Ionization energy

Periodic trends for ionization energy (Ei) vs. atomic number: note that within each of the seven periods the Ei (colored circles) of an element begins at a minimum for the first column of the periodic table (the alkali metals), and progresses to a maximum for the last column (the noble gases) which are indicated by vertical lines and labelled with a noble gas element symbol, and which also serve as lines dividing the 7 periods. The maximum ionization energy for each row diminishes as one progresses from row 1 to row 7 in a given column, due to the increasing distance of the outer electron shell from the nucleus as inner shells are added.

In physics and chemistry, ionization energy (American English spelling) or ionisation energy (British English spelling), denoted Ei, is the minimum amount of energy required to remove the most loosely bound electron, the valence electron, of an isolated neutral gaseous atom or molecule. It is quantitatively expressed as

X + energy → X+ + e

where X is any atom or molecule capable of ionization, X+ is that atom or molecule with an electron removed, and e is the removed electron. This is generally an endothermic process. Generally, the closer the outermost electrons are to the nucleus of the atom , the higher the atom's or element's ionization energy.

The sciences of physics and chemistry use different measures of ionization energy. In physics, the unit is the amount of energy required to remove a single electron from a single atom or molecule, expressed as electronvolts. In chemistry, the unit is the amount of energy required for all of the atoms in a mole of substance to lose one electron each: molar ionization energy or enthalpy, expressed as kilojoules per mole (kJ/mol) or kilocalories per mole (kcal/mol).[1]

Comparison of Ei of elements in the periodic table reveals two periodic trends:

  1. Ei generally increases as one moves from left to right within a given period (that is, row).
  2. Ei generally decreases as one moves from top to bottom in a given group (that is, column).

The latter trend results from the outer electron shell being progressively farther from the nucleus, with the addition of one inner shell per row as one moves down the column.

The nth ionization energy refers to the amount of energy required to remove an electron from the species with a charge of (n-1). For example, the first three ionization energies are defined as follows:

1st ionization energy
X → X+ + e
2nd ionization energy
X+ → X2+ + e
3rd ionization energy
X2+ → X3+ + e

The term ionization potential is an older name for ionization energy,[2] because the oldest method of measuring ionization energy was based on ionizing a sample and accelerating the electron removed using an electrostatic potential. However this term is now considered obsolete.[3]Some factors affecting the ionization energy include:

  1. Nuclear charge: the greater the magnitude of nuclear charge the more tightly the electrons are held by the nucleus and hence more will be ionization energy.
  2. Number of electron shells: the greater the size of the atom, the less tightly the electrons are held by the nucleus and ionization energy will be less.
  3. Effective nuclear charge (Zeff): the greater the magnitude of electron shielding and penetration the less tightly the electrons are held by the nucleus, the lower the Zeff of the electron, and hence less will be the ionization energy.[4]
  4. Type of orbital ionized: the atom having a more stable electronic configuration has less tendency to lose electrons and consequently has high ionization energy.
  5. Occupancy of the orbital matters: if the orbital is half or completely filled then it is harder to remove electrons

Values and trends

Electron binding energy vs Z.jpg

Generally, the (n+1)th ionization energy is larger than the nth ionization energy. When the next ionization energy involves removing an electron from the same electron shell, the increase in ionization energy is primarily due to the increased net charge of the ion from which the electron is being removed. Electrons removed from more highly charged ions of a particular element experience greater forces of electrostatic attraction; thus, their removal requires more energy. In addition, when the next ionization energy involves removing an electron from a lower electron shell, the greatly decreased distance between the nucleus and the electron also increases both the electrostatic force and the distance over which that force must be overcome to remove the electron. Both of these factors further increase the ionization energy.

Some values for elements of the third period are given in the following table:

Successive ionization energy values / kJmol−1
(96.485 kJ/mol ≡ 1 eV)
Element First Second Third Fourth Fifth Sixth Seventh
Na 496 4,560
Mg 738 1,450 7,730
Al 577 1,816 2,881 11,600
Si 786 1,577 3,228 4,354 16,100
P 1,060 1,890 2,905 4,950 6,270 21,200
S 999.6 2,260 3,375 4,565 6,950 8,490 27,107
Cl 1,256 2,295 3,850 5,160 6,560 9,360 11,000
Ar 1,520 2,665 3,945 5,770 7,230 8,780 12,000

Large jumps in the successive molar ionization energies occur when passing noble gas configurations. For example, as can be seen in the table above, the first two molar ionization energies of magnesium (stripping the two 3s electrons from a magnesium atom) are much smaller than the third, which requires stripping off a 2p electron from the neon configuration of Mg2+. That electron is much closer to the nucleus than the previous 3s electron.

Ionization energy is also a periodic trend within the periodic table organization. Moving left to right within a period, or upward within a group, the first ionization energy generally increases, with some exceptions such as aluminum and sulfur in the table above. As the nuclear charge of the nucleus increases across the period, the atomic radius decreases and the electron cloud becomes closer towards the nucleus.