Chemistry 120
Chapter 5: Atomic Theory:
The Nuclear Model of the
Atom
Fig. 5-CO, p. 123
Atomic Theory
• Elements (matter) is composed of small, indivisible
particles called atoms.
• Atoms of a given element are identical in mass and
behavior.
• Atoms of different elements differ in mass and
behavior.
• Chemical combination of elements to make different
substances occurs when atoms join together in small
whole number ratios.
• Chemical reactions only rearrange the way the atoms
are combined; the atoms themselves are not
changed
Law of Multiple Proportions (John Dalton)
• When the same two elements combine to
form more than one compound:
the ratios of the mass of one element in the
first compound to its mass in the second
compound, (as it combines with the same
mass of the other element), can always be
expressed as ratios of small whole numbers
• Carbon combines with oxygen to form CO and
CO2 .
Mass of
Carbon(g)
Mass of
Oxygen(g)
CO
12.01
16.00
CO2
12.01
32.00
Ratio of O in CO2
to O in CO
2:1
Example of the Law of Multiple
Proportions
• That of nitrogen and oxygen forming three
different compounds that contain different
relative amounts of nitrogen and oxygen.
Mass of Nitrogen that
combines with 1 g of Oxygen
• Compound A
1.750 g
• Compound B
0.8750 g
• Compound C
0.4375 g
Example of the Law of Multiple
Proportions continued
The ratios of the masses of nitrogen combining with 1
gram of oxygen in each pair of compound should be
small whole integer number
• A/B = 1.750/0.8750 = 2 for nitrogen
0.8750/0.8750= 1 for nitrogen, a ratio 2:1
• B/C= 0.8750/0.4375 = 2 for nitrogen
0.4375/ 0.4375= 1 for nitrogen, a ratio 2:1
• A/C = 1.750/0.4375 = 4 for nitrogen
0.4375/0.4375= 1 for nitrogen, a ratio 4:1
Sub Atomic Particles
Example – Chemical Symbols
• How many protons does a sulfur atom have?
Example – Chemical Symbols
• How many electrons does a sulfur atom have?
Example – Chemical Symbols
• How many electrons does a sulfide ion, S2-,
have?
Example – Chemical Symbols
• How many protons does an aluminum ion,
Al3+, have?
Example – Chemical Symbols
• How many electrons does an aluminum ion,
Al3+, have?
Example – Chemical Symbols
• An atom has 5 protons, 5 electrons, and 6
neutrons. What is the atomic number?
Example – Chemical Symbols
• An atom has 5 protons, 5 electrons, and 6
neutrons. What is the name of the element?
Example – Chemical Symbols
• An atom has 5 protons, 5 electrons, and 6
neutrons. What is the mass number?
Example – Chemical Symbols
• An atom has 5 protons, 5 electrons, and 6
neutrons. What is the charge?
Example – Chemical Symbols
• An atom has 20 protons, 18 electrons, and 20
neutrons. What is the atomic number?
Example – Chemical Symbols
• An atom has 20 protons, 18 electrons, and 20
neutrons. What is the name of the element?
Example – Chemical Symbols
• An atom has 20 protons, 18 electrons, and 20
neutrons. What is the mass number?
Example – Chemical Symbols
• An atom has 20 protons, 18 electrons, and 20
neutrons. What is the charge?
Example – Chemical Symbols
• What is the atomic notation for an atom that
has 17 protons, 20 neutrons, and 17
electrons?
A.
B.
C.
D.
E.
37
17 𝐶𝑙
34
17 𝐶𝑙
17
37𝐶𝑙
37
17 𝐶𝑙
17
37𝑅𝑏
Example – Chemical Symbols
• What is the atomic notation for an atom that
has 15 protons, 16 neutrons, and 18
electrons?
A.
B.
C.
D.
E.
31
15 𝑃
15
31 𝐺𝑎
31 3−
15 𝑃
15
3−
𝐺𝑎
31
31 3+
15 𝑃
Isotopes
• Atoms which differ only in the number of
neutrons present in the nucleus.
Example – Atomic Mass
• Calculate the atomic mass of magnesium
given
Mass (amu)
%abundance
Mg-24
23.985
78.70
Mg-25
24.986
10.13
Mg-26
25.983
11.17
Example – Atomic Mass
• Calculate the atomic mass of chlorine given
Mass (amu)
%abundance
Chlorine-35
34.969
75.78
Chlorine-37
36.966
24.22
Example – Atomic Mass
• Lithium has two isotopes: lithium-6 at 7.420%
and lithium-7 at 92.58%. If the atomic mass of
lithium is 6.941 amu and the mass of lithium-6
is 6.0151 amu, what the mass of lithium-7?
Example – Atomic Mass
• Bromine has two isotopes. Bromine-79 the
most abundant isotope at 50.69% has a mass
of 78.9183 amu. What is the mass of bromine81?