What Happens to the Ionization Energy as the Atomic Number Increases Down a Family

3.3: Trends in Ionization Energy

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Skills to Develop

• Describe and explain the observed trends in ionization free energy of the elements

Video \(\PageIndex{i}\): A brief overview of ionization energy.

Variation in Ionization Energies

The amount of energy required to remove the most loosely leap electron from a gaseous atom in its basis land is chosen its first ionization energy (IE_ane). The start ionization energy for an chemical element, 10, is the free energy required to grade a cation with +1 charge:

\[\ce{X}(thou)⟶\ce{X+}(grand)+\ce{eastward-}\hspace{20px}\ce{IE_1}\]

The energy required to remove the 2d virtually loosely spring electron is chosen the second ionization free energy (IE₂).

\[\ce{X+}(g)⟶\ce{X^two+}(1000)+\ce{due east-}\hspace{20px}\ce{IE_2}\]

The energy required to remove the 3rd electron is the third ionization energy, and so on. Energy is always required to remove electrons from atoms or ions, and then ionization processes are endothermic and IE values are ever positive. For larger atoms, the most loosely leap electron is located further from the nucleus and so is easier to remove. Thus, equally size (diminutive radius) increases, the ionization energy should decrease. Relating this logic to what nosotros have simply learned near radii, we would expect first ionization energies to decrease down a group and to increment beyond a period.

Effigy \(\PageIndex{ane}\): The first ionization free energy of the elements in the starting time 5 periods are plotted against their atomic number.

Figure \(\PageIndex{1}\) graphs the relationship between the first ionization energy and the diminutive number of several elements. Inside a period, the values of first ionization energy for the elements (IE₁) generally increases with increasing Z. Down a group, the IE_i value generally decreases with increasing Z. There are some systematic deviations from this trend, however. Notation that the ionization energy of boron (atomic number 5) is less than that of beryllium (diminutive number iv) even though the nuclear charge of boron is greater past one proton. This can be explained because the energy of the subshells increases as fifty increases, due to penetration and shielding (as discussed previously in this chapter). Within any one shell, the s electrons are lower in energy than the p electrons. This means that an southward electron is harder to remove from an atom than a p electron in the same shell. The electron removed during the ionization of beryllium ([He]2due south ⁱⁱ) is an s electron, whereas the electron removed during the ionization of boron ([He]2s ²2p ¹) is a p electron; this results in a lower first ionization energy for boron, even though its nuclear accuse is greater by one proton. Thus, nosotros see a pocket-sized deviation from the predicted trend occurring each time a new subshell begins.

Effigy \(\PageIndex{2}\): This version of the periodic table shows the first ionization energy of (IE: ₁), in kJ/mol, of selected elements.

Another departure occurs as orbitals become more 1-one-half filled. The showtime ionization energy for oxygen is slightly less than that for nitrogen, despite the trend in increasing IE₁ values across a period. Looking at the orbital diagram of oxygen, we tin can encounter that removing one electron will eliminate the electron–electron repulsion caused past pairing the electrons in the twop orbital and will effect in a half-filled orbital (which is energetically favorable). Coordinating changes occur in succeeding periods (note the dip for sulfur later phosphorus in Figure \(\PageIndex{two}\).

Removing an electron from a cation is more than difficult than removing an electron from a neutral cantlet because of the greater electrostatic attraction to the cation. Likewise, removing an electron from a cation with a higher positive charge is more than hard than removing an electron from an ion with a lower charge. Thus, successive ionization energies for one chemical element e'er increment. As seen in Tabular array \(\PageIndex{1}\), there is a large increase in the ionization energies (colour modify) for each element. This spring corresponds to removal of the core electrons, which are harder to remove than the valence electrons. For case, Sc and Ga both have iii valence electrons, so the rapid increase in ionization energy occurs afterward the third ionization.

Table \(\PageIndex{1}\): Successive Ionization Energies for Selected Elements (kJ/mol)

Element	IEone	IE2	IE3	IE4	IEfive	IE6	IE7
K	418.8	3051.eight	4419.6	5876.9	7975.5	9590.vi	11343
Ca	589.8	1145.4	4912.4	6490.6	8153.0	10495.7	12272.9
Sc	633.i	1235.0	2388.7	7090.6	8842.9	10679.0	13315.0
Ga	578.eight	1979.four	2964.6	6180	8298.7	10873.nine	13594.8
Ge	762.2	1537.5	3302.i	4410.six	9021.4	Not available	Non available
As	944.5	1793.6	2735.5	4836.8	6042.nine	12311.5	Not available

Example \(\PageIndex{1}\): Ranking Ionization Energies

Predict the order of increasing free energy for the following processes: IE₁ for Al, IE₁ for Tl, IE₂ for Na, IE_three for Al.

Solution

Removing the vip ¹ electron from Tl is easier than removing the 3p ¹ electron from Al because the higher north orbital is farther from the nucleus, then IE₁(Tl) < IE_i(Al). Ionizing the third electron from

\[\ce{Al}\hspace{20px}\ce{(Al^two+⟶Al^3+ + e- )} \nonumber\]

requires more energy because the cation Al^two+ exerts a stronger pull on the electron than the neutral Al atom, so IE_one(Al) < IE₃(Al). The second ionization energy for sodium removes a core electron, which is a much higher energy process than removing valence electrons. Putting this all together, we obtain:

IE_ane(Tl) < IE₁(Al) < IE₃(Al) < IE₂(Na).

Do \(\PageIndex{1}\)

Which has the everyman value for IE₁: O, Po, Pb, or Ba?

Answer

Ba

Summary

Video \(\PageIndex{2}\): A brief review of ionization energy.

Ionization energy (the energy associated with forming a cation) decreases downwards a group and mostly increases beyond a menstruum considering information technology is easier to remove an electron from a larger, higher energy orbital.

Glossary

ionization energy

energy required to remove an electron from a gaseous atom or ion. The associated number (due east.g., second ionization energy) corresponds to the charge of the ion produced (X²⁺)

Contributors

• Paul Flowers (Academy of N Carolina - Pembroke), Klaus Theopold (University of Delaware) and Richard Langley (Stephen F. Austin State University) with contributing authors.Textbook content produced by OpenStax Higher is licensed under a Creative Commons Attribution License four.0 license. Download for free at http://cnx.org/contents/85abf193-2bd...a7ac8df6@9.110).

• Adelaide Clark, Oregon Plant of Engineering
• Crash Course Chemical science: Crash Class is a division of Complexly and videos are free to stream for educational purposes.
• Guillotined Chemistry is a product of Marking Anticole and available for gratis on Youtube.

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