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Unit 2: Atoms, Ions and Ionic
Compounds
 Atoms
 Nuclear Symbols
 Ions
 Names and Formulas of Ionic Compounds
 Atomic Models
Atoms
 Atom: the smallest representative particle
of an element
 Democritus (460-370 BC) first suggested the
concept of atoms
 Rejected by Aristotle and the Greek thinkers
in favor of fire, earth, air, water & heavenly
ether as the basic components of matter
 Dalton’s Atomic Theory (1803 – 1807) led to
the re-emergence of the idea that atoms are
the building blocks of all matter.
Dalton’s Atomic Theory
 Dalton: Each element is composed of
extremely small, indivisible, indestructible
particles called atoms.
 We now know that atoms are not
indivisible. Each atom contains subatomic
particles including:
 Protons
 Neutrons
 Electrons
Dalton’s Atomic Theory
 Dalton: All atoms of a given element are
identical to each other in mass and other
properties but different from the atoms of
all other elements.
 Dalton wasn’t quite correct. Some atoms
of the same element have different
masses. They are, however, chemically
identical to each other.
Dalton’s Atomic Theory
 Dalton: Atoms of one element cannot be
changed into atoms of another element.
 Dalton was correct when talking about
ordinary chemical reactions.
 Nuclear reactions, however, can convert
one element into another.
Dalton’s Atomic Theory
 Dalton: Compounds are formed when atoms
of more than one element combine.
 A given compound contains atoms combined
in definite whole number ratios.
 Fe2O3 (rust)
 H 2O
(water)
 H2O2 (hydrogen peroxide)
 These ratios are the same for any sample
of a particular pure substance.
Dalton’s Atomic Theory
 Dalton’s Atomic Theory explained several
laws that were already known:
 Law of Conservation of Mass (Matter)
 Matter cannot be created or
destroyed.
 The total mass of substances present
before and after a chemical reaction is
constant.
5.0 g of reactants  5.0 g of products
Dalton’s Atomic Theory
 Dalton’s Atomic Theory explained several
laws that were already known (cont.):
 Law of Constant Composition
 A given compound always contains the
same relative numbers and kinds of
atoms.
Ethyl alcohol
C2H6O
Methane always contains 25% H and 75% C
Dalton’s Atomic Theory
 Dalton’s atomic theory was based on evidence
gained in the macroscopic world.
 Today, scientists can actually see individual
atoms using special techniques.
 See figure 2.2 of your text for a scanning
tunneling micrograph of gallium arsenide
 http://www.almaden.ibm.com/vis/stm/gallery.html
Modern View of Atomic Structure
 Atoms are tiny spherical particles with a:
1. nucleus:
 The small positively charged center of
an atom
 contains most of the mass of the atom
 contains two important subatomic
particles: protons and neutrons
2. diffuse, negatively charged outer layer
that contains a third type of subatomic
particle, the electron
Modern View of Atomic Structure
 Most of the volume of an atom is empty
space.
Diffuse layer
containing electrons
in constant motion
Modern View of Atomic Structure
 Important subatomic particles
 Protons
 positively charged subatomic particles
found in the nucleus
 Neutrons
 neutral (uncharged) subatomic particles
found in the nucleus
 Electrons
 very small, negatively charged
subatomic particles found in a diffuse
layer surrounding the nucleus
Modern View of Atomic Structure
Particle
Symbol
Relative
Charge
Mass
Proton
p
+1
1.0073 amu
Neutron
n
0
1.0087 amu
Electron
e-
-1
5.468 x 10-4 amu
amu (atomic mass unit):
 a unit used to express very small masses
 1.66054 x 10-24 g
Modern View of Atomic Structure
 If all atoms are composed of protons,
neutrons, and electrons, then why is an atom
of gold different from an atom of carbon?
 The atoms of each element have a unique
number of protons in the nucleus.
 Atomic number: the number of protons in
the nucleus
 Carbon: 6 p (atomic # = 6)
 Gold:
79 p (atomic # = 79)
Modern View of Atomic Structure
 Atoms are electrically neutral (i.e. the total
charge on an atom is zero).
 Each atom contains the same number of
protons and electrons.
 Helium:
 Magnesium:
 Iron:
2 p, 2 e12 p, 12 e26 p, 26 e-
 The atomic number tells you both the number
of protons and electrons in an atom.
Modern View of Atomic Structure
 The number of neutrons found in atoms of
the same element can vary.
 Hydrogen atoms:
 Always 1 p and 1 e May have 0, 1, or 2 n
 Atoms that have the same number of protons
but different numbers of neutrons are
referred to as isotopes of an element.
Modern View of Atomic Structure
 Isotopes:
 Atoms with the same number of protons
but different numbers of neutrons
 Atoms with the same atomic number but
different mass numbers
 Mass number = # protons + # neutrons
 a calculated value
 A hydrogen atom with 2 neutrons:
 Mass number = 1 p + 2 n = 3
Modern View of Atomic Structure
 Atoms are small pieces of matter and
therefore have mass.
 Atomic weight (atomic mass)
 The average atomic mass of an element
expressed in amu
 Calculated using the weighted average of
each of the naturally occurring isotopes of
an element
Modern View of Atomic Structure
 Finding information on the periodic table:
Atomic
number
Atomic weight
(atomic mass)
5
10.81
B
electronegativity
Element
symbol
en = 2.04
boron
Element
name
 So where do you find mass number???
Nuclear Symbols
 Information about a particular isotope of an
element is often depicted using its nuclear
symbol:
mass
number
atomic
number
X
charge
 For example, the nuclear symbol for a
hydrogen atom with 2 neutrons would be: 3
 Since atoms are electrically neutral,
the charge is zero and is not shown.
H
1
Nuclear Symbols
Example: Use the following nuclear symbol to
determine the information listed below:
35
S
16
Mass number =
Atomic number =
# protons =
# neutrons =
# electrons =
Charge =
Nuclear Symbols
Example: An atom has 21 neutrons and 19
protons. Identify the missing information.
Mass number =
Atomic number =
# protons =
# neutrons =
# electrons =
Charge =
Nuclear symbol:
Ions
 An atom can gain or lose electrons to become
a charged particle called an ion.
 A positively or negatively charged chemical
particle that is formed when an atom (or
group of atoms) gains or loses one or more
electrons
 Cation: a positively charged ion
 Formed when an atom loses one or
more electrons
Cats have
“pos” (paws)
 Anion: a negatively charged ion
 Formed when an atom gains one or more
electrons
Ions
Formation of a cation
e- 3p e4n
e-
Lithium atom
3 p, 4 n, 3 e7
3
Li
Net charge = 0
e
3p
e-
4n
+ e-
Lithium ion (cation)
3 p, 4 n, 2 e7
3
Li
+
Net charge = +1
Ions
Formation of an anion
8p
8n
8e-
Oxygen atom
8p, 8 n, 8e-
16
O
8
Net charge = 0
+
2e-
8p
8n
10e-
Oxide ion (anion)
8p, 8n, 10e-
16 28O
Net charge = -2
Nuclear Symbols of Ions
 Since protons  electrons for an ion, the
charge of the ion must be shown in the
nuclear symbol.
mass
number
atomic
number
X
charge
137
56
Ba
2+
 Charge = # p - # e-
 Show the magnitude of the charge
first and then show the sign (+ or -)
Nuclear Symbols of Ions
Example: Use the following nuclear symbol to
determine the missing information.
137
56
Mass number =
Atomic number =
# protons =
# neutrons =
# electrons =
Charge =
Ba
2+
Nuclear Symbols of Ions
Example: Write the nuclear symbol.
1) 53 p, 74 n, 54 e2) 23 e-, 30 n, charge = +3
3) 10 e-, 10 n, charge = -1