Lesson 10: A Final Trend of the Periodic Table: Atom Size
(See pages 135-137)
How large are the atoms of elements found on the periodic table? While we understand that individual atoms are too
small for us to see, chemists have devised ways to measure how large they are in relationship to each other. In this
lesson, we will examine how the atom sizes vary as you move across the table through elements which make up a
period and also how size varies as you move down through members of a family. The table on the next page tells the
approximate size of atoms of each element measured from the nucleus to the outer edge in units called angstroms. This
measurement is known as the atomic radius. Look at the diagram below to help you understand the measurement of
atomic radius. Use the information in the table to complete the graph like you did when you graphed ionization energy
values in Lesson 9.
The atomic radius is measured from the
nucleus to the outermost layer of
electrons.
To see how the size of the elements varies as you move across a series of elements on the periodic table of elements,
look at your graph and find the series of elements which begins with potassium and ends with krypton. Note that a
series of elements is a row of elements going from left to right across the table. Note that potassium is at a peak and as
you move across through the series the radius tends to decrease until you get to the smallest atom in the series which is
krypton
It is theorized that the atoms decrease in size as you move across through a series because of the increased
attractive forces between the nucleus and the electron cloud. This is thought to be due to the increased number of
positively charged protons in the nucleus and negatively charged electrons in orbit. Like opposite poles on a magnet
which attract each other, opposite charges in an atom attract each other. As you increase the number of oppositely
charged particles (protons and electrons) the force between them tends to increase which results in the electron cloud
being more compact.
Element
Atomic Radius
Element
Atomic Radius
Element
Atomic Radius
Hydrogen
0.32
Potassium
2.03
Rubidium
2.16
Helium
0.31
Calcium
1.74
Strontium
1.91
Lithium
1.23
Scandium
1.44
Yttrium
1.62
Beryllium
0.89
Titanium
1.32
Zirconium
1.45
Boron
0.82
Vanadium
1.22
Niobium
1.34
Carbon
0.77
Chromium
1.18
Molybdenum
1.30
Nitrogen
0.74
Manganese
1.17
Technetium
1.27
Oxygen
0.70
Iron
1.17
Ruthenium
1.25
Fluorine
0.68
Cobalt
1.16
Rhenium
1.25
Neon
0.67
Nickel
1.15
Palladium
1.28
Sodium
1.54
Copper
1.17
Silver
1.34
Magnesium
1.36
Zinc
1.25
Cadmium
1.48
Aluminum
1.18
Gallium
1.26
Indium
1.44
Silicon
1.11
Germanium
1.22
Tin
1.72
Phosphorus
1.06
Arsenic
1.20
Antimony
1.53
Sulfur
1. 11
Selenium
1.17
Tellurium
1.42
Chlorine
0.99
Bromine
1.14
Iodine
1.32
Argon
0.98
Krypton
1.12
Xenon
1.24
The overall trend is as you move across the table (from left to right) through a series of elements, atomic size tends to
decrease. Let's examine how atomic size varies as you move down through a family of elements. Recall that as you move
down through a family of elements, an additional energy level is added with each downward step you take. Look at the
sodium family, for example.
H: 1s
Li: 1s 2s
Na: 1s 2s 2p 3s
K: 1s 2s 2p 3s 3p 4s
Rb: 1s 2s 2p 3s 3p 4s 3d 4p 5s
C5: 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s
Note: With each step down through the alkali metal family, one additional energy level is added.
Now, examine the atomic size of each member of the alkali metals family by finding them on your graph from the
previous page. Note that as you move down through the sodium family, with each step, the radius measurement grows.
Theorists believe that with each addition of energy level, the radius increases. In other words, as each energy level is
added, the size of the atom increases. Therefore, as you move down through members within the same family, the
atomic radius increases.
Size decreases as you move across
through a series or period of elements
Size increases as you
move down through a
family of elements
Notice the Nobles Gases
are an exception and
are large
Lesson 10 Practice - Atomic size
Listed below are pairs of elements. By looking at your periodic table of elements, determine
first if they are members of the same family or are found in the same series on the periodic
table. Based upon that information, tell which of the two elements that has the greater atomic
radius.
Element A
Element B
Which element has the larger atomic radius?
Write it's symbol here.
(The first one has been done for you!)
1. Hydrogen
Sodium
2. Sodium
Silicon
3. Potassium
Lithium
4. Chromium
Cobalt
5. Boron
Gallium
6. Fluorine
Nitrogen
7. Argon
Xenon
8. Iron
Osmium
9. Chromium
Calcium
10. Krypton
Copper
11. Rubidium
12. Zinc
Francium
Nickel
13. Sulfur
Tellurium
14. Arsenic
Bromine
15. Technetium
Manganese
16. Magnesium
Chlorine
17. Helium
Radon
18. Potassium
Scandium
19. Calcium
Chromium
20. Chlorine
Aluminum
Na