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Chemistry-140
Lecture 4
Chapter 2: Atoms & Elements
 Chapter Highlights

atomic theory

structure of the atom; electrons, protons, neutrons

atomic mass, mass number and isotopes

atomic number

periodic table

allotropes
Chemistry-140
Lecture 4
Philosophy
Chemistry is a pyramidal science, that is, all higher
learning draws on the material already absorbed.
A large base of knowledge provides a stable platform
for understanding higher concepts.
Chemistry-140
Lecture 4
Dalton’s Atomic Theory of Matter
 Each element is composed of small particles called atoms.
 All atoms of a given element are identical
 Atoms are neither created nor destroyed in chemical
reactions.
 Compounds are formed when atoms of more than one
element combine
Chemistry-140
Lecture 4
Law’s From Dalton’s Theories
 Law of constant composition: all samples of a given substance
must have the same composition (#4).
 Law of conservation of matter: matter cannot be created nor
destroyed, only rearranged (#3).
 Law of multiple proportions: if two samples have different
compositions, they must be samples of two different
substances.
Chemistry-140
Lecture 4
Consequences of Dalton’s Theories
 Combinations of elements are in
ratios of small whole numbers.
 In forming water, 8.0 g of oxygen combines with 1.0 g of
hydrogen.
In forming hydrogen peroxide 16.0 g of oxygen combines with
1.0 g of hydrogen.
 Using atomic theory, we conclude that hydrogen peroxide
contains twice as many atoms of oxygen per hydrogen.
H2O2 versus H2O
Chemistry-140
Lecture 4
The Discovery of Atomic Structure
 Cathode rays were discovered in the 1800s and were found to
be beams of electrons streaming from the negatively charged
cathode toward the positively charged anode.
 curvature is related to the mass and velocity of the particles
Chemistry-140
Lecture 4
The Discovery of Atomic Structure
 J. J. Thompson: A balancing of the magnetic and electrical
fields allows measurement of the charge-to-mass ratio of the
electron
Chemistry-140
Lecture 4
The Discovery of Atomic Structure
 Robert Millikan: a balancing of gravity with attraction to the
positively charged plate allowed measurement of the charge
on the electron (and the mass of the electron)
Chemistry-140
Lecture 4
Radioactivty and Atomic Theory
 Radioactivity is the spontaneous emission of radiation from a
substance. eg. X-rays, g-rays, a-particles, and b-particles
 X-rays and g-rays are high-energy light
 a-radiation is a stream of helium nuclei, He2+ known as aparticles.
 b-radiation is a stream of high speed electrons that originate
in the nucleus (b-particles)
Chemistry-140
Lecture 4
The Nuclear Atom
 Most of the mass and all of the positive charge was
concentrated in the nucleus while the negative charge and
most of the volume was accounted for by the electrons
outside the nucleus.
Chemistry-140
Lecture 4
The Nuclear Atom; Continued
 Subsequent experiments
led to the discovery of
both positive (protons)
and neutral (neutrons)
particles in the nucleus.
 Protons were discovered
in 1919 by Rutherford.
 Neutrons were discovered
by the British scientist
James Chadwick in 1932
Chemistry-140
Lecture 4
Modern View of Atomic Structure
 In a chemical context,
there are three important subatomic particles
 The proton is found in the nucleus
charge = + 1.602 x 10-19 C (+1 electronic charge)
mass = 1.67262 x 10-24 g (1.0073 amu)
 The neutron is found in the nucleus
charge = 0
mass = 1.67495 x 10-24 g (1.0087 amu)
 The electron is found outside the nucleus
charge = -1.602 x 10-19 C (-1 electronic charge)
mass = 9.10939 x 10-28 g (5.486x10-4 amu)
Chemistry-140
Lecture 4
Putting a Scale on Atoms
 The heaviest atom has a mass of only 4 x 10-22 g
and a diameter of only 5 x 10-10 m.
Useful units:
 1 amu (atomic mass unit) = 1.66054 x 10-27 kg
 1 pm (picometer) = 1 x 10-12 m
 1 Å (Angstrom) = 1 x 10-10 m (not SI unit but very common)
Chemistry-140
Lecture 4
A Simple Calculation With Atoms
Question:
The diameter of a penny is 19 mm. The diameter of a
copper atom is 2.6 Å. How many copper atoms could be
arranged side by side in a straight line across the diameter
of a penny?
Chemistry-140
Lecture 4
A Simple Calculation With Atoms
Answer:
The unknown is the number of Cu atoms. We can
take the relationship that 1 Cu atom = 2.6 Å as a conversion
factor relating number of atoms and distance.
 103 m   1 A   1 Cu atom 

  10  

 1 mm   10 m   2.6 A 
Thus: # Cu atoms
= (19mm)(3.8 x 106 Cu atom/mm)
= 7.3 x 107 Cu atoms
Chemistry-140
Lecture 4
Isotopes, Atomic Numbers and Mass Numbers
 Atomic number is the number of protons in the nucleus.
All atoms of the same element have the same atomic number.
 Mass number is the sum of the number of protons & neutrons
The number of neutrons in the nucleus is given by the mass
number minus the atomic number.
 Isotopes are atoms of the same element that have different
numbers of neutrons. Therefore isotopes have the same
atomic number but different mass numbers.
 An atom of a specific isotope is called a nuclide.
Chemistry-140
Lecture 4
Isotopes, Atomic Numbers and Mass Numbers
 In reference to a specific isotope, we use the symbolism;
where
mass number = (protons + neutrons)
atomic number = (protons)
 usually do not include atomic number in symbol as the
symbol implies a particular atomic number
example:
14C
is "carbon fourteen"
 also some isotopes have special names:
1H
= hydrogen, 2H = deuterium, 3H = tritium
Chemistry-140
Lecture 4
Measuring Atomic Masses
 A mass spectrometer is used to ionize samples and separate
the ions produced according to their mass
Chemistry-140
Lecture 4
Measuring Atomic Masses
 A sample of the element antimony (Sb) is separated into ions
according to isotopic mass and natural abundance
Chemistry-140
Lecture 4
A Question of Isotopes
Example 2.3:
Silver has two isotopes, one with 60 neutrons
and the other with 62 neutrons. What are the mass
numbers and symbols of these isotopes?
Chemistry-140
Lecture 4
A Question of Isotopes
Answer:
Silver has an atomic number of 47, so it has 47
protons in the nucleus. The two isotopes therefore
have mass numbers of
Isotope 1: A = 47 protons + 60 neutrons = 107
Isotope 2: A = 47 protons + 62 neutrons = 109
The first isotope has a symbol
the second is
Chemistry-140
Lecture 4
Atomic Mass
 Atomic mass of an element is the average of all the
isotopes weighted by their natural abundance
Example 2.4:
Bromine has two naturally occurring
isotopes, one with a mass of 78.918336 amu and a
percent abundance of 50.69%, the other with mass
80.916289 and a percent abundance of 49.31%.
Calculate the atomic mass of bromine.
Chemistry-140
Lecture 4
Atomic Mass
Answer:
The atomic mass of any element is the average of
the masses of all the isotopes in a representative
sample. To calculate the atomic mass, you multiply the
mass of each isotopes by its percent abundance divided
by 100.
 % abundance of isotope 1
Atomic mass = 
  mass of isotope 1


100
 % abundance of isotope 2

 mass of isotope 2  ...


100
Average atomic mass of bromine = atomic mass of
bromine
Chemistry-140
Lecture 4
Textbook Questions From Chapter #2
Atoms:
24, 25
Isotopes:
30
Atomic Mass:
32, 34
Good Problems: 56, 62
Fun(?):
46
Chemistry-140
Lecture 5
The Periodic Table
 Elements are listed in ascending order of atomic number
 Periods are the horizontal rows in the periodic table.
 Groups (Families) are the vertical columns in the periodic table.
Periodic Table of the Elements
Chemistry-140
Lecture 5
The Periodic Table
 Metals: elements to the left of the periodic table (excluding
hydrogen). Generally lustrous, ductile, malleable and
conduct heat and electricity.
 Nonmetals: elements to the right of the periodic table. Lack
the properties of metals, are brittle as solids, do not conduct
well, and are often dull in appearance. Many are gases.
 Semimetals (metalloids): elements at the boundary between
metals and nonmetals. Properties are intermediate between
those of the metals and nonmetals. Boundary is often shown
as a "stepped" line on the periodic table, from B to At
Chemistry-140
Lecture 5
The Periodic Table
 Some Groups have special names:
Examples
Group 1
alkali metals
Group 2
alkaline earth metals
Group 17
halogens
Group 18
noble gases
1
Group 1: Alkali Metals
Group 2: Alkaline Earth Metals
2
Group 17: Halogens
17
Group 18: Noble Gases
18
Chemistry-140
Lecture 5
Allotropes
 Some elements exists in more than one form:
Example:
C:
O:
P:
graphite, diamond, fullerene(s)
dioxygen, ozone
red, white, black
 Different forms of the same element are known as allotropes
Chemistry-140
Lecture 5
Chemistry-140
Lecture 5
Chemistry-140
Lecture 5
C60 Fullerene