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.

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.

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.

/ Which element has the larger atomic radius?
Element A / Element B / Write it's symbol here.
(The first one has been done for you!)
1. Hydrogen / Sodium / Na
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 / Francium
12. Zinc / 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

Lesson 10 Practice - Atomic size