1.008 grams. A mole of
H2 is 2.016 grams. A mole of CO2 is 44.011
grams. A mole of CO is 28.01 grams. A mole of
octane (C8H18) is 114.22 grams. A mole of copper
(Cu) is 63.54 grams. A mole of table salt (NaCl)
is 58.44 grams. A mole of sodium bicarbonate
(NaHCO3) is 84.01 grams. A mole of Ag2O is 231.74
grams. A mole of glucose (C6H12O6) is 180.16
grams. A mole of chlorophyll (C55H72MgN4O5) is
893.51 grams.
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Molecules and Molecular Compounds
Picturing Molecules
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Ions and Ionic Compounds
When an atom or molecule loses electrons, it
becomes positively charged.

• For example, when Na loses an electron, it
  becomes Na.
• Positively charged ions are called cations.

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Ions and Ionic Compounds
When an atom or molecule gains electrons, it
becomes negatively charged.
For example when Cl gains an electron it becomes
Cl-. Negatively charged ions are called
anions. An atom or molecule can lose more than
one electron.
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Ions and Ionic Compounds
When an atom or molecule loses two electrons,
it becomes doubly positively charged.
For example, when Mg loses two electrons, it
becomes Mg2
Mg2 ion
Mg atom
magnesium atom magnesium
ion
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Ions and Ionic Compounds
When an atom or molecule gains two electrons,
it becomes doubly negatively charged.
For example, when S gains 2 electrons, it becomes
S2-
S2- ion
S atom
sulfur atom
sulfide ion
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Ions and Ionic Compounds
Important note that there are no easily
identified NaCl molecules in the ionic lattice.
Therefore, we cannot use molecular formulas to
describe ionic substances.
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Ions and Ionic Compounds
Ionic Compounds I.e., 3Mg atoms need to form
3Mg2 ions (total 3x2 charges) and 2 N atoms
need to form 2N3- ions (total 2x3-
charges). Therefore, the formula is Mg3N2.
MgO
Mg N
3
2
Be careful! whats the ionic compound formed
between magnesium and oxygen?)
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Chemical Equations

• Lavoisier mass is conserved in a chemical
  reaction.
• Chemical equations descriptions of chemical
  reactions.
• Two parts to an equation reactants and products

2H2 O2
2H2O
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H2
H2O
O2
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• Stoichiometric coefficients numbers in front of
  the chemical formulas give numbers of molecules
  or atoms reacting (and numbers being produced).

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Law of Conservation of Mass All reactions must
be balanced
CH4 O2 CO2 H2O
is not balanced. (Why?)
Count atoms
Reactants Products
1 C
1 C
4 H
2 H
2 O
3 O
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Balance reactions only by changing coefficients,
not by altering chemical formula