Elemental Symbols
The Periodic Table of Elements is a table with a series of small boxes which contain
valuable information about all of the existing elements in our universe. For review,
please look at the elements below and answer the following questions.
Ca
20
40.08
1. The atomic symbol for this element is ________________. This element is
______________.
2. The atomic number for this element is ________________.
3. The atomic number is consist with the number of which subatomic particle in the
element? __________________.
4. What is the atomic mass or weight of this element? ________________ (no unit is
necessary)
5. How many protons and neutrons does this element have?
!
protons ______________! !
neutrons ______________
Se
34
78.96
1. The atomic symbol for this element is ________________. This element is
______________.
2. The atomic number for this element is ________________.
3. The atomic number is consist with the number of which subatomic particle?
__________________.
4. What is the atomic mass or weight of this element? ________________ (no unit is
necessary)
5. How many protons and neutrons does this element have?
!
protons ______________! !
neutrons ______________
Elemental Symbols and Isotopes
While the periodic table boxes above display information about an elements number,
symbol and weight it does not speak directly for its isotopes. Please look at the symbols
below to learn how isotopes are represented.
!
!
!
!
These three symbols all speak for the element Carbon. The bottom numbers represent
the atomic number and the top numbers the atomic weight. In writing we would name
these isotopes Carbon-14, Carbon-13 and Carbon-12 accordingly. Please write out the
information for the these isotopes below.
1. Here again are three isotopes for the element Carbon.
a. What is the atomic weight of Carbon-14? _____________
b. What is the atomic weight of Carbon-12? _____________
c. How many neutrons does Carbon-13 have? _____________
d. How many neutrons does Carbon-14 have? _______________
2. Here are two isotopes of the element Phosphorus:
24
15
P
39
15
P
a. What is the atomic weight of Phosphorus-24? _____________
b. What is the atomic weight of Phosphorus-39? _____________
c. How many neutrons does Phosphorus-24 have? _____________
d. How many neutrons does Phosphorus-39 have? _______________
Atomic Weights
A considerable amount of time has been spent researching atomic weight. In 1900 two
English scientists, J.J. Thomson and F.W. Aston worked on a piece of equipment to
learn how to prove and measure the existence of various isotopes of elements. They
were ultimately successful and invented a device called the mass spectrograph (also
called a mass spectrometer) which could separate atoms of different masses of the
same element by accelerating them with an electron beam. The electron beam
accelerated the atoms towards a photographic plate. This with the additional presence
of a strong magnetic field would bend atoms of a different masses in different ways.
Lighter atoms would bend to a greater degree and heavier atoms would bend to a
lesser degree. This can be compared to a basket ball and bowling ball traveling in a
straight line and encountered by a strong wind. The basketball would bend to a greater
degree, as its lighter weight would make it easier to influence. The bowling bowls line of
movement would bend to a lesser degree because of its greater mass. For this
experiment neon gas was used.
The atomic weights represented on the periodic table are the average weights of the
isotopes of each element. We can determine the average atomic weight of an element if
we know the approximate mass of each isotope and the proportion of each isotope
within the element.
For example, Fluorine-19 is much more abundant than its isotope Fluorine-18. The
proportions of each are: 99.7% for Fluorine-19 and 0.3% for Fluorine-18. To determine
the average weight you can follow this calculation:
Example:
Isotope!
19
F
9
Mass of Isotope!
Proportion in element!
Mass x proportion! Sum
19.000
0.997
18.943
x
=
+
18
9
F
18.000
x
0.003
=
=
18.997
.054
This calculation takes the mass of each isotope and multiplies it by its percentage
abundance (represented in decimals). It then adds the two (mass x proportion) answers
and gets a sum. This sum is the relative atomic weight for the element. Here that sum is
18.997.
Problems:
Please do the next two problems on your own:
1. Sodium has two isotopes, 1123Na and 1122Na. The isotope 1123Na has an atomic mass
of approximately 23.000 and its proportion in the element is 99.2%. The isotope
22
Na has a mass approximately 22.000 and a proportion within the element of
11
0.8%,
Isotope!
Mass of Isotope!
Proportion in element!
Mass x proportion! Sum
23
Na
11
23.000
0.992
__________
x
=
+
=
______
22
Na
11
22.000
x
0.008
=
__________
2. Cobalt has an isotope, 2760Co which has an approximate mass of 60.00 and
constitutes 48.0% of the element. Another isotope 2758Co has an approximate
mass of approximately 58.00 and constitutes 52.0% of the element. Please
calculate the atomic weight of Co using this data.
Isotope!
60
Co
27
58
27
Co
Mass of Isotope!
Proportion in element!
Mass x proportion! Sum
60.00
0.480
=
__________
=
+
__________
58.00
x
x
0.520
=
______