Download ATOMIC THEORY WORKSHEET 1.

ATOMIC THEORY WORKSHEET
1. Which of the following statements of the atomic theory proposed by John Dalton at
the beginning of the 19th century are not quite true in light of modern atomic
physics? Rewrite each statement to reflect the current understanding of the atomic
theory.
An element is made up of atoms. All
atoms of a given element are identical.
Atoms cannot be created or destroyed.
All atoms of a given element have the
same number of protons in the nucleus.
They may differ in the number of
neutrons. Chemically the atoms of a
given element are virtually
indistinguishable: the types of chemical
reactions are the same; the rates may
slightly differ for different isotopes.
Atoms cannot be created or destroyed
in chemical reactions. In chemical
reactions the old bonds between atoms
are broken down and new bonds are
formed. Atoms, however, can be
created or destroyed in nuclear
reactions: radioactive decays, nuclear
fission and fusion.
All atoms of one element have the
same mass. Atoms of two different
elements have different masses.
Atoms of different elements may
combine in the ratio of small, whole
numbers to form compounds.
The main characteristic of an element is
the atomic number. Two elements differ
from each other by their atomic
numbers. In fact, atoms of two different
elements may have masses that are
very close to each other. For example,
39.9624 amu for an atom of argon-40
and 39.9640 amu for an atom of
potassium-40.
This is often the case for many
compounds, especially inorganic. It
was always the case for compounds
known to Dalton. Most organic
compounds (whose composition was
not known to Dalton) do not follow that
rule: C 12 H 22 O 11 (sugar), C 27 H 46 O
(cholesterol), C 17 H 21 NO 4 (cocaine).
2. Calculate the atomic weight of neon using the data on its isotopes provided in the
table below.
Isotope
Number of
Protons
Number of
Neutrons
Mass
Number
Atomic Mass
(amu)
Natural
Abundance
(atom %)
20
10
10
20
19.9924
90.48
21
10
11
21
20.9938
0.27
22
10
12
22
21.9914
9.25
Ne
Ne
Ne
Weighted Average of Atomic Masses = 19.9924 amu × 0.9048 +
+ 20.9938 amu ×0.0027 + 21.9914 amu × 0.0925 = 20.18 amu = Atomic Weight or
Relative Atomic Mass (the number below the symbol in the Periodic Table of the
Elements)
ATOMS, ISOTOPES, ELEMENTS
Symbol
of the
Atom
(Isotope)
Mass
Number
Number of
Protons
in the
Atom
Number of
Neutrons
in the
Atom
Number of
Electrons
in the
Atom
Atomic
Number
235
92
U
92
235
92
143
92
235.0439
amu
uranium
uranium-235
238
92
U
92
238
92
146
92
238.0508
amu
uranium
uranium-238
34
16
S
16
34
16
18
16
33.9679
amu
sulfur
sulfur-34
37
17
Cl
17
37
17
20
17
36.9659
amu
chlorine
chlorine-37
39
19
K
19
39
19
20
19
38.9637
amu
potassium
potassium-39
12
6
C
6
12
6
6
6
12 amu
(exactly)
carbon
carbon-12
13
6
C
6
13
6
7
6
13.0034
amu
carbon
carbon-13
Na
11
23
11
12
11
22.9898
amu
sodium
sodium-23
23
11
Mass of
the Atom
Name of
the Element
Name of
the Isotope