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6.17: Periodic Trends - Ionization Energy

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    372933
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    Sheep herds are like protons and electrons
    Figure \(\PageIndex{1}\) (Credit: Paul Englefield; Source: http://commons.wikimedia.org/wiki/File:Sheep_herd.jpg; License: CC BY 2.0)

    Why do sheep travel in herds?

    Like many other animals, sheep travel in herds. The tendency is for each individual sheep to stay with the herd. However, a sheep may sometimes wander off, depending on how strong the attraction is for a particular food or water supply. At other times, a sheep may become frightened and run off. Whether a sheep chooses to stay with the herd or go its own way depends on the balance between attraction to the herd and attraction to the outside influence.

    There is an on-going tension between the electrons and protons in an atom. Reactivity of the atom depends in part on how easily the electrons can be removed from the atom. We can measure this quantity and use it to make predictions about the behaviors of atoms.

    Ionization Energy

    Ionization energy is the energy required to remove an electron from a specific atom. It is measured in kJ/mol, which is an energy unit, much like calories. The ionization energies associated with some elements are described in table below. For any given atom, the outermost valence electrons will have lower ionization energies than the inner-shell kernel electrons. As more electrons are added around a nucleus, the outer electrons become shielded from the nucleus by the inner shell electrons. This is called electron shielding.

    Table \(PageIndex{1}\): Ionization Energies (kJ/mol) of the First 18 Elements

    Element

    IE1

    IE2

    IE3

    IE4

    IE5

    IE6

    H

    1312

     

     

         

    He

    2373

    5251

     

     

     

     

    Li

    520

    7300

    11,815

     

     

     

    Be

    899

    1757

    14,850

    21,005

     

     

    B

    801

    2430

    3660

    25,000

    32,820

     

    C

    1086

    2350

    4620

    6220

    38,000

    47,261

    N

    1400

    2860

    4580

    7500

    9400

    53,000

    O

    1314

    3390

    5300

    7470

    11,000

    13,000

    If we plot the first ionization energies vs. atomic number for the main group elements, we would have the following trend

    Graph of ionization energy versus atomic number
    Figure \(\PageIndex{2}\): Ionization energy and atomic number. (Credit: User:RJHall/Wikimedia Commons; Source: http://commons.wikimedia.org/wiki/File:Ionization_energies.svg; License: Public Domain)

    Moving from left to right across the periodic table, the ionization energy for an atom increases. We can explain this by considering the nuclear charge of the atom. The more protons in the nucleus, the stronger the attraction of the nucleus to electrons. This stronger attraction makes it more difficult to remove electrons. 

    Within a group, the ionization energy decreases as the size of the atom gets larger. On the graph, we see that the ionization energy increases as we go up the group to smaller atoms. In this situation, the first electron removed is farther from the nucleus as the atomic number (number of protons) increases.  Being farther away from the positive attraction makes it easier for that electron to be pulled off.

    Trends of ionization energy, electron affinity, and metallic character
    Figure \(\PageIndex{3}\) (Credit: User:Mirek2/Wikimedia Commons; Source: http://commons.wikimedia.org/wiki/File:Periodic_trends.svg; License: Public Domain)

    Summary

    • Ionization energy refers to the amount of energy needed to remove an electron from an atom.
    • Ionization energy decreases as we go down a group.
    • Ionization energy increases from left to right across the periodic table.

    Review

    1. Define “ionization energy.”
    2. Do valence electrons have larger or smaller ionization energies that the inner-shell kernel electrons?
    3. What is electron shielding?
    4. Describe the trends in ionization energy from left to right across the periodic table.
    5. Describe the trends in ionization energy from top to bottom of a group in the periodic table.
    6. Why is the second ionization energy for lithium so much larger than the first ionization energy?

    6.17: Periodic Trends - Ionization Energy is shared under a CK-12 license and was authored, remixed, and/or curated by CK-12 Foundation via source content that was edited to conform to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.

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