Unit 2
Atomic Number, Mass Number and
Isotopes
Homework
 Complete worksheet 4A
 Read Pages 29-31
 Chapter 2 problems: 8, 10, 14,16
 Test scheduled for Nov. 17, 18 and 19
(Wed., Thursday or Friday)
Distinguishing Between Atoms
Atomic Number
– Unique for each element.
– Equal to the number of protons in the
nucleus of an atom of that element.
– The number of protons must equal the
number of electrons.
Distinguishing Between Atoms
Mass Number
– Mass is concentrated in the nucleus
– Depends on the number of protons and
neutrons.
Distinguishing Between Atoms
Determining atomic composition
from mass number and atomic
number
– Number of protons = number of electrons
= Atomic number
– Number of neutrons = mass number –
number of protons.
Distinguishing Between Atoms
 Representing atomic composition using
atomic number and mass number
Distinguishing Between Atoms
Elements can also be represented
using name and mass number
– Carbon - 12
Distinguishing Between Atoms
Element
Nitrogen-15
Neon-22
Beryllium-9
Symbol Atomic # Mass # # of Neutrons
Distinguishing Between Atoms
Element
Symbol
Atomic
#
Carbon
Mass #
12
K
2
5
Mercury
80
# of
Electrons
# of
Neutrons
6
19
21
12
Helium
# of
Protons
12
4
12
2
5
6
120
Distinguishing Between Atoms
Isotopes
– Equal number of protons and electrons.
– Different number of neutrons
– Different mass number
Chemically alike
Distinguishing Between Atoms
Atomic Mass
– Mass of proton or neutron is small
(1.67 x 10-24 g)
– Mass of electron is negligible in
comparison (9.11 x 10-28 g)
The mass of even the largest atom is
incredibly small
Distinguishing Between Atoms
Compare relative mass using a
reference isotope as a standard
Isotope C-12 assigned a mass of
12 amu
Atomic mass unit (amu) defined as
one-twelfth the mass of carbon-12
Distinguishing Between Atoms
If atomic mass is determined by
the number of protons and
neutrons, why isn’t the mass of an
element a whole number?
Most elements occur as a mixture
of two or more isotopes.
Isotopes of hydrogen
Hydrogen – 1 (protium 99.98%)
Hydrogen – 2 (deuterium
0.0156%)
Hydrogen – 3 (tritium trace
quantities)
Protium
1 proton
1 electron
0 neutrons
Deuterium
1 proton, 1electron, 1neutron
D2O = heavy water
Concentrated with neutrons during
slow electrolysis of water
Greater density and higher boiling
point then H2O
Not radioactive
Tritium
1 proton, 1 electron, 2 neutrons
Formed continuously in upper
atmosphere in nuclear reactions by
cosmic rays
Compounds can be labeled with D or T
by comparing the location of the heavy
Hydrogen
Distinguishing Between Atoms
The atomic mass of an element is a
weighted average mass of the
atoms in a naturally occurring
sample of the element.
Distinguishing Between Atoms
You can calculate the atomic mass
if you know three values:
– The number of stable isotopes of the
element.
– The mass of each isotope.
– The natural percent abundance of each
isotope.
Distinguishing Between Atoms
Chlorine, symbol Cl, has two
isotopes
– Cl-35 (34.97) abundance 75.77%
– Cl-37 (36.97) abundance 24.23%
Calculate the atomic mass of chlorine
4A Atomic Structure
1. 12
2. 12
3. 12.0 amu
4a. 5 b. 16 c. 38 d. 79
5a. hydrogen c. lithium
b. iron
d. argon
7. 6.95 amu
4A Atomic Structure
# of p+ # of e- # of n0 atomic # mass #
element
manganese Mn
25
25
30
25
55
sodium
Na
11
11
12
11
23
bromine
Br
35
35
45
35
80
yttrium
Y
39
39
50
39
89
arsenic
As
33
33
42
33
75
actinium
Ac
89
89
138
89
227
Chapter 2
8. 86 protons, 134 neutrons
10. 54 Fe
26
56
26
Fe
14. a. 34
b. 41
d. 41 n0, 34p+, 36e-
c. 34
Chapter 2 #16
symbol
79
35
Br
charge
0
14
7
N3-
-3
7
7
10
As5+
+5
33
42
28
Zr4+
+4
40
50
36
75
33
90
40
protons neutrons electrons
35
44
35
Homework
Read Chapter 2 pages 33-35
WS: 5.4 The Periodic Table
Chapter 2: problems 20, 22, 24 and 26
Webassign Due Nov. 19
Test: Nov. 17, 18 19 (Wed. Thurs. Fri)
The Periodic Table
Dmitri Mendeleev (1834-1907)
– Russian chemist
– Listed elements in columns in order of
increasing atomic mass.
– Arranged columns so that elements with
similar properties were side by side.
The Periodic Table
Mendeleev left blank spaces where
there were no known elements with
the appropriate properties or mass.
Predicted the properties of the
missing elements.
THE PERIODIC TABLE
Dmitri Mendeleev (1834-1907)
THE PERIODIC TABLE
Henry Mosely (1887-1915)
Determined atomic numbers of
the elements.
Arranged elements in table by
atomic number instead of mass.
THE PERIODIC TABLE
Henry Mosely (1887-1915)
HORIZONTAL ROWS:
PERIODS
 seven periods
 2 to 32 elements in a period
 properties of the elements change
as you move across a period
 this pattern repeats from period to
period: The Periodic Law
VERTICAL COLUMNS:
GROUPS/FAMILIES
Elements in a group have similar
physical and chemical properties.
Groups identified by A or B and a #
Group A are the representative
elements
Group A can be divided into three
broad classes
1. METALS
high electrical conductivity
high luster when clean
ductile
malleable
1. METALS
divided into:
alkali metals – group 1A
alkaline earth metals – group 2A
transition metals – group B
inner transition metals
2. NONMETALS
poor electrical conductivity
non-lustrous
group 7A – halogens
group 8A – noble gases
3. METALLOIDS
properties are intermediate
between metals and nonmetals
5.4 The Periodic Table…
1. groups
2. periods
3. atomic #
4. group
5. rep. elements
6. noble gases
7. transition metals
8. inner tr. metals
9. alkali metals
10. alkaline earth
metals
11. group 7A
12. metalloids
13. ST
14. AT
15. AT
16. NT
5.4 The Periodic Table…
17. E
18. G
19. A
20. F
21. C
22. D
23. H
24. B
25. nonmetals: N, P
metalloids: As, Sb
metal: Bi
26. malleable, ductile,
conduct electricity, etc.
Mass Defect – missing mass?
• Masses of particles that make up an
atom are added, the sum is always
larger than the actual mass of the
atom.
• The missing mass is the matter
converted into energy when the
nucleus was formed from its
component protons and neutrons.
Mass Defect
• Calculate the mass defect of a
chlorine-35 atom.
• The actual mass of chlorine-35
is 5.81 x 10-23 grams
• e = 9.11 x 10-28 g
• p = 1.67 x 10-24 g
• n = 1.67 x 10-24 g