We have actually seen the when aspects react, they frequently gain or lose enough electrons to achieve the valence electron construction of the nearest noble gas. Why is this so? In this section, we construct a much more quantitative technique to predicting such reactions by assessing periodic fads in the energy changes that accompany ion formation.

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Ionization Energies

Because atoms execute not spontaneously shed electrons, energy is required to remove an electron from an atom to form a cation. Cg-tower.comists define the ionization power (\(I\)) that an element as the lot of energy needed to eliminate an electron indigenous the gaseous atom \(E\) in its soil state. \(I\) is as such the energy required for the reaction

\< E_(g) \rightarrow E^+_(g) +e^- \;\;\ \textenergy required=I \label7.4.1\>

Because an input of power is required, the ionization energy is always positive (\(I > 0\)) because that the reaction as written in Equation \(\PageIndex1\). Bigger values the I average that the electron is much more tightly bound come the atom and harder come remove. Typical units because that ionization energies space kilojoules/mole (kJ/mol) or electron volts (eV):

\<1\; eV/atom = 96.49\; kJ/mol\>

If one atom possesses more than one electron, the quantity of power needed to remove successive electrons boosts steadily. Us can define a first ionization energy (\(I_1\)), a 2nd ionization power (\(I_2\)), and in basic an nth ionization energy (\(I_n\)) according to the following reactions:

\< \ceE(g) \rightarrow E^+(g) +e^- \;\;\ I_1=\text1st ionization energy \label7.4.2\>

\< \ceE^+(g) \rightarrow E^2+(g) +e^- \;\;\ I_2=\text2nd ionization energy \label7.4.3\>

\< \ceE^2+(g) \rightarrow E^3+(g) +e^- \;\;\ I_3=\text3rd ionization energy \label7.4.4\>

Values for the ionization energies of \(Li\) and \(Be\) listed in Table \(\PageIndex1\) show that successive ionization energies for an aspect increase as they go; that is, the takes an ext energy to eliminate the second electron indigenous an atom 보다 the first, and also so forth. There are two factors for this trend. First, the second electron is being eliminated from a positively charged species rather 보다 a neutral one, therefore in accordance through Coulomb’s law, an ext energy is required. Second, removing the very first electron reduces the repulsive forces among the staying electrons, for this reason the attraction the the remaining electrons come the nucleus is stronger.

Successive ionization energies because that an facet increase.

Table \(\PageIndex1\): Ionization Energies (in kJ/mol) because that Removing successive Electrons from Li and also Be. Source: Data from CRC Handbook the cg-tower.comistry and also Physics (2004). ReactionElectronic Transition\(I\)ReactionElectronic Transition\(I\)
\(\ceLi (g)\rightarrow Li^+ (g) + e^-\) \(1s^22s^1 \rightarrow 1s^2\) I1 = 520.2 \(\ceBe (g) \rightarrow Be^+(g) + e^-\) \(1s^22s^2 \rightarrow 1s^22s^1\) I1 = 899.5
\(\ceLi^+(g) \rightarrow Li^2+(g) +e^-\) \(1s^2 \rightarrow 1s^1\) I2 = 7298.2 \(\ceBe^+(g) \rightarrow Be^2+(g) + e^-\) \(1s^22s^1 \rightarrow 1s^2\) I2 = 1757.1
\(\ceLi^2+ (g) \rightarrow Li^3+(g) + e^-\) \(1s^1 \rightarrow 1s^0\) I3 = 11,815.0 \(\ceBe^2+(g) \rightarrow Be^3+(g) + e^-\) \(1s^2 \rightarrow 1s^1\) I3 = 14,848.8
\(\ceBe^3+(g) \rightarrow Be^4+(g) + e^-\) \(1s^1 \rightarrow 1s^0\) I4 = 21,006.6

The increase in succeeding ionization energies, however, is no linear, but increases drastically when removing electrons in reduced \(n\) orbitals closer to the nucleus. The many important consequence of the values noted in Table \(\PageIndex1\) is the the cg-tower.comistry of \(\ceLi\) is dominated by the \(\ceLi^+\) ion, while the cg-tower.comistry that \(\ceBe\) is overcame by the +2 oxidation state. The power required to remove the second electron native \(\ceLi\):

\<\ceLi^+(g) \rightarrow Li^2+(g) + e^- \label7.4.5\>

is an ext than 10 times better than the energy needed to remove the an initial electron. Similarly, the energy required to remove the third electron native \(\ceBe\):

\<\ceBe^2+(g) \rightarrow Be^3+(g) + e^- \label7.4.6\>

is about 15 times better than the power needed to remove the very first electron and also around 8 times greater than the energy required to remove the 2nd electron. Both \(\ceLi^+\) and also \(\ceBe^2+\) have 1s2 closed-shell configurations, and much more energy is required to eliminate an electron indigenous the 1s2 core than from the 2s valence orbital of the very same element. The cg-tower.comical consequences are enormous: lithium (and all the alkali metals) creates compounds through the 1+ ion but not the 2+ or 3+ ions. Similarly, beryllium (and every the alkaline earth metals) forms compounds through the 2+ ion but not the 3+ or 4+ ions. The energy required to eliminate electrons native a filled main point is prohibitively big and just cannot be achieved in normal cg-tower.comical reactions.

The energy required to remove electrons from a filled core is prohibitively large under normal reaction conditions.

Ionization Energies the s- and also p-Block Elements

Ionization energies that the aspects in the 3rd row of the periodic table exhibition the same pattern together those of \(Li\) and also \(Be\) (Table \(\PageIndex2\)): succeeding ionization energies rise steadily together electrons are gotten rid of from the valence orbitals (3s or 3p, in this case), followed by an especially large increase in ionization power when electrons are eliminated from filled core levels as indicated by the bold diagonal heat in Table \(\PageIndex2\). Thus in the third row the the periodic table, the largest increase in ionization energy synchronizes to removing the fourth electron native \(Al\), the 5th electron indigenous Si, and so forth—that is, removing an electron from an ion that has actually the valence electron construction of the coming before noble gas. This pattern explains why the cg-tower.comistry of the aspects normally requires only valence electrons. Too much energy is compelled to either remove or share the inner electrons.

Table \(\PageIndex2\): successive Ionization Energies (in kJ/mol) because that the aspects in the third Row the the periodic Table.Source: Data indigenous CRC Handbook of cg-tower.comistry and also Physics (2004). Element\(I_1\)\(I_2\)\(I_3\)\(I_4\)\(I_5\)\(I_6\)\(I_7\) *Inner-shell electron
Na 495.8 4562.4*
Mg 737.7 1450.7 7732.7
Al 577.4.4 1816.7 2744.8 11,577.4.4
Si 786.5 1577.1 3231.6 4355.5 16,090.6
P 1011.8 1907.4.4 2914.1 4963.6 6274.0 21,267.4.3
S 999.6 2251.8 3357 4556.2 7004.3 8495.8 27,107.4.3
Cl 1251.2 2297.7 3822 5158.6 6540 9362 11,018.2
Ar 1520.6 2665.9 3931 5771 7238 8781.0 11,995.3

Example \(\PageIndex1\): Highest 4th Ionization Energy

From their locations in the regular table, predict which of these aspects has the highest fourth ionization energy: B, C, or N.

Given: three elements

Asked for: element with highest 4th ionization energy


list the electron construction of every element. Recognize whether electrons space being removed from a filled or partially filled valence shell. Suspect which aspect has the highest 4th ionization energy, recognizing that the highest energy coincides to the removal of electron from a fill electron core.


A These facets all lie in the 2nd row that the periodic table and have the adhering to electron configurations:

B: 2s22p1 C: 2s22p2 N: 2s22p3

B The fourth ionization power of an element (\(I_4\)) is identified as the energy required to eliminate the fourth electron:


Because carbon and nitrogen have four and also five valence electrons, respectively, their 4th ionization energies correspond to removing an electron indigenous a partly filled valence shell. The 4th ionization power for boron, however, synchronizes to removing an electron from the fill 1s2 subshell. This have to require much much more energy. The actual values space as follows: B, 25,026 kJ/mol; C, 6223 kJ/mol; and N, 7475 kJ/mol.

Exercise \(\PageIndex1\): Lowest 2nd Ionization Energy

From their areas in the routine table, predict i m sorry of these aspects has the lowest second ionization energy: Sr, Rb, or Ar.



The very first column of data in Table \(\PageIndex2\) reflects that first ionization energies tend to increase throughout the third row the the periodic table. This is due to the fact that the valence electrons perform not screen each other an extremely well, enabling the efficient nuclear charge to boost steadily throughout the row. The valence electron are because of this attracted more strongly come the nucleus, so atomic sizes decrease and also ionization energies increase. These impacts represent 2 sides the the same coin: stronger electrostatic interactions between the electrons and also the nucleus additional increase the power required to remove the electrons.

Figure \(\PageIndex1\): A Plot of regular Variation of very first Ionization energy with atom Number for the an initial Six Rows of the routine Table. There is a to decrease in ionization energy within a group (most conveniently seen right here for groups 1 and 18).

However, the an initial ionization energy decreases in ~ Al (3s23p1) and also at S (3s23p4). The electron configurations of these "exceptions" provide the answer why. The electron in aluminum’s to fill 3s2 subshell are better at screening the 3p1 electron 보다 they are at screening each other from the atom charge, therefore the s electrons pass through closer come the nucleus than the p electron does and the p electron is more easily removed. The decrease in ~ S occurs because the two electrons in the exact same p orbital repel each other. This makes the S atom slightly much less stable than would otherwise be expected, together is true of every the team 16 elements.

Figure \(\PageIndex2\): an initial Ionization Energies of the s-, p-, d-, and f-Block Elements

The very first ionization energies that the aspects in the very first six rows of the periodic table room plotted in figure \(\PageIndex1\) and also are presented numerically and graphically in number \(\PageIndex2\). These numbers illustrate three vital trends:

The changes seen in the 2nd (Li to Ne), fourth (K come Kr), fifth (Rb to Xe), and also sixth (Cs to Rn) rows of the s and also p block follow a pattern similar to the pattern described for the third row that the regular table. The change metals are included in the fourth, fifth, and sixth rows, however, and also the lanthanides are contained in the sixth row. The very first ionization energies of the change metals space somewhat similar to one another, as space those the the lanthanides. Ionization energies rise from left come right throughout each row, through discrepancies emerging at ns2np1 (group 13), ns2np4 (group 16), and also ns2(n − 1)d10 (group 12). An initial ionization energies typically decrease under a column. Back the principal quantum number n increases down a column, filled within shells are reliable at screening the valence electrons, so over there is a fairly small boost in the efficient nuclear charge. Consequently, the atoms become larger together they acquire electrons. Valence electron that space farther native the nucleus are much less tightly bound, making them simpler to remove, which causes ionization energies to decrease. A bigger radius typically synchronizes to a reduced ionization energy. Because of the very first two trends, the elements that form positive ion most conveniently (have the lowest ionization energies) lied in the reduced left edge of the routine table, conversely, those that space hardest come ionize lie in the top right corner of the regular table. Consequently, ionization energies generally increase diagonally from reduced left (Cs) come upper best (He).
The darkness the the shading within the cell of the table indicates the relative magnitudes the the ionization energies. Elements in gray have actually undetermined first ionization energies. Source: Data native CRC Handbook of cg-tower.comistry and also Physics (2004).

Generally, \(I_1\) rises diagonally native the reduced left that the routine table to the upper right.

Gallium (Ga), i beg your pardon is the first element following the first row of shift metals, has the complying with electron configuration: 4s23d104p1. Its first ionization power is substantially lower than that the the immediately preceding element, zinc, because the fill 3d10 subshell the gallium lies inside the 4p subshell, shielding the solitary 4p electron native the nucleus. Experiments have revealed other of even greater interest: the 2nd and 3rd electrons that space removed as soon as gallium is ionized come native the 4s2 orbital, not the 3d10 subshell. The cg-tower.comistry of gallium is conquered by the result Ga3+ ion, with its 3d10 electron configuration. This and similar electron construction are specifically stable and also are frequently encountered in the heavier p-block elements. They are periodically referred to together pseudo noble gas configurations. In fact, for elements that exhibit this configurations, no cg-tower.comical compounds are recognized in which electrons are removed from the (n − 1)d10 filled subshell.

Ionization Energies of change Metals & Lanthanides

As us noted, the very first ionization energies that the transition metals and also the lanthanides adjust very tiny across every row. Differences in their 2nd and third ionization energies are additionally rather small, in sharp comparison to the sample seen v the s- and also p-block elements. The factor for these similarities is that the transition metals and also the lanthanides form cations by shedding the ns electrons before the (n − 1)d or (n − 2)f electrons, respectively. This way that change metal cations have actually (n − 1)dn valence electron configurations, and lanthanide cations have (n − 2)fn valence electron configurations. Since the (n − 1)d and also (n − 2)f shells space closer to the nucleus than the ns shell, the (n − 1)d and (n − 2)f electrons display the ns electrons fairly effectively, reduce the efficient nuclear fee felt by the ns electrons. As Z increases, the raising positive charge is mostly canceled by the electrons included to the (n − 1)d or (n − 2)f orbitals.

That the ns electrons space removed prior to the (n − 1)d or (n − 2)f electrons might surprise you because the orbitals were filled in the turning back order. In fact, the ns, the (n − 1)d, and also the (n − 2)f orbitals space so close to one one more in energy, and interpenetrate one another so extensively, that very small changes in the effective nuclear fee can adjust the stimulate of their energy levels. Together the d orbitals space filled, the efficient nuclear charge causes the 3d orbitals to it is in slightly lower in power than the 4s orbitals. The 3d2 electron construction of Ti2+ tells united state that the 4s electron of titanium room lost before the 3d electrons; this is shown by experiment. A similar pattern is seen with the lanthanides, creating cations with an (n − 2)fn valence electron configuration.

Because your first, second, and third ionization energies change so little across a row, these aspects have necessary horizontal similarities in cg-tower.comical nature in enhancement to the meant vertical similarities. For example, all the first-row transition metals except scandium kind stable compounds as M2+ ions, vice versa, the lanthanides primarily kind compounds in which lock exist together M3+ ions.

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Exercise \(\PageIndex2\): Highest an initial Ionization Energy

Use their locations in the periodic table come predict which aspect has the highest an initial ionization energy: As, Bi, Ge, Pb, Sb, or Sn.