Earlier in this lesson, you learned that potassium has a lower ionization energy than sodium. This means that it is easier to remove an electron from potassium than from sodium.
Reminder
If the ionization energy is low, that means that it does not take as much energy to remove the electron, so it is easier to remove.
Look at this example
The electron configurations for sodium and potassium are:
Sodium: 1s22s22p63s1
Potassium: 1s22s22p63s23p64s1
The valence electron in potassium is farther away from the nucleus than the valence electron in sodium.
Which element has less pull on its valence electron by the nucleus?
How does this relate to the reaction of potassium with water and sodium with water?
The valence electron in potassium feels less pull from the nucleus because it is farther from the nucleus.
Since the valence electron in potassium feels less pull from the nucleus, it will be easier to remove it, and thus potassium will react more vigorously with water than sodium.
Many properties of the elements tend to gradually change in a predictable way as you move down a group or across a period of the periodic table. Each of these tendencies is known as a periodic trend.
The patterns that ionization energy exhibit on the periodic table are summarized in this table.
| General Trend for Ionization Energy in a Group | Generally, those elements that are lower in a group will have lower ionization energy so it will be easier to remove electrons. |
| General Trend for Ionization Energy in a Period | A pattern is also seen when going across a period. Since there are more protons and electrons, the pull between them is more, so it is harder to remove an electron, and the ionization energy will be higher. |
Let's Practice
Show your understanding of the trend of ionization energies by completing this activity. Complete both ordering activities.
If you need a periodic table, click below to open an interactive periodic table or to download a PDF.
Put the alkali metals in order from the least ionization energy to the most.
Cesium
Rubidium
Potassium
Sodium
Lithium
Put the following third period elements in order from the least ionization energy to the most.
Sodium
Magnesium
Aluminum
Silicon
Phosphorus
Sulfur
Chlorine
Reflection
How does the pattern in ionization energy in the alkali metals explain the difference between sodium and potassium in their reaction with water?
