Download The evolution of Atomic Theory

The evolution of Atomic Theory
Dalton’s Atomic Theory
Development of structure of the atom
Nucleus
Dimitri Mendeleev(Periodicity)
Modern periodic table
New model of the atom
Chemistry Timeline #1
B.C.
400 B.C. Demokritos and Leucippos use the term "atomos”

2000 years of Alchemy
1500's
 Georg Bauer: systematic metallurgy
 Paracelsus: medicinal application of minerals
1600's
Robert Boyle:The Skeptical Chemist. Quantitative experimentation, identification of
elements
1700s'
 Georg Stahl: Phlogiston Theory
 Joseph Priestly: Discovery of oxygen
 Antoine Lavoisier: The role of oxygen in combustion, law of conservation of
mass, first modern chemistry textbook
Chemistry Timeline #2
1800's
Joseph Proust: The law of definite proportion (composition)
 John Dalton: The Atomic Theory, The law of multiple proportions
Joseph Gay-Lussac: Combining volumes of gases, existence of diatomic molecules
Amadeo Avogadro: Molar volumes of gases
Jons Jakob Berzelius: Relative atomic masses, modern symbols for the elements
 Dmitri Mendeleyev: The periodic table
 J.J. Thomson: discovery of the electron
 Henri Becquerel: Discovery of radioactivity
1900's
 Robert Millikan: Charge and mass of the electron
 Ernest Rutherford: Existence of the nucleus, and its relative size
 Meitner & Fermi: Sustained nuclear fission
 Ernest Lawrence: The cyclotron and trans-uranium elements
Dalton’s Atomic Theory (1808)
 All matter is composed of extremely
small particles called atoms
 Atoms of a given element are
identical in size, mass, and other
properties; atoms of different
John Dalton
elements differ in size, mass, and
other properties
 Atoms cannot be subdivided, created, or destroyed
 Atoms of different elements combine in simple
whole-number ratios to form chemical compounds
 In chemical reactions, atoms are combined,
separated, or rearranged
Modern Atomic Theory
Several changes have been made to Dalton’s theory.
Dalton said:
Atoms of a given element are identical in
size, mass, and other properties; atoms of
different elements differ in size, mass, and
other properties
Modern theory states:
Atoms of an element have a characteristic
average mass which is unique to that
element.
Modern Atomic Theory #2
Dalton said:
Atoms cannot be subdivided, created, or destroyed
Modern theory states:
Atoms cannot be subdivided, created, or destroyed
in ordinary chemical reactions. However, these
changes CAN occur in nuclear reactions!
Adding Electrons to the Model
Materials, when rubbed, can develop a charge difference. This
electricity is called “cathode rays” when passed through an
evacuated tube (demos).
These rays have a small mass and are negative.
Thompson noted that these negative subatomic particles were a
fundamental part of all atoms.
Dalton’s “Billiard ball” model (18001900)
Atoms are solid and indivisible.
2) Thompson “Plum pudding” model (1900)
Negative electrons in a positive framework.
3) The Rutherford model (around 1910)
Atoms are mostly empty space.
Negative electrons orbit a positive nucleus.
1)
Law of conservation of matter
When a chemical change (reaction)
takes place, matter is neither created
nor destroyed.
Antoine Lavoisier (1775)
Law of definite Proportions
When two elements react to form a
compound, the total amount(mass)
of the compound formed is
determined by the composition of the
compound and not by the masses of
the elements used.
Example
If 50.0 g of water is decomposed into
5.6 g of hydrogen gas and 44. 4g of
oxygen gas. What is the percent by
mass of these elements?
Law of multiple proportions

When two elements (a and b) can combine to
form more than one compound, then for a fixed
weight of a, the weights of b in two different
compounds always form a ratio that is
expressible in small whole numbers.
Development of the structure of
the atom

Electron (e-) Thomson,1897 present in
all atoms, negative charge(-1), 1/1836
mass of H atom
 Proton (p+)
Thomson and
Goldstein,1907
Present in all atoms, about the same mass
of H, charge (+1)
 Neutron (no) Chadwick, 1932
Same mass as proton , 0 charge
Discovery of the Electron
In 1897, J.J. Thomson used a cathode ray tube
to deduce the presence of a negatively charged
particle.
Cathode ray tubes pass electricity through a gas
that is contained at a very low pressure.
Thomson’s Experiment
Voltage source
+
A stream of charged particles flows from cathode to the anode
Vacuum tube
Metal Disks
Thomson Experiment
Voltage source
+
-
Passing
an electric current makes a beam
appear to move from the negative to the
positive end
Thomson’s Experiment
Voltage source
+
By
adding an electric field
Thomson’s Experiment
Voltage source
+
By
adding an electric field he found that
the moving pieces were negative
Thomson’s Atomic Model
Thomson believed that the electrons were like plums
embedded in a positively charged “pudding,” thus it was
called the “plum pudding” model.
What did Thomson conclude from
his experiment?

He concluded that a cathode ray is
composed of particles with a
negative charge. These particles are
electrons.
Mass of the Electron
1909 – Robert Millikan
determines the mass of
the electron.
The oil drop apparatus
Mass of the
electron is
9.109 x 10-31 kg
Conclusions from the Study of
the Electron
 Cathode rays have identical properties regardless
of the element used to produce them. All elements
must contain identically charged electrons.
Atoms are neutral, so there must be positive
particles in the atom to balance the negative
charge of the electrons
 Electrons have so little mass that atoms must
contain other particles that account for most of
the mass
Atomic Particles
Particle
Charge
Mass (kg)
Location
Electron
-1
9.109 x 10-31
Electron
cloud
Proton
+1
1.673 x 10-27
Nucleus
0
1.675 x 10-27
Nucleus
Neutron
Rutherford’s Gold Foil Experiment
 Alpha particles are helium nuclei
 Particles were fired at a thin sheet of gold foil
 Particle hits on the detecting screen (film) are
recorded
Nucleus: holds protons and
neutrons.
Rutherford’s gold foil experiment
“It was… as if you fired a 15-inch
shell at a piece of tissue paper and it
came back and hit you”

Try it Yourself!
In the following pictures, there is a target hidden by
a cloud. To figure out the shape of the target, we
shot some beams into the cloud and recorded where
the beams came out. Can you figure out the shape of
the target?
The Answers
Target #1
Target #2
The Atomic
Scale
 Most of the mass of the
atom is in the nucleus
(protons and neutrons)
 Electrons are found
outside of the nucleus (the
electron cloud)
 Most of the volume of
the atom is empty space
“q” is a particle called a “quark”
About Quarks…
Protons and neutrons are
NOT fundamental particles.
Protons are made of
two “up” quarks and
one “down” quark.
Neutrons are made of
one “up” quark and
two “down” quarks.
Quarks are held together
by “gluons”
Rutherford’s Findings
 Most of the particles passed right through
 A few particles were deflected
 VERY FEW were greatly deflected
Conclusions:
 The nucleus is small
 The nucleus is dense
 The nucleus is positively charged
Isotopes
Isotopes are atoms of the same element having
different masses due to varying numbers of neutrons.
Isotope
Protons
Electrons
Neutrons
Hydrogen–1
(protium)
1
1
0
Hydrogen-2
(deuterium)
1
1
1
Hydrogen-3
(tritium)
1
1
2
Nucleus
Atomic Masses
Atomic mass is the average of all the naturally
isotopes of that element.
Carbon = 12.011
Symbol
Composition of
the nucleus
% in nature
Carbon-12
12C
6 protons
6 neutrons
98.89%
Carbon-13
13C
6 protons
7 neutrons
1.11%
Carbon-14
14C
6 protons
8 neutrons
<0.01%
Isotope
The structure of the atom
Atomic number (Z) = number of protons in nucleus
Mass number (A) = number of protons + number of neutrons
= atomic number (Z) + number of neutrons
Isotopes are atoms of the same element (X) with different
numbers of neutrons in their nuclei
Mass Number
A
ZX
Atomic Number
1
1H
235
92
2
1H
U
Element Symbol
(D)
238
92
3
1H
U
(T)
Learning Check
An atom has 14 protons and 20 neutrons.
A. Its atomic number is
1) 14
2) 16
B. Its mass number is
1) 14
2) 16
C. The element is
1) Si
3) 34
2) Ca
3) Se
D. Another isotope of this element is
1) 34X
2) 34X
3) 36X
16
14
3) 34
14
Law of Mendeleev
Properties of the elements recur in
regular cycles (periodically) when the
elements are arranged in order of
increasing atomic weight
Mendeleev’s Periodic Table
Dmitri Mendeleev
Modern Periodic Table
Vertical columns (groups)
 Horizontal rows (periods)
 Group IA-VIIIA representative or main
group of elements
 Group IB-VIIIB are transition metals
 Green rows at the bottom: (rare earth
metals)
Lanthanides and Actinides

Group or Family
Period
The Periodic Table
Group or
family
Period
The Properties of a Group:
the Alkali Metals
Easily lose valence electron
(Reducing agents)
React violently with water
Large hydration energy
React with halogens (group viiA)
to form salts
Properties of Metals
 Metals are good
conductors of heat and
electricity
 Metals are malleable
 Metals are ductile
Metals have luster
Properties of Nonmetals
Carbon, the graphite in “pencil lead” is a great
example of a nonmetallic element.
• Nonmetals are poor conductors of heat and
electricity
• Nonmetals tend to be brittle
• Many nonmetals are gases at room
temperature
Properties of Metalloids
Metalloids straddle the
border between metals
and nonmetals on the
periodic table.
 They have properties of both metals and
nonmetals.
Metalloids are more brittle than metals, less
brittle than most nonmetallic solids
 Metalloids are semiconductors of electricity
 Some metalloids possess metallic luster
Silicon, Si – A Metalloid
 Silicon has metallic luster
 Silicon is brittle like a nonmetal
 Silicon is a semiconductor of
electricity
Other metalloids include:





Boron, B
Germanium, Ge
Arsenic, As
Antimony, Sb
Tellurium, Te
Atomic size
Atomic radius(measured by experimental
techniques) measured in angstroms(10-10
meter), nanometers(nm, 10-9) or
picometers(pm, 10-12)
 In a group, as you move down the atoms
get larger(there are exceptions)
 Across a period from left to right the
atoms get smaller

Determination of Atomic Radius:
Half of the distance between nuclei in
covalently bonded diatomic molecule
"covalent atomic radii"
Periodic Trends in Atomic Radius
Radius decreases across a period
Radius increases down a group
Ionic Radii
Cations
(+)
Positively charged ions formed when
an atom of a metal loses one or
more electrons
Smaller than the corresponding
atom
 Negatively charged ions formed
when nonmetallic atoms gain one
Anions
or more electrons
(-)
 Larger than the corresponding
atom (the extra repulsion produced by the
incoming electron causes the atom to expand)
Forming Cations & Anions
A CATION forms
when an atom
loses one or
more electrons.
Mg -->
Mg2+
+ 2 e-
An ANION forms
when an atom
gains one or
more electrons
F + e- --> F-
Predicting Ionic Charges
Group 1: Lose 1 electron to form 1+ ions
H+
Li+ Na+
K+
Predicting Ionic Charges
Group 2: Loses 2 electrons to form 2+ ions
Be2+
Mg2+
Ca2+
Sr2+
Ba2+
Predicting Ionic Charges
B3+
Al3+
Ga3+
Group 13: Loses 3
electrons to form
3+ ions
Predicting Ionic Charges
Caution! C22- and C4are both called carbide
Group 14: Loses 4
electrons or gains
4 electrons
Predicting Ionic Charges
N3- Nitride
P3- Phosphide
As3- Arsenide
Group 15: Gains 3
electrons to form
3- ions
Predicting Ionic Charges
O2- Oxide
S2- Sulfide
Se2- Selenide
Group 16: Gains 2
electrons to form
2- ions
Predicting Ionic Charges
F1- Fluoride
Br1- Bromide
Cl1-Chloride
I1- Iodide
Group 17: Gains 1
electron to form
1- ions
Predicting Ionic Charges
Group 18: Stable
Noble gases do not
form ions!
Ionization Energy
Minimum amount of energy required
to remove the outermost electron
from an atom (IE)
 IE decreases as you go down a
group
 IE increases as you go from left to
right in a period

Electronegativity
A measure of the ability of an atom in a chemical
compound to attract electrons
Electronegativities tend to increase across
a period
Electronegativities tend to decrease down a
group or remain the same
Summation of Periodic Trends
New model of the atom
New model came from closer examinations
of properties of light given off by
gaseous atoms
Light could be defined as particles or
pockets of energy called photons or as
waves of energy.
As a wave it is characterized by lambda(λ)
Lambda= distance between identical adjacent
points on the wave(distance between one crest
or trough)
Visible light ranges from
380-750nm
The visible and invisible wavelengths
are called electromagnetic radiation
 The entire
spectrum=electromagnetic spectrum
 The white light dispersed by a
prism into different wavelengths
gives the continuous spectrum.
