Atomic Structure
& Compounds
(loosely based on Chapter 3 Sec 1 thru 4
of Jespersen 6th ed)
Dr. C. Yau
Fall 2013
1
Dalton's Atomic Theory (p.10)
1. Matter consists of tiny particles called atoms.
2. In any sample of a pure element, all the atoms are
identical in mass (?) and other properties.
3. The atoms of different elements differ in mass and
other properties.
4. When atoms of different elements combine they form
compounds with the elements in a fixed ratio by
mass.Chemical Reactions are merely rearrangement
of atoms to form different compounds.
5. Atoms are indestructible (?) . In chemical reactions,
the atoms rearrange but they do not themselves
2
break apart.
The 3 major subatomic particles
Know this well !
Name &
Symbol
Mass
Charge
proton (p)
~1u
+1
inside nucleus
neutron (n)
~1u
0
inside nucleus
electron (e-)
~0u
-1
outside nucleus
Location
u (amu) = atomic mass unit
(a very small unit of mass)
1 gram = 602,000,000,000,000,000,000, 000 u
3
Atomic Mass
• Dalton’s atomic theory states that atoms of
an element have a constant, characteristic
atomic mass or atomic weight measured
in u.
• Atomic masses are based on a standard
mass, that of an atom of C.
• 1 atom of carbon-12 weighs 12 u exactly.
• Thus 1 u = 1/12 the mass of a carbon-12
atom. This is the definition of u.
4
Isotopes
• Most elements in nature are uniform mixtures of two
or more kinds of atoms with slightly different masses.
• For example: There are 3 isotopes of hydrogen, 4
isotopes of iron and 10 isotopes of tin.
• Isotopes
KNOW WELL
are atoms of the same element with different masses.
• Isotopes
are atoms with the same #p but with different #n.
• Chemically, isotopes have virtually identical
properties. (There are slight differences that we will
note be concerned with at this level of chemistry.)
5
A
X
Z
An element is a substance whose atoms all
Atomic Notation
•
contain the identical number of protons, called
the atomic number (Z)
• Isotopes are distinguished by the
mass number (A):
– Atomic number, Z = number of protons
– Mass number, A = (# of protons) + (# of neutrons)
– Note that for atoms, A is greater than Z: the symbol
is "top-heavy."
• For neutral atoms, the number of electrons and
protons must be equal.
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Example: uranium-235
This indicates the mass number.
Mass number, (protons + neutrons) 
Chemical Symbol 
Atomic number, (number of protons) 
235
U
92
From the nuclear symbol we can determine
the # protons, electrons and neutrons:
# protons = 92
# electrons = 92 (since no charge is shown)
#neutrons = 235 – 92 = 143
7
Fill in the blanks:
symbol
neutrons
60Co
33
81Br
46
65
29
36
Cu
protons
electrons
27
27
35
35
29
29
8
This is how Fe appears
in the periodic table.
How many neutrons are
there in Fe?
26
Fe
Trick question! You can’t tell!
55.85
How many neutrons are there
in 53Fe?
A) 26
B) 55
This is NOT the atomic mass number.
C) 27
D) none of these
Ans. 53 – 26 = 27
mass number - # protons = # neutrons
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Hydrogen has 3 isotopes:
1
1H
protium
2
1H
deuterium
3
1H
tritium
99.985%
abundance
1 proton
0.015 %
abundance
1 proton
0.000 %
abundnace
1 proton
no neutron
1 neutron
2 neutrons
approx. mass of each
atom = 1 u
2u
3u
10
What we call "heavy water" is D2O.
(Regular water being H2O, with practically all
the hydrogen atoms being protium.)
One molecule of H2O weighs 18 u.
One molecule of D2O weighs 20 u.
Hence D2O is known as “heavy water.”
Tritium is the only radioactive isotope of hydrogen.
Such isotopes are referred to as a radioisotope.
NOTE: The term "isotope" does NOT mean that it is
radioactive.
11
Naturally occurring chlorine is a mixture of two
isotopes. In every sample of this element,
75.77% of the atoms are chlorine-35 and
24.23% are chlorine-37. The measured mass of
chlorine-35 is 34.9689 u and that of chlorine-37
is 36.9659 u. Calculate the average atomic mass
of chlorine.
0.7577 x 34.9689 u = 26.495 u (4 sig. fig.)
0.2423 x 36.9659 u = 8.9568 u (4 sig. fig.)
atomic mass
= 35.4518 u (2 decimal places)
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Cl
35.45
35.45 u
This is the atomic mass
or atomic weight.
12
There are 2 isotopes of element Z. The first
is 56.5% in abundance and has a mass of
152.3 u. If the atomic mass is 155.5 u,
what is the mass of the other isotope?
Hint: Let x be the mass of the other isotope.
What % in abundance is this other isotope?
159.6 u = 160. u (3 sig.fig.)
13
Some Important Classifications:
•A groups = representative elements
or main group elements
I A = alkali metals
VII A = halogens
II A = alkaline earth metals
VIII = noble (inert) gases
•B groups = transition elements
• Inner transition elements = elements 58 – 71
and 90 – 103
58 – 71 = lanthanide elements
90 – 103 = actinide elements
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Metals, Nonmetals, And Metalloids
15
Properties Of Metals
Metals...
• reflect light (have metallic luster)
• can be hammered or rolled into thin sheets
(are malleable) and can be drawn into a
wire (are ductile)
• are solids at room temperature
(except Hg)
• conduct electricity and heat
16
Nonmetals And Metalloids
Nonmetals...
– lack the properties of metals
– tend to pulverize when struck with a hammer
– Are non-conductors of electricity and heat
– Many are gases, a few solids,
and one liquid (Br2)
– react with metals to form (ionic) compounds
• Metalloids
– have properties between metals and
nonmetals
17
EXTREMELY IMPORTANT!
nonmetal + nonmetal
covalent molecules
nonmetal + metal
ionic compounds
metal + metal
alloys
Note: Alloys are not compounds. They
are homogeneous mixtures of metals,
sometimes with a small amount of
nonmetals, such as C in steel.
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Learning Check:
Count The Atoms In A Chemical Formula
•
•
•
•
•
Na2CO3
(NH4)2SO4 •
Mg3(PO4)2 •
CuSO4•5H2O •
1 C, ___
2
3 O
___Na,
___
2
8
1
4
___N,
___H,
___S,
____O
8
3
2
___Mg,
___P,
____O
1
1
10
9
___Cu,
___S,
___O,
___H
What exactly does this
dot tell you?
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Covalent Molecules
The subscripts in the formula of covalent
molecules tell you exactly how many
atoms of each element is present.
They do not merely state a ratio.
e.g. CO2 tells us the molecule contains one
C atom and two O atoms.
Ionic Compounds
• Positively charged ions are called cations
• Negatively charged ions are called anions
• Subscripts in the formula always specify the
smallest whole-number ratio of the ions
needed to make a neutral combination
(formula unit, or f.u.)
2 Fe3+ + 3 O2-
Fe2 O 3
It does not mean 2 Fe ions are bonded to 3 O ions.
Subscripts tell us the ions are in a ratio of 2Fe to 3 O.
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The subscripts in the formula of
• covalent molecules tell you the exact
number of the atoms of each element is
present.
• ionic compounds tell you the ratio of the
atoms of each element that is present.
Overview: Molecules vs. Formula Units
• Electrically neutral, discrete particles
called molecules. (H2O)
• Neutral groups of charged particles called
formula units.
(NaCl)
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Why are ions charged?
•
•
•
•
# p+ = # e- if neutral
# p+ < # e- if negative
# p+ > # e- if positive
The number of p+ never changes when
ions form.
• Ions are formed when the atom gains or
loses electrons. Protons are not affected.
How does Ca form Ca2+? Ca lost 2 electrons
How is N3- formed?
N gained 3 electrons
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Fill in the blanks:
Symbol
neutrons
60Co3+
33
81Br-
Cu 2 
65
29
46
36
protons
electrons
27
24
35
36
29
27
25
Summary of Properties
Hardness and brittleness
–Molecular compounds tend to be soft and easily
crushed because the attractions between
molecules are weak and molecules can slide past
each other.
–Ionic compounds are hard and brittle because of
the strong attractions and repulsions between ions
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Melting Points
• To melt the a solid, there must be
sufficient kinetic energy to overcome the
attractions between particles
– Molecular compounds have weak attractions
between particles and so tend to have low
melting points.
– Many molecular compounds are gases at
room temperature.
– Ionic compounds tend to have strong
attractions so they have high melting points.
– Nearly all ionic compounds are solids at
room temperature.
27
Electrical Conductivity
• Requires the movement of electrical charge
• Ionic compounds:
– Do not conduct electricity in the solid state
– Do conduct electricity in the liquid and
aqueous states-The ions are free to move.
• Molecular compounds:
– Do not conduct electricity in any state.
– Molecules are comprised of uncharged
particles.
– Exception: Strong acids are molecular
substances that become ionic when dissolved
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in water.
Which of the following is likely true of NO2?
a) It conducts electricity well.
b) It has a low melting point.
c) It is likely a solid in its pure form.
d) None of these
The formula tells you it’s a molecular
compound, so it is not expected to conduct
electricity, should have a relatively low mp,
and therefore not likely to be a solid in its
pure form.
You should know how the formula tells you that
it’s a molecular formula. See Slide 18!
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