Download Historical development of the nature of matter Democritus (~460

Chemistry 115 Note outlines
Historical development of the nature of matter
Democritus (~460-370 BC)
♦ Known by peers as the ‘laughing philosopher,’ because he
emphasized the value of ‘cheerfulness’
♦ Elaborated the “atomist theory” originated by his teacher,
Leucippus
−
Democritus proposed that atomos.....
− were small hard particles
− were made of a single material
− were always moving
− formed different materials by joining together
♦ Apocryphal legend of theory inspiration: Horses on a distant hill
− Indisputably individual entities-yet they looked like one continuous mass
− Thought experiment: Maybe if had we had better eyes we could see these atomos
♦ He was ridiculed by scientists of his day
♦ His theory largely forgotten for 2000 years
Aristotle (~384-322 BC)
♦ Proponent of continuous theory of matter
♦ Largely responsible from dismissal of
Democritus’ theory
John Dalton (~1805)
♦ British school teacher and chemist
♦ Performed experiments to determine how elements join
together to form new substances
♦ Recognized his results were inconsistent with existing
theory
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“We are what we repeatedly do. Excellence, then, is not an act, but a habit.”
Aristotle
Page 1 of 10
Chemistry 115 Note outlines
Dalton’s Atomic Theory
Dalton postulated that:
1. All matter consists of atoms; tiny indivisible particles of an element that cannot be created or
destroyed.
2. Atoms of one element cannot be converted into atoms of another element.
3. Atoms of an element are identical in mass and other properties to all other atoms of the same
element and are different from the atoms of any other element.
4. Compounds result from the chemical combination of a specific ratio of atoms of different
elements.
The law of mass conservation
♦
Arisotle’s and Dalton’s theories were both consistent with the Law of Mass Conservation
Law of Definite (or Constant) Composition
♦ No matter the source, a particular compound is composed of the same elements in the same
parts (fractions) by mass
Purified calcium carbonate obtained from grinding
up coral from the south Mediterranean sea
Purified calcium carbonate obtained from
limestone dug up in Preston county, WV
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“We are what we repeatedly do. Excellence, then, is not an act, but a habit.”
Aristotle
Page 2 of 10
Chemistry 115 Note outlines
Law of Multiple Proportions
♦ A pure compound always contains the same elements in the same proportions by mass
♦ If particles were infinitely divisible we would see an infinite number of mass ratios. Again, we
do not!
Dalton’s interpretation:
If there are two elements that can combine(for example carbon and oxygen), their combinations will
occur in a set sequence.
− The first compound will have one atom of carbon and one of oxygen
− Another, one atom of carbon and two atoms of oxygen
− And so on
A theory must be discarded or refined when it can't explain new experimental results
♦ Continuous matter; the idea that matter is infinitely divisible could account for the
BUT......
♦ It is inconsistent with the Law of Definite (or Constant) Composition and the Law of
Multiple Proportions
♦ So we must conclude that the continuous theory of matter is invalid so must be discarded
Concept Check
A long standing and time tested, consistent explanation of known observations is called
a. an experiment
b. a hypothesis
c. a prediction
d. a theory
“We are what we repeatedly do. Excellence, then, is not an act, but a habit.”
Aristotle
Page 3 of 10
Chemistry 115 Note outlines
The atomic world according to Dalton.....
The Components of Matter
Element
Molecule
♦ the simplest type of substance with unique
♦ an entity that consists of two or more atoms
physical and chemical properties
that are chemically bound together and thus
behave as an independent unit
♦ consists of only one type of atom
♦ It cannot be broken down into any simpler
substances by physical or chemical means
Compound
♦ a substance composed of two or
more elements which are chemically
combined
Mixture
♦ a group of two or more elements
and/or compounds that are
physically intermingled
The following scenes represent an atomic-scale view of three samples of matter. Describe each sample
as an element, compound, or mixture.
mixture
element
compound
“We are what we repeatedly do. Excellence, then, is not an act, but a habit.”
Aristotle
Page 4 of 10
Chemistry 115 Note outlines
Don’t forget to be working on memorizing all the elements and polyatomic ions listed on the syllabus!
Polyatomic ions are ions consisting of a group of atoms. This group of atoms stays together as a
unit in many reactions. Students should memorize the names and formulas for the following
polyatomic ions.
Ion
Name
Ion
Name
Ion
Name
2−
2−
−
Acetate
Chromate
Sulfate
C2H3O2
CrO4
SO4
+
2NH4
Ammonium
Cyanide
SO3
Sulfite
CN−
2
−
−
−
Bicarbonate
Dichromate
Nitrate
HCO3
Cr2O7
NO3
+
−
(or hydrogen carbonate)
H3O
Hydronium
Nitrite
NO2
2−
−
−
Peroxide
Bisulfate (or hydrogen
Hydroxide
O2
HSO4
OH
sulfate)
Bisulfite (or hydrogen
Permanganate PO43−
Phosphate
HSO3−
MnO4−
sulfite)
Carbonate
Oxalate
Phosphite
CO32−
C2O42−
PO33−
Oxy anion series
Ion
ClO4−
ClO3−
ClO2−
ClO−
Name
Perchlorate
Chlorate
Chlorite
Hypochlorite
Ion
BrO4−
BrO3−
BrO2−
BrO−
Name
Perbromate
Bromate
Bromite
Hypobromite
Ion
IO4−
IO3−
IO2−
IO−
Name
Periodate
Iodate
Iodite
Hypoiodite
Question: Of what is matter made?
Answer: Chemists assert that all matter is composed of tiny particles called atoms and molecules
Question:
Answer:
♦
Pentacene: C22H46 [Theory- textbook]
Pentacene: C22H46 [AFM-atomic force microscopy]
Images courtesy of IBM Research – Zurich
“We are what we repeatedly do. Excellence, then, is not an act, but a habit.”
Aristotle
Page 5 of 10
Chemistry 115 Note outlines
Question:
Answer:
♦ Experiments from 1810 to 1910 provided evidence that atoms did exist, but suggested that they
were composed of smaller (subatomic) particles
Cathode-Ray Tubes: J. J. Thomson (1897)
♦ Totally changed the view of an atom by discovering the electron
♦ Thomson’s atomic theory suggested that the was composed of smaller subatomic particles
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Observation
Conclusion
♦ Plum pudding model of the atom
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“We are what we repeatedly do. Excellence, then, is not an act, but a habit.”
Aristotle
Page 6 of 10
Chemistry 115 Note outlines
Ernest Rutherford (1871-1937): Scattering experiment and discovery of the atomic nucleus
♦ Proposed a new and revolutionary concept of the atom based on his experiments
− Results of these experiments were completely unexpected
♦ New model of the atom
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Rutherford’s Gold foil experiment (http://www.youtube.com/watch?v=iuU4Ayom4b0)
“You can learn a lot by throwing rocks”
♦
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♦
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“Dalton! You forgot isotopes!!!”
♦
♦
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Subatomic Particles (summary)
Relative charge
−1
Relative mass
1/1800 amu
Proton
+1
1 amu
Inside nucleus
neutron
0
1 amu
Inside nucleus
Electron
location
Outside nucleus
Atomic mass unit (amu) ≡ a mass equal to exactly 1/12 the mass of a carbon-12 atom
“We are what we repeatedly do. Excellence, then, is not an act, but a habit.”
Aristotle
Page 7 of 10
Chemistry 115 Note outlines
Notes
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♦
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Isotopes ≡ atoms with same number of protons but different numbers of neutrons
♦ Isotopes of an element have slightly different weights but the same chemical properties.
Notation
Atomic number, Z ≡ the number of protons in an atom of an element
Mass number, A ≡ the number of protons plus neutrons in an atom
Concept Check
Which of the following gives the correct number of protons, neutrons, and electrons for a
a.
b.
c.
d.
protons
8
8
8
8
electrons
10
10
8
8
18
O −2 ion.
neutrons
8
10
10
8
Concept Check
In which of the following sets do all species have the same number of electrons?
a. Br−, Kr, Sr2+
b. C, N3−, O2−
c. Mg2+, Sr2+, Ba2+
d. O, O2−, O2+
All elements have at least two different isotopes. Since they have a different number of neutrons, they
have different weights. How do we report the atomic weight of an element?
We report a weighted average
An example of a weighted average: Your overall course average
( )
( )
( )
( )
( )
( )
( )
10%
10%
10%
10%
25%
10%
25%
ave = (E1) 100%
+ (E2 ) 100%
+ (E3) 100%
+ (E3) 100%
+ (Lab ave) 100%
+ (HW ) 100%
+ (Final ) 100%
“We are what we repeatedly do. Excellence, then, is not an act, but a habit.”
Aristotle
Page 8 of 10
Chemistry 115 Note outlines
For atomic weight
 % isotope 1 
 % isotope 2 
At Wt = 
 (mass of isotope 1) + 
 (mass of isotope 2 ) + .... etc
 100% 
 100% 
 % isotope i 
or in mathematical language .... At Wt = ∑ 
(mass of isotope i )
100% 
i 
Silver (Ag, Z = 47) has two naturally occurring isotopes, 107Ag and 109Ag. From the percent abundance
provided, calculate the atomic mass of Ag.
Isotope
Mass (amu)
Abundance (%)
107
Ag
106.90509
51.84
109
Ag
108.90476
48.16
Elements can be classified by their relative positions within the table
Period ≡ a horizontal row in the periodic table
s block
p block
Group or Family ≡ a vertical column in the
periodic table
d block
♦ Elements in same family have similar
chemical properties
f block
Main group/ Representative elements≡ the s-block plus the p-block elements above the mettaloids
Transition Metals ≡ the d-block elements plus the p-block elements below the mettaloids
Inner Transition Metals ≡ the f-block elements
“We are what we repeatedly do. Excellence, then, is not an act, but a habit.”
Aristotle
Page 9 of 10
Chemistry 115 Note outlines
Properties of Metals
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Properties of Non-metals
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Metalloids
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Elements that occur as molecules under standard conditions
Valence electrons ⇒ those electrons in the outermost shell (highest value of n) of an atom
♦ For representative elements this is the same as its group number
♦ For transition metals it is more complicated (not discussed until Chem 422)
Octet Rule⇒ atoms tend to ionize or combine such that they achieve eight valence electrons
♦ This would be a electronic configuration the same as the nearest noble gas
♦ Ions with charges in excess of ± 3 are unstable and do not typically form
Predict the monoatomic ion formed by each of the following elements
Iodine (Z = 53)
Aluminum (Z = 13)
Calcium (Z = 20)
Potassium (Z = 19)
Oxygen (Z = 8)
Ar (Z=18)
“We are what we repeatedly do. Excellence, then, is not an act, but a habit.”
Aristotle
Page 10 of 10