Development of chemistry, organization of the elements and the determination of atomic weights

1
Development of chemistry, organization of the
elements and the determination of atomic weights

• A second dose of chemical history
• Making inferences from chemical information
• Trends in properties of elements and compounds
• Classifying matter elements
• Atomic weights, isotopes
• Determining chemical formulas

2
A second dose of chemistry history

• Recap discovery of oxygen and Lavosier
  contributions
• Humphrey Davy and use of electrolysis to discover
  additional elements
• Suggestion of connection of electrical properties
  to properties of matter
• Continued era of elemental discovery through use
  of new technology
• Pick up story after about 1810
• Macroscopic developments first
• Microscopic developments next (actual order)

3
Jons Jakob Berzelius (17791848) 2000 formulas

• Credited with discovering four elements
• cerium (1803), selenium (1817), silicon (1824),
  thorium (1829)
• Students isolated lithium, vanadium and some rare
  earths
• Developed symbolism for the elements and
  compounds (1811)
• Superscripts instead of subscripts
• Accurate relative masses of 2000 substances and
  elements
• Table of atomic weights (1828)
• Developed idea of ions and ionic compounds
• A second father of chemistry

4
Dimitri Mendeleev (1834-1907) Putting it all
together

• Cataloged physical and chemical properties of
  elements and compounds of elements
• Arranged elements in groups according to common
  properties
• Reassigned properties placed elements elsewhere
• Molar mass of Be
• Rearranged order of some elements
• Cobalt nickel tellurium iodine
• Changes left gaps in order of elements
• Predicted the existence of three undiscovered
  elements that belonged in gaps and gave their
  properties
• Eka-boron eka-aluminum eka-silicon
• Elements discovered later
• Eka-boron scandium eka-aluminum gallium
  eka-silicon germanium
• Father of Periodic Table (1869)

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Making inferences from chemical information

• Looking at the properties of the elements
• Physical Properties
• Melting point, boiling point, conductivity,
  color, hardness, density
• Comparison to neighbors
• Chemical Properties
• How elements react
• What formulas are formed in reactions
• Comparison to neighbors

6
Some properties of element in the first full row
of the PT
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Some properties of element in the second full row
of the PT
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Chemical and physical properties of compounds
formed from elements
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Melting Points of Oxides
10
Melting Point of the Hydrides
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Melting Points of Sulfides
12
Melting Points of Chlorides
13
Products of reactions of highest oxides with
water the actual compounds

• Most products are as predicted from patterning
  exercise
• Higher oxidation number oxides tend to form acids
• Definite border between metal nonmetal
  products
• Atypical products at borders lower in periodic
  table

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Criteria for Classifying Matter

• Level of complexity
• Elements vs compounds
• Physical and chemical properties
• State at STP
• Standard conditions 25oC and sea level ( 1.0
  atmosphere pressure)
• Melting and boiling points, color, hardness
• Solubility in different solvent particularly
  water
• Reactions (for instance of oxides with water)
• Formation of other unique species
• How organized (LATER)

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Broad classification of the elements

• Metals (generally)
• Shiny luster good conductors of heat
  electricity solids (exception Hg)
• Malleable (pounded into sheets) ductile (drawn
  into wires)
• Nonmetals (generally)
• No shiny luster brittle gas or liquid, soft as
  solid (exception C)
• Poor conductors of heat and electricity
  (insulators)
• Semimetals (metalloids)
• In between properties
• No elements are ionic !!!!
• Na is NOT an element
• Ions form only when some elements react with
  other elements

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Conclusions about organization of elements

• Elements that are gases and have very low boiling
  points (all non-metallic in character)
• These elements must exist on the microscopic
  level as atoms or as very small combinations of
  atoms
• DECLARED TRUTH (at this time)
• Atomic elements ? He, Ne, Ar, Kr, Xe
• Diatomic elements ? H2, N2, O2, F2, Cl2
• Elements that are low melting AND low boiling
  (all non-metallic in character)
• These elements must exist on the microscopic
  level as small combinations of atoms (also
  DECLARED TRUTH)
• Br2, I2 (BP183) P4 (BP280) S8 (BP445)
• Elements that are higher melting
• Must be organized in networks of connected
  atoms
• Strength of the network should be related to
  melting point
• Should be different kinds of networks for metals
  and non-metals/metalloids based on differences in
  conductivity

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Classifying Elements
ELEMENTS
Is the element a metal?
Examples Na, Al, Pb, Hg, Cr, Ca, U
DISCRETE NON-METALLIC ELEMENTS
NETWORK NON-METALLIC ELEMENTS
Examples H2 O2 P4 S8
Examples C, Si, B
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Atomic Weights

• No atomic number concept prior to1913 (Moseley)
• There was no upper number in the element box of
  Periodic Tables until after this!
• Elements were ordered in terms of increasing
  atomic weight EXCEPT
• Where the properties of the elements and their
  periodic relationships overruled increasing
  weight
• Atomic weights were obtained experimentally by
  combining elements with each other knowing
  combining weights
• Some problems early on since the fact that
  certain elements were diatomic was not recognized
• Resolved by 1860
• We will not discuss how atomic weights are
  determined

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The other number in the element box Atomic
Weight (mass)

• The non integer number
• This number is always larger than the atomic
  number
• An average weight for an average atom of the
  element
• Atomic mass units (amus) ? 10 -24 g each atom!!!
• The mass of 6.02 x 1023 atoms in the isotopic
  proportions found in nature
• g units ? lowest value for H 1.008 g/6.02 x
  1023 H atoms

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Isotopes

• Discovered after Chadwicks discovery of neutron
• Same in every atom of an element
• Number of protons (1) (in nucleus)
• Number of electrons (-1) (outside nucleus)
• Protons Electrons
• Potentially different in atoms of an element
• Number of neutrons (in nucleus)
• Relative stability
• Types of nuclei
• Most common isotope
• Other stable isotopes (heavier and lighter)
• Unstable (radioactive) isotopes
• Stability can vary tremendously
• Recall creation story

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Isotopes an example
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What does the Atomic Weight of an Element Mean?

• Weighted Averages (easy)
• 5 blocks weighing 10 g 5 blocks weighing 20 g
• Weighted average 15 g

• Weighted Averages (harder)
• 6 blocks weighing 10.2 g 8 blocks weighing 15.3
  g 4 blocks weighing 19.8 g

• Are any of the blocks an average block?
• For any collection of atoms of any element that
  is weighable on a real scale, the average weight
  represents contributions from different types of
  atoms in the proportion they are found in nature
  although there are no average atoms present!

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Average Atomic Mass (Atomic Weight )

• Atomic Weight (AW) - The average mass of all of
  the isotopes of an element that occur in nature.
• AW fractional abundances x masses
• Abundance Mass
• 35Cl 75 35.0 amu
• 37Cl 25 37.0 amu
• AW 0.75 x 35.0 0.25 x 37.0 35.5 amu
• (75/100 x 35.0 25/100 x 37.0)

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The Dreaded Mole first round

• A mole
• A small blind rodent
• ? this is a pest
• An imperfection on skin
• ? this is sometimes unsightly
• 6.02 x 1023 of anything
• ?this is very useful
• When 1.00 mole of any collection of atoms
  organized as a formula is weighed, the mass in g
  the sum of the weighted atomic masses of the
  formula
• It is far more convenient to work with masses we
  can measure easily than to count large numbers of
  individual atoms
• The mole correlates numbers to masses
• If the collection of atoms is 1.00 mole of a
  representative sample of isotopes of an element,
  the amount in grams is the ATOMIC WEIGHT

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Atomic Weights

• Atomic weights are a type of conversion factor
• Weight (g) ?? Numbers (moles)
• The mole can be used as a conversion factor in
  problem solving.
• 1 average atom H 1.008 amu 1mol H 1.008 g H
• 1 average atom Cu 63.546 amu 1 mol Cu 63.546
  g Cu
• 1 average atom U 238.07 amu 1mol U 238.07 g
  U
• These equalities can be changed into conversion
  factors

• Conversion factors can be flipped as needed

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Using atomic weights to determine formulas of
compounds

• From weights of each component in a pure
  substance
• convert mass of component element to moles of
  component element
• compare moles of the component elements
• determine integer ratios
• From percent composition of elements in a pure
  substance
• Change percentages to mass values
• convert mass of component element to moles of
  component element
• compare moles of the component elements
• determine integer ratios

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Switching Yard First application determining
formulas
g of element A
ATOMIC WEIGHT
mole of A g of A
moles of element A
COMPARE
g of A mole of A
moles of B moles of A
moles of A moles of B
mole of B g of B
moles of element B
g of B mole of B
g of element B
ATOMIC WEIGHT
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Determining Formulas

• A gaseous compound contains 63.6 g of N and 36.4
  g of O. What is the empirical formula
• 63.6 g of N ? moles of N
• 36.4 g of O ? moles of O
• An oxide of arsenic is composed of 75.8 arsenic.
  What is the formula?
• Same strategy
• A product formed from the reaction of sulfur
  trioxide (SO3) and water contains 2.06 H, 32.71
  S and 65.23 O. What is its formula?
• Tougher but same strategy

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Switching Yard In operation
63.6 g N
1mole of N 14.00 g of N
4.54 moles of N
4.54 moles N 2.27 moles O
2 moles N 1 moles O
N2O
2.27 moles of O
1 mole of O 16.00 g of O
36.4 g O
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Switching Yard In operation
75.8 g As
1mole of As 74.92 g of As
1.01 moles of As
1.51 moles O 1.01 moles As
1.5 moles O 1.0 moles As
As2O3
3 moles O 2 moles As
1.51 moles of O
1 mole of O 16.00 g of O
24.2 g O
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Switching Yard In operation
2.06 g H
g of species A
32.71 g S
1 mole of H 1.008 g of H
1mole of S 32.06 g of S
moles of species A
1.020 moles of S
2.043 moles of H
3.41 mole C 3.41 mole O 4.54 mole H 3.41 mole
1.00 mole C 1.00 mole O 1.33 mole H
1.00 mole S 2.00 mole H 4.00 mole 0
H2SO4
moles of species B
4.077 moles of O
g of species B
1 mole of O 16.00 g of O
65.23 g O