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Atoms, Ions and Molecules
The Building Blocks of Matter
Chapter 2
Chapter Outline
2.1
2.2
2.3
2.4
2.5
2.6
2.7
The Rutherford Model of Atomic Structure
Nuclides and Their Symbols
Navigating the Periodic Table
The Masses of Atoms, Ions, and Molecules
Moles and Molar Mass
Making Elements
Artificial Nuclides
Experiments in Atomic Structure
• J. J. Thompson (1906 Nobel Prize in Physics)
- cathode ray tube experiments; discovery of the
electron; measurement of the charge-to-mass
ratio.
• Robert Millikan (1923 Nobel Prize in Physics)
- oil-drop experiments; measured the mass of the
electron, therefore calculate the charge
• Ernest Rutherford (1908 Nobel Prize in Physics)
- gold-foil experiments; the nuclear atom
• James Chadwick (1935 Nobel Prize in Physics)
- discovery of the neutron
J.J. Thomson Cathode Ray Tube
Experiments - Electrons
Results of “Cathode Ray” Experiments
•
•
•
•
Travel in straight lines
invisible
independent of cathode composition
bend in a magnetic field like a
negatively-charged particle would
• charge/mass = -1.76 x 108 C/g
Thompson’s “Plum Pudding” Model of the Atom
electrons distributed throughout a diffuse, positively charged sphere.
Robert Millikan’s oil drop Experiment measured the mass of the electron
Millikan’s Results
• The air molecules in the chamber were ionized by a beam of X-rays,
producing electrons and positively-charged fragments
• Fine mist of oil introduced into chamber; electrons adhere to the
droplets
• Negatively-charged droplets settle to bottom of chamber under
influence of gravity
• Charged repeller plates adjusted until droplets were suspended in
mid-air
• From the physics and knowledge of the size of the gravitational and
electrostatic forces, the charge on each droplet could be calculated
• Discovered that each droplet was a whole-number multiple of 1.60 X
10-19 C, so the mass = 9.11 X 10-28 g
Radioactivity and the Nuclear Atom
Spontaneous emission of particles and/or radiation from a
decaying, unstable nucleus
-particles =
-particles =
-rays =
Ernest Rutherford - the nuclear atom
Rutherford's Observations
b) Expected results from “plum
pudding” model.
c) Actual results.
1. the majority of particles penetrated undeflected
2. some particles were deflected at small angles
3. occasionally -particles scattered back at large angles
Rutherford’s Conclusions
• The atom is mainly empty space
because most of the -particles passed
through undeflected
• The nucleus is very dense and
positively charged because some of the
-particles were repulsed and deflected
• Electrons occupy the space around the
nucleus
• The atom is electrically neutral
Rutherford’s Model of the Atom
atomic radius ~ 100 pm = 1 x 10-10 m
nuclear radius ~ 5 x 10-3 pm = 5 x 10-15 m
If the nucleus was the size of an orange, then the radius of the atom would
be 2.5 miles
mass p  mass n = 1840 x mass e-
Chapter Outline
2.1
2.2
2.3
2.4
2.5
2.6
2.7
The Rutherford Model of Atomic Structure
Nuclides and Their Symbols
Navigating the Periodic Table
The Masses of Atoms, Ions, and Molecules
Moles and Molar Mass
Making Elements
Artificial Nuclides
Atomic Mass Units
• Atomic Mass Units (amu)
• Comprise a relative scale to express the
masses of atoms and subatomic particles.
• Scale is based on the mass of 1 atom
of carbon:
» 6 protons + 6 neutrons = 12 amu.
• 1 amu = 1 Dalton (Da)
Isotopes: Experimental Evidence
Atomic number (Z) = number of protons in nucleus
Mass number (A) = number of protons + number of neutrons
= atomic number (Z) + number of neutrons
Isotopes (nuclides) are atoms of the same element (X) with
different numbers of neutrons in the nucleus
Mass Number
Atomic Number
A
ZX
Element Symbol
1
1H
2
1H
235
92
U
(D)
3
1H
238
92
U
(T)
Practice: Isotopic Symbols
• Use the format AX to write the symbol for the
nuclides having 28 protons and 31 neutrons.
• Collect and Organize:
• Analyze:
• Solve:
• Think about It:
Practice: Identifying Atoms and Ions
• Complete the missing information in the
table.
• Collect and Organize:
• Analyze:
• Solve:
• Think about It:
Chapter Outline
2.1
2.2
2.3
2.4
2.5
2.6
2.7
The Rutherford Model of Atomic Structure
Nuclides and Their Symbols
Navigating the Periodic Table
The Masses of Atoms, Ions, and Molecules
Moles and Molar Mass
Making Elements
Artificial Nuclides
The Periodic Table of the Elements
Mendeleev’s Periodic Table
Dmitrii Mendeleev (1872):
• Ordered elements
by atomic mass.
• Arranged elements in
columns based on similar
chemical and physical
properties.
• Left open spaces in the
table for elements not yet
discovered.
The Modern Periodic Table
• Also based on a classification of elements in
terms of their physical and chemical
properties.
• Horizontal rows: called periods (1 → 7).
• Columns: contain elements of the same
family or group (1 →18).
• Several groups have names as well as
numbers.
Navigating the Modern Periodic Table –
Groups and Families
Groups of Elements (cont.)
These 7 elements occur naturally as diatomics (memorize) -
H2
N2
F2
O2
I2
Cl2
Br2
Metals
• found to the left of the “diagonal
line”
• lose electrons in chemical
reactions
• solids (except for Hg, Cs, and Fr)
• conduct electricity
• ductile (draw into a wire)
• malleable (roll into sheets)
• form alloys ("solid-solution" of
one metal in another)
Nonmetals
• found to the right of the “diagonal line”
• like to gain electrons from metals, or
share electrons among themselves
• found as solids, liquids (Br), and gases
(Inert gases, H, N, O, F, Cl)
• “diatomics” - H2, N2, F2, O2 ,I2, Cl2, Br2
• oxygen also exist as ozone, O3
• insulators (except for graphite or C)
Helium-Neon lasers
Metalloids
• elements next to the
“diagonal line”
• B, Si, Ge, As, Sb, and Te
• physical properties of a
metal (can be “convinced”
to conduct electricity) and
chemical properties of a
nonmetal
Elemental Si is used in
the semiconductor
industry
Chapter Outline
2.1
2.2
2.3
2.4
2.5
2.6
2.7
33
The Rutherford Model of Atomic Structure
Nuclides and Their Symbols
Navigating the Periodic Table
The Masses of Atoms, Ions, and Molecules
Moles and Molar Mass
Making Elements
Artificial Nuclides
Average Atomic Mass
Weighted average mass of natural sample of an element,
calculated by multiplying the natural abundance of each
isotope by its exact mass in amu’s and then summing up
these products.
AM = (mass 1)(abn) + (mass 2)(abn) + (mass 3)(abn) +………
One Mole of:
C
S
Hg
Cu
Fe
Molecular Mass
Molecular Mass – the sum of the average atomic masses of the atoms in it.
e.g. H2SO4
NOTE: the terms mass and weight are used interchangeably, e.g.
molecular weight (MW) or atomic weight (AW)
Formula Units and Formula Mass
Formula Units – for ionic compounds,
the smallest electrically neutral unit in
an ionic compound
Formula Mass – the sum of the
average atomic masses of the cations
and anions that make up a neutral
formula unit
e.g. NaCl
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Chapter Outline
2.1
2.2
2.3
2.4
2.5
2.6
2.7
38
The Rutherford Model of Atomic Structure
Nuclides and Their Symbols
Navigating the Periodic Table
The Masses of Atoms, Ions, and Molecules
Moles and Molar Mass
Making Elements
Artificial Nuclides
The Mole - The mole is the
Chemist’s counting unit
dozen
= 12
pair
=2
ream =
500
gross
= 144
Avogadro’s Number
(NA) = 6.022 X 1023
= 1 mole of atoms,
molecules, ions, etc.
Experiment –
how many atoms must be added together so
that the mass in grams = mass in amu’s?
Analogy
using
coins:
Mass ratio = 1 : 5 : 25
Significance of the Mole
Mass in
amu’s
Equivalent to
Mass in
grams/mole
NA of carbon atoms weighs
__________
NA of iron atoms weighs
__________
Moles, Mass, and
Particles
• To convert between number of particles
and an equivalent number of moles.
4
Sample Exercise 2.5
The silicon used to make computer chips has to be extremely pure. Fpr
example, it must contain less than 3 x 10-10 moles of phosphorus (a
common impurity in Si) per mole of silicon. What is this level of impurity
expressed in atoms of phosphorus per mole of Si?
43
Using the Molar Mass as a Conversion
Factor for Atoms & Molecules
e.g. carbon
12.011gram s
mole
1 mole
12.011grams
e.g. H2SO4 sulfuric acid
98.0 grams
mole
1 mole
98.0grams
Moles, Mass, and
Particles
grams of
atoms or
molecules
45
moles of
atoms or
molecules
Numbers of
atoms or
molecules
Practice: Mole Calculations
#1
How many moles of K atoms are present in 19.5
g of potassium?
46
Practice: Mole Calculations
#2
How many moles are present in 58.4 g of chalk
(CaCO3)?
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Practice: Mole Calculations
#3
The uranium used in nuclear fuel exists in nature
in several minerals. Calculate how many moles of
uranium are found in 100.0 grams of carnotite,
K2(UO2)2(VO4)2•3H2O.
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