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CHAPTER 2
Atoms and the Periodic Table
General, Organic, & Biological
Chemistry
Janice Gorzynski Smith
CHAPTER 2: Atoms & the Periodic Table
Learning Objectives:
! Elemental Symbols
! Metals vs Nonmetals vs Metalloids or Semimetals
! Subatomic Particles: properties & location
! Formulae of Compounds
! Models to represent particles
! Nucleus and structure of atom
! Atomic number, Mass Number, Isotopes, Atomic Weight, & Atomic Mass
! Periodic Table: groups & periods: similar properties within groups
! Electron structure: valence electrons and electron dot symbols
! Periodic trends: atomic size and ionization energy
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Matter
Definition
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Matter
Elements
• An element is a pure substance that cannot be
broken down into simpler substances by a
chemical reaction.
• Each element is identified by a one- or two-letter
symbol.
• Elements are arranged in the periodic table.
• The position of an element in the periodic table
tells us much about its chemical properties.
diamond = carbon
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gold
sulfur
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Definition
Matter
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Compounds
Matter
Compound: a pure substance formed by chemically
combining two or more elements together.
A chemical formula consists of:
•  Element symbols to show the identity of the
elements forming a compound.
•  Subscripts to show the ratio of atoms in the
compound.
H 2O
2 H atoms 1 O atom
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C 3H 8
3 C atoms
8 H atoms
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Compounds
Matter
Compounds can be drawn many ways:
Different elements are represented by different
colors:
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Smith. General Organic & Biolocial Chemistry 2nd Ed.
Depicting Compounds
Matter
CH4
methane
H
H
C
H
H
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Atoms
Subatomic Particles
All matter is composed of the same
basic building blocks called atoms.
Atoms are composed of three
subatomic particles:
Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E
Atoms
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Subatomic Particles
Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E
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Subatomic Particles
Atoms
Opposite charges attract while like charges
repel each other.
Protons and electrons attract each other, but
two electrons repel each other.
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Subatomic Particles
Atoms
From the periodic table:
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Li
Atomic number (Z) is
the number of protons
in the nucleus.
• Every atom of a given element has the same
number of protons in the nucleus.
• Different elements have different atomic numbers.
• A neutral atom has no net overall charge, so
Z = number of protons = number of electrons
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Atoms
Subatomic Particles
Isotopes are atoms of the same element that have
a different number of neutrons.
Mass number (A) =
the number of protons (Z)
+
the number of neutrons
Mass number (A)
35
Atomic number (Z)
17
# of protons
Cl
=
17
# of electrons =
17
# of neutrons
35 – 17 = 18
=
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Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E
Atoms
Atomic Weight
The atomic weight is the weighted average of the
masses of the naturally occurring isotopes of a
particular element reported in atomic mass units.
From the periodic table:
6
C
12.01
Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E
atomic number
element symbol
atomic weight (amu)
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Determine the Atomic Weight of an Element
Atoms
Example
What is the atomic weight of chlorine?
Step [1]
List each isotope, it s mass in atomic
mass units, and it s abundance in nature.
Isotope
Mass (amu)
Isotopic Abundance
Cl-35
34.97
75.78% = 0.7578
Cl-37
36.97
24.22% = 0.2422
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Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E
Determine the Atomic Weight of an Element
Atoms
Step [2]
Multiply the isotopic abundance by the mass
of each isotope, and add up the products.
The sum is the atomic weight of the element.
34.97 x 0.7578 =
26.5003 amu
36.97 x 0.2422 =
8.9541 amu
35.4544 amu =
4 sig. figs.
Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E
35.45 amu
Answer
4 sig. figs.
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Periodic
Table
Elements
Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E
Periodic
Table
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Elements
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Periodic
Table
Groups
1A
2A
B
B
7A
8A
Alkali Metals
Alkaline
Earth Metals
Transition
Metals
Lanthanide
& Actinide
Halogens
Nobel
Gases
Very reactive Reactive
Metals
Metals
except for H
Form ions
with several
different
charges
(oxidation
states)
+1 ions
React with
Oxygen to
form
compounds
that dissolve
into alkaline
solutions in
water
+2 ions
Oxygen
compounds
are strongly
alkaline
Many are not
water soluble
Tend to form
+2 and +3
ions
Lanthanides
58 – 71
Actinides
90 – 103
Reactive
Inert
Form
diatomic
molecules in
elemental
state
Heavier
elements
have limited
reactivity
-1 ions
Actinides are Salts with
radioactive
alkali metals
Smith. General Organic & Biolocial Chemistry 2nd Ed.
Periodic
Table
Do not form
ions
Monoatomic
gases
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Metals, Nonmetals, Metalloids
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Periodic
Table
Metals, Nonmetals, Metalloids
Metals
Nonmetals
Metalloids
•  Brittle, dull
•  Metallic luster,
malleable, ductile,
hardness variable
•  Properties
intermediate between
metals and nonmetals
•  Insulators, nonconductors of
electricity and heat
•  Conduct heat and
electricity
•  Chemical reactivity
varies
•  Metallic shine but
brittle
FIRST PAGES
ELECTRONIC STRUCTURE
•  Solids at room
temperature with the
exception of Hg
•  Semiconductors:
conduct electricity
but
•  Exist mostly as
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compounds rather
not as well as metals:
then pure elements
examples are silicon
2.4C The Unusual Nature of Carbon
•  Chemical reactivity
and germanium
Carbon, a second-row element in
4A ofare
the periodic
•  group
Many
gases,table, is different from most other elevaries
greatly:
Au,
Pt
ments in that it has three elemental forms
(Figure
The two
some
are2.6).
solids
at most common forms of carbon are
unreactive
Na,
K is hard because it contains a dense three-dimensional network of
diamond andwhile
graphite.
Diamond
room temp, only Br2 is
carbon
atoms in six-membered rings. Graphite, on the other hand,
is a slippery black substance
very
reactive
a liquid.
used as a lubricant. It contains parallel sheets of carbon atoms in flat six-membered rings.
Buckminsterfullerene, also referred to as a bucky ball, is a third form that contains 60 carbon
atoms joined together in a sphere of 20 hexagons and 12 pentagons in a pattern that resembles a
Smith. General Organic & Biolocial Chemistry 2nd Ed.
soccer ball. A component of soot, this form of carbon was not discovered until 1985. Its unusual
name stems from its shape, which resembles the geodesic dome invented by R. Buckminster Fuller.
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Carbon’s ability to join with itself and other elements gives it versatility not seen with any
other element in the periodic table. In the unscientific but eloquent description by writer Bill
Bryson in A Short History of Nearly Everything, carbon is described as “the party animal of
the atomic world, latching on to many other atoms (including itself) and holding tight, forming
molecular conga lines of hearty robustness—the very trick of nature necessary to build proteins
and DNA.” As a result, millions of compounds that contain the element carbon are known. The
chemistry of these compounds is discussed at length in Chapters 11–24.
Carbon
Atoms
Figure 2.6ability
Three Elemental
Forms
of Carbon
Carbon’s
to join
with
itself and other elements
a. Diamond
b. Graphite
c. Buckminsterfullerene
gives it a versatility not seen with any
other
element in the periodic table.
Elemental forms of carbon include the following
carbon-only structures:
• Diamond consists of an
intricate three-dimensional
network of carbon atoms.
diamond
• Graphite contains parallel
sheets of carbon atoms.
graphite
• Buckminsterfullerene
contains a sphere with
buckminsterfullerene
60 carbon atoms.
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2.5
Electronic Structure
Why do elements in a group of the periodic table have similar chemical properties? The chemical
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Atoms
Electron Configurations & Orbitals
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Atoms
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Electron Configurations & Orbitals
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Electron Configurations & Orbitals
Atoms
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http://chemwiki.ucdavis.edu/
Valence Electrons
Atoms
• The chemical properties of an element depend on
the number of electrons in the valence shell.
• The valence shell is the outermost shell (the highest
value of n).
• The electrons in the valence shell are called valence
electrons.
Be
Cl
1s22s2
1s22s22p63s23p5
valence shell: n = 2
valence shell: n = 3
# of
valence electrons = 2
# of
valence electrons = 7
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Valence Electrons
Atoms
• Elements in the same group have similar
electron configurations.
• Elements in the same group have the same
number of valence electrons.
• The group number, 1A–8A, equals the number of
valence electrons for the main group elements.
• The exception is He, which has only 2 valence
electrons.
• The chemical properties of a group are therefore
very similar.
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Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E
Valence Electrons
Atoms
Group number:
1A
2A
3A
4A
Period 1:
H
1s1
5A
6A
7A
8A
He
1s2
Period 2:
Li
2s1
Be B
C
N
O
F
Ne
2
2
1
2
2
2
3
2
4
2
5
2s 2s 2p 2s 2p 2s 2p 2s 2p 2s 2p 2s22p6
Period 3:
Na
Mg Al
Si
P
S
Cl
Ar
1
2
2
1
2
2
2
3
2
4
2
5
3s
3s 3s 3p 3s 3p 3s 3p 3s 3p 3s 3p 3s23p6
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Atoms
Valence Electrons
• Dots representing valence electrons are placed
on the four sides of an element symbol.
• Each dot represents one valence electron.
• For 1 to 4 valence electrons, single dots are used.
With > 4 valence electrons, the dots are paired.
H
C
O
Cl
# of Valence electrons:
1
4
6
7
Electron-dot symbol:
H
C
O
Cl
Element:
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Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E
Atoms
Periodic Trends
Increases
The size of atoms
increases down a
column, as the
valence e! are
farther from the
nucleus.
Decreases
• The size of atoms decreases across a row, as
the number of protons in the nucleus increases,
pulling the valence electrons in closer.
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Atoms
Periodic Trends
The ionization energy is the energy needed to remove
an electron from a neutral atom.
Na + energy
Na+ + e–
Decreases
Increases
• Ionization energies
decrease down a
column as the
valence e! get
farther away from
the positively
charged nucleus.
• Ionization energies increase across a row as the
number of protons in the nucleus increases.
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