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Trends in Ionization (ionisation) Energy

Key Concepts

  • Ionization energy is given a number of symbols including I, E, and I.E.

  • Ionization energy is measured in kilojoules per mole (kJ mol-1) or electronvolts per atom (eV)*.
    (1 kJ/mol = 96.4869 x eV)

  • Ionization energy, or ionisation energy, is the energy required to remove an electron from a gaseous atom or ion.
    First Ionization energy : energy required to remove an electron from the gaseous atom
    First Ionization for the element M, M(g) → M+(g) + e, first ionization energy is I1
    First Ionization for Hydrogen: H(g) → H+(g) + e

    First Ionization for Carbon: C(g) → C+(g) + e

    Second Ionization energy : energy required to remove an electron from the gaseous ion
    Second Ionization of element M, M+(g) → M2+(g) + e, second ionization energy is I2
    Second Ionization for Carbon: C+(g) → C2+(g) + e

    The second ionization energy of an element will be higher than the first ionization energy.

  • General trends in the ionization energy of elements in the Periodic Table:
  1. Ionization energy decreases down a group.
  2. Ionization energy increases across a period from left to right.
  • Successive ionization energies for an element provide evidence for the number of electrons occupying the highest energy level, or valence shell, of the atom.

Animated Tutorial

Trends in First Ionization (ionisation) Energy in Groups of the Periodic Table

As you go down a Group in the Periodic Table from top to bottom, the electron being removed occupies a higher energy and is therefore further away from the nucleus.
The force of attraction between the negatively charged electron being removed and the positively charged nucleus decreases as you go down the Group.

Group 1 (IA, Alkali Metals)

Element Atomic
Number
(Z)
Symbol Ionization Reaction First
Ionization
Energy
(kJ/mol)
Trend Energy Level
of Electron
being removed
lithium 3 Li Li(g) → Li+(g) + e 520 (highest)
|
2nd
sodium 11 Na Na(g) → Na+(g) + e 496 | 3rd
potassium 19 K K(g) → K+(g) + e 419 | 4th
rubidium 37 Rb Rb(g) → Rb+(g) + e 403 | 5th
cesium 55 Cs Cs(g) → Cs+(g) + e 376 | 6th
francium 87 Fr Fr(g) → Fr+(g) + e 393 |
\/
(lowest)
7th
First ionization energy decreases as you go down Group 1 because the electron being removed is further from the nucleus (in a higher energy level).

Group 17 (VIIA, Halogens)

Element Atomic
Number
(Z)
Symbol Ionization Reaction First
Ionization
Energy
(kJ/mol)
Trend Energy Level
of Electron
being removed
fluorine 9 F F(g) → F+(g) + e 1681 (highest)
|
2nd
chlorine 17 Cl Cl(g) → Cl+(g) + e 1251 | 3rd
bromine 35 Br Br(g) → Br+(g) + e 1140 | 4th
iodine 53 I I(g) → I+(g) + e 1008 | 5th
astatine 85 At At(g) → At+(g) + e 897 |
\/
(lowest)
6th
First ionization energy decreases as you go down Group 17 because the electron being removed is further from the nucleus (in a higher energy level).

Group 18 (VIIIA, 0, Nobel Gases)

Element Atomic
Number
(Z)
Symbol Ionization Reaction First
Ionization
Energy
(kJ/mol)
Trend Energy Level
of Electron
being removed
helium 2 He He(g) → He+(g) + e 2372 (highest)
|
1st
neon 10 Ne Ne(g) → Ne+(g) + e 2081 | 2nd
argon 18 Ar Ar(g) → Ar+(g) + e 1521 | 3rd
krypton 36 Kr Kr(g) → Kr+(g) + e 1351 | 4th
xenon 54 Xe Xe(g) → Xe+(g) + e 1170 | 5th
radon 86 Rn Rn(g) → Rn+(g) + e 1037 |
\/
(lowest)
6th
First ionization energy decreases as you go down Group 18 because the electron being removed is further from the nucleus (in a higher energy level).

Trends in First Ionization (ionisation) Energy in Periods of the Periodic Table

In general, the first ionization energy of elements increases as you go across a Period from left to right as the nuclear charge increases across a period the electron being removed is more strongly bound to the nucleus, and, as the atomic radius decreases, the negatively charged electron being removed is closer to the positively charged nucleus.
Irregularities in this trend are due to the location of the electron being removed in terms of its orbital and how many electrons are present in that orbital.

Period 2

Element Li Be B C N O F Ne
Electron
Configuration
1s22s1 1s22s2 1s22s22p1 1s22s22p2 1s22s22p3 1s22s22p4 1s22s22p5 1s22s22p6
Location of electron being removed 2s
orbital
2s
orbital
2p
orbital
2p
orbital
2p
orbital
2p
orbital
2p
orbital
2p
orbital
First Ionization
Energy (kJ/mol)
520 899 801 1086 1402 1314 1681 2081
Notes on Irregularities higher energy p orbital 2 electrons in same p orbital
General Trend (lowest)- ----- ----- ----- ----- ----- ----> (highest)
Ionization energy generally increases across Period 2 from left to right as the increasing nuclear charge on successive atoms more tightly binds the electron being removed to the nucleus.

Period 3

Element Na Mg Al Si P S Cl Ar
Electron
Configuration
[Ne]3s1 [Ne]3s2 [Ne]3s23p1 [Ne]3s23p2 [Ne]3s23p3 [Ne]3s23p4 [Ne]3s23p5 [Ne]3s23p6
Location of electron being removed 3s
orbital
3s
orbital
3p
orbital
3p
orbital
3p
orbital
3p
orbital
3p
orbital
3p
orbital
First
Ionization
Energy
(kJ/mol)
496 738 578 787 1012 1000 1251 1521
Notes on Irregularities higher energy p orbital 2 electrons in same p orbital
General Trend (lowest)- ----- ----- ----- ----- ----- ----> (highest)
Ionization energy generally increases across Period 3 from left to right as the increasing nuclear charge on successive atoms more tightly binds the electron being removed to the nucleus.

Trends in Successive Ionization (ionisation) Energy of Elements in the Periodic Table

  • Second ionization energy is greater than first ionization energy for an element as it is harder to remove a negatively charged electron from the positively charged ion.
  • Third ionization energy is greater than second ionization energy for an element because you are now trying to remove a negatively charged electron from ion with a 2+ charge.
  • Removing successive electrons from the same energy level requires a little more energy each time, but removing an electron from a lower energy level requires much, much, more energy because the electron being removed is then so much closer to the positively charged nucleus.

Group 1 (IA, Alkali Metals)

Element First Ionization Energy (kJ/mol)
M(g)→M+(g)+e
Second Ionization Energy (kJ/mol)
M+(g)→M2+(g)+e
Third Ionization Energy (kJ/mol)
M2+(g)→M3+(g)+e
Trend
Li to Cs
Ratio I1:I2 Ratio I2:I3
Li 520 7300 11800 (highest) 1:14.0 1:1.6
Na 496 4570 6920 | 1:9.2 1:1.5
K 419 3080 4400 | 1:7.4 1:1.4
Rb 403 2660 3900 \/ 1:6.6 1:1.5
Cs 376 2430 3400 (lowest) 1:6.5 1:1.4
Trend
I1 to I3
(lowest)-------------------------------->(highest)  
  • Going down the group, the trend is that ionization energy decreases because as you go down the group you are removing an electron from a higher energy level which is further from the nucleus.
    First ionization energy decreases as you go down the group.
    Second ionization energy decreases as you go down the group.
    Third ionization energy decreases as you go down the group.

  • For each element in the Group, the first ionization energy is less than the second ionization energy which is less than the third ionization energy.
    I1 < I2 < I3

    Each time an electron is removed, it results in there being 1 more proton in the nucleus with its positive charge not being balanced by a negatively charged electron, so the relative nuclear charge is increasing.
    As this relative nuclear charge increases, the remaining electrons are more strongly attracted to the nucleus making it even harder to remove the next electron.

  • The ratio of the first ionization energy to the second is much, much, less than the ratio of the second ionization energy to the third.
    I1 << I2 < I3
    or
    I1/I2 << I2/I3

    It is much, much, easier to remove one electron from a Group 1 atom than it is to remove an electron from a Group 1 ion with charge +1.
    This suggests that the second electron being removed is in a lower energy level and therefore closer to the nucleus and much more strongly attracted to it.
    The first electron being removed must be in a higher energy level.

Period 2

Element Electron Configuration I1 (kJ/mol) I2 (kJ/mol) I3 (kJ/mol) I4 (kJ/mol) I5 (kJ/mol) I6 (kJ/mol) I7 (kJ/mol) I8 (kJ/mol)
Li 1s22s1 520 7300 10950 - - - - -
Be 1s22s2 899 1721 14513 20550 - - - -
B 1s22s22p1 801 2403 3617 24931 32294 - - -
C 1s22s22p2 1086 2327 4551 6128 31028 46542 - -
N 1s22s22p3 1402 2830 4544 7063 9348 52712 63618 -
O 1s22s22p4 1314 3390 5308 7490 10617 13324 71745 84097
F 1s22s22p5 1681 3237 5797 8030 9485 13801 17063 87826
General Trend (lowest)- ----- ----- ----- ----- ----- ----> (highest)
  • For each element: I1 < I2 < I3 etc as it becomes increasingly difficult to remove a negatively charged electron from a positively charged ion of charge +1 then +2 etc

  • Removing electrons from a higher energy level is much easier than removing electrons from the lower energy levels which are closer to the nucleus:
    Li(g)Li+(g) + e I1 = 520 kJ/mol
    1s22s11s2  
    Li+(g)Li2+(g) + e I2 = 7300 kJ/molI2/I1 = 14
    1s21s1  
    Removing electrons from the first energy level (1s electrons) is much harder than removing electrons from the second energy level (2s electrons).
    Li2+(g)Li3+(g) + e I3 = 10950 kJ/molI3/I2 = 1.5
    1s1 Li nucleus  
    Removing the second 1s electron is only slightly harder than removing the first.

    Be(g)Be+(g) + e I1 = 899 kJ/mol
    1s22s21s22s1  
    Be+(g)Be2+(g) + e I2 = 1721 kJ/molI2/I1 = 1.9
    1s22s11s2  
    I2 is only slightly greater than I1 because both electrons are being removed from the same energy level.
    Be2+(g)Be3+(g) + e I3 = 14513 kJ/molI3/I2 = 8.4
    1s2 1s1  
    Removing an electron from the first energy level is much, much harder than removing the electrons from the second energy level.
    Be3+(g)Be4+(g) + e I4 = 20550 kJ/molI3/I2 = 1.4
    1s1 Be nucleus  
    Removing the second electron from the first energy is only slighly harder than removing the first one.


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*Another unit of measurement defines an atomic unit of energy, also known as the hartree.
1 hartree = 27.21eV
To convert hartrees to kJ/mol: kJ/mol = hartrees x 27.21 x 96.4869
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