Greek Model
“To understand the very large,
we must understand the very small.”
Democritus
• Greek philosopher
• Idea of ‘atomos’
– Atomos = ‘indivisible’
– ‘Atom’ is derived
• No experiments to support
idea
Democritus’s model of atom
No protons, electrons, or neutrons
Solid and INDESTRUCTABLE
Democritus
DEMOCRITUS (400 BC) – First Atomic Hypothesis
Atomos: Greek for “uncuttable”. Chop up a piece of matter until you reach the atomos.
Properties of atoms:
• indestructible.
• changeable, however, into different forms.
• an infinite number of kinds so there are an infinite number of elements.
• hard substances have rough, prickly atoms that stick together.
• liquids have round, smooth atoms that slide over one another.
• smell is caused by atoms interacting with the nose – rough atoms hurt.
• sleep is caused by atoms escaping the brain.
• death – too many escaped or didn’t return.
• the heart is the center of anger.
• the brain is the center of thought.
• the liver is the seat of desire.
“Nothing exists but atoms and space, all else is opinion”.
Four Element Theory
FIRE
• Aristotle was an atomist
• Thought all matter was
composed of 4 elements:
–
–
–
–
–
Earth (cool, heavy)
Water (wet)
Fire (hot)
Air (light)
Ether (close to heaven)
Hot
Dry
‘MATTER’
AIR
Wet
EARTH
Cold
WATER
Relation of the four elements and the four qualities
Blend these “elements” in different proportions to get all substances
Foundations of Atomic Theory
Law of Conservation of Mass
Mass is neither destroyed nor created during ordinary chemical
reactions.
Law of Definite Proportions
The fact that a chemical compound contains the same elements
in exactly the same proportions by mass regardless of the size
of the sample or source of the compound.
Law of Multiple Proportions
If two or more different compounds are composed of the
same two elements, then the ratio of the masses of the
second element combined with a certain mass of the first
elements is always a ratio of small whole numbers.
Legos are Similar to Atoms
H
H2
H
H
O
+
H2
H
H
O2
H
O
H 2O
H
O
O
H
H 2O
Lego's can be taken apart and built into many different things.
Atoms can be rearranged into different substances.
Law of Multiple Proportions
John Dalton (1766 – 1844)
If two elements form more than one
compound, the ratio of the second element
that combines with 1 gram of the first
element in each is a simple whole number.
e.g. H2O &
H2O2
water hydrogen peroxide
Ratio of oxygen is 1:2 (an exact ratio)
The Atomic Theory of Matter
• In 1803, Dalton proposed that elements consist of
individual particles called atoms.
• His atomic theory of matter contains four hypotheses:
1. All matter is composed of tiny particles called atoms.
2. All atoms of an element are identical in mass and
fundamental chemical properties.
3. A chemical compound is a substance that always
contains the same atoms in the same ratio.
4. In chemical reactions, atoms from one or more
compounds or elements redistribute or rearrange in
relation to other atoms to form one or more new
compounds. Atoms themselves do not undergo a
change of identity in chemical reactions.
Copyright © 2007 Pearson Benjamin Cummings. All rights reserved.
The Atomic Theory of Matter
• Dalton’s atomic theory is essentially correct, with four
minor modifications:
1. Not all atoms of an element must have precisely the same mass.
2. Atoms of one element can be transformed into another through
nuclear reactions.
3. The composition of many solid compounds are somewhat
variable.
4. Under certain circumstances, some atoms can be divided
(split into smaller particles: i.e. nuclear fission).
Copyright © 2007 Pearson Benjamin Cummings. All rights reserved.
Dalton’s Symbols
John Dalton
1808
A Cathode Ray Tube
Source of
Electrical
Potential
Stream of negative
particles (electrons)
Metal Plate
Gas-filled
glass tube
Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 58
Metal plate
J.J. Thomson
• He proved that atoms of
any element can be
made to emit tiny
negative particles.
• He knew that atoms did
not have a net negative
charge and so there must
be balancing the negative
charge.
J.J. Thomson
William Thomson
(Lord Kelvin)
• In 1910 proposed
the Plum Pudding
model
– Negative electrons
were embedded into
a positively charged
spherical cloud.
Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 56
Spherical cloud of
Positive charge
Electrons
Rutherford’s Gold Foil
Experiment
Rutherford received the 1908 Nobel Prize in Chemistry for his pioneering work in nuclear chemistry.
beam of alpha particles
radioactive
substance
circular ZnS - coated
fluorescent screen
gold foil
Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 120
What he expected…
What he got…
richocheting
alpha particles
Interpreting the
Observed Deflections
.
.
.
.
.
.
beam of
alpha
particles
.
.
.
.
.
undeflected
particles
.
.
.
.
.
gold foil
Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 120
.
deflected particle
Rutherford Scattering (cont.)
Rutherford interpreted this result by suggesting that
the a particles interacted with very small and heavy
particles
Particle bounces off
of atom?
Case A
Case B
Particle goes through
atom?
Particle attracts
to atom?
Case C
Case D
.
Particle path is altered
as it passes through atom?
Explanation of Alpha-Scattering Results
Alpha particles
Nucleus
+
+
-
-
+
+
-
+
+
-
+
-
+
-
-
Plum-pudding atom
Nuclear atom
Thomson’s model
Rutherford’s model
Bohr’s Model
Nucleus
Electron
Orbit
Energy Levels
Quantum Mechanical Model
Niels Bohr &
Albert Einstein
Modern atomic theory describes the
electronic structure of the atom as the
probability of finding electrons within certain
regions of space (orbitals).
Modern View
• The atom is mostly empty space
• Two regions
– Nucleus
• protons and neutrons
– Electron cloud
• region where you might find an electron
mass p = mass n = 1840 x mass e2.2
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
(T)
U
2.3
Do You Understand Isotopes?
How many protons, neutrons, and electrons are
14
in 6 C ?
6 protons, 8 (14 - 6) neutrons, 6 electrons
How many protons, neutrons, and electrons are
11
in 6 C ?
6 protons, 5 (11 - 6) neutrons, 6 electrons
2.3
Noble Gas
Halogen
Group
Alkali Metal
Alkali Earth Metal
Period
2.4
A molecule is an aggregate of two or more atoms in a
definite arrangement held together by chemical bonds
H2
H2O
NH3
CH4
A diatomic molecule contains only two atoms
H2, N2, O2, Br2, HCl, CO
A polyatomic molecule contains more than two atoms
O3, H2O, NH3, CH4
2.5
ELEMENTS THAT EXIST
AS DIATOMIC MOLECULES
Remember:
BrINClHOF
P: 1 or 4
S: 1 or 8
These elements
only exist as
PAIRS. Note that
when they
combine to make
compounds, they
are no longer
elements so they
are no longer in
pairs!
An ion is an atom, or group of atoms, that has a net
positive or negative charge.
cation – ion with a positive charge
If a neutral atom loses one or more electrons
it becomes a cation.
Na
11 protons
11 electrons
Na+
11 protons
10 electrons
anion – ion with a negative charge
If a neutral atom gains one or more electrons
it becomes an anion.
Cl
17 protons
17 electrons
Cl-
17 protons
18 electrons
2.5
A monatomic ion contains only one atom
Na+, Cl-, Ca2+, O2-, Al3+, N3-
A polyatomic ion contains more than one atom
OH-, CN-, NH4+, NO3-
2.5
Do You Understand Ions?
How many protons and electrons are in
27 3+
13 Al
?
13 protons, 10 (13 – 3) electrons
How many protons and electrons are in
78 234 Se
?
34 protons, 36 (34 + 2) electrons
2.5
2.6
A molecular formula shows the exact number of
atoms of each element in the smallest unit of a
substance
An empirical formula shows the simplest
whole-number ratio of the atoms in a substance
molecular
empirical
H2O
H2O
C6H12O6
CH2O
O3
O
N2H4
NH2
2.6
ionic compounds consist of a combination of
cation(s) and an anion(s)
• the formula is always the same as the empirical formula
• the sum of the charges on the cation(s) and anion(s) in each
formula unit must equal zero
The ionic compound NaCl
2.6
Formula of Ionic Compounds
2 x +3 = +6
3 x -2 = -6
Al2O3
Al3+
1 x +2 = +2
Ca2+
1 x +2 = +2
Na+
O22 x -1 = -2
CaBr2
Br1 x -2 = -2
Na2CO3
CO322.6
2.6
Examples of Older Names of Cations
formed from Transition Metals
(memorize these!!)
From Zumdahl
Chemical Nomenclature
• Ionic Compounds
– often a metal + nonmetal
– anion (nonmetal), add “ide” to element name
BaCl2
barium chloride
K2O
potassium oxide
Mg(OH)2
magnesium hydroxide
KNO3
potassium nitrate
2.7
• Transition metal ionic compounds
– indicate charge on metal with Roman numerals
FeCl2
2 Cl- -2 so Fe is +2
iron(II) chloride
FeCl3
3 Cl- -3 so Fe is +3
iron(III) chloride
Cr2S3
3 S-2 -6 so Cr is +3 (6/2) chromium(III) sulfide
2.7
• Molecular compounds
• nonmetals or nonmetals + metalloids
• common names
• H2O, NH3, CH4, C60
• element further left in periodic table
is 1st
• element closest to bottom of group is
1st
• if more than one compound can be
formed from the same elements, use
prefixes to indicate number of each
kind of atom
• last element ends in ide
2.7
Molecular Compounds
HI
hydrogen iodide
NF3
nitrogen trifluoride
SO2
sulfur dioxide
N2Cl4
dinitrogen tetrachloride
NO2
nitrogen dioxide
N2O
dinitrogen monoxide
TOXIC!
Laughing Gas
2.7
An acid can be defined as a substance that yields
hydrogen ions (H+) when dissolved in water.
HCl
•Pure substance, hydrogen chloride
•Dissolved in water (H+ Cl-), hydrochloric acid
An oxoacid is an acid that contains hydrogen,
oxygen, and another element.
HNO3
nitric acid
H2CO3
carbonic acid
H2SO4
sulfuric acid
HNO3
2.7
2.7
2.7
A base can be defined as a substance that yields
hydroxide ions (OH-) when dissolved in water.
NaOH
sodium hydroxide
KOH
potassium hydroxide
Ba(OH)2
barium hydroxide
2.7
2.7
Mixed Practice
1.
2.
3.
4.
5.
6.
7.
8.
9.
Dinitrogen monoxide
Potassium sulfide
Copper (II) nitrate
Dichlorine heptoxide
Chromium (III) sulfate
Ferric sulfite
Calcium oxide
Barium carbonate
Iodine monochloride
1.
2.
3.
4.
5.
6.
7.
8.
9.
N2O
K2S
Cu(NO3)2
Cl2O7
Cr2(SO4)3
Fe2(SO3)3
CaO
BaCO3
ICl
Mixed Practice
1.
2.
3.
4.
5.
6.
7.
8.
9.
BaI2
P4S3
Ca(OH)2
FeCO3
Na2Cr2O7
I2O5
Cu(ClO4)2
CS2
B2Cl4
1.
2.
3.
4.
5.
6.
7.
8.
9.
Barium iodide
Tetraphosphorus trisulfide
Calcium hydroxide
Iron (II) carbonate
Sodium dichromate
Diiodine pentoxide
Cupric perchlorate
Carbon disulfide
Diboron tetrachloride
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Atomic Structure