What is Ionisation energy: Definition and 14 Discussions

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

X(g) + energy ⟶ X+(g) + e−where X is any atom or molecule, X+ is the resultant ion when the original atom was stripped of a single electron, and e− is the removed electron. This is generally an endothermic process. As a rule, the closer the outermost electrons are to the nucleus of the atom, the higher the atom's ionization energy.
The sciences of physics and chemistry use different units for 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 approximately enthalpy, expressed as kilojoules per mole (kJ/mol) or kilocalories per mole (kcal/mol).Comparison of ionization energies of atoms in the periodic table reveals two periodic trends which follow the rules of Coulombic attraction:
Ionization energy generally increases from left to right within a given period (that is, row).
Ionization energy generally decreases 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 having a charge of (n-1). For example, the first three ionization energies are defined as follows:

1st ionization energy is the energy that enables the reaction X ⟶ X+ + e−2nd ionization energy is the energy that enables the reaction X+ ⟶ X2+ + e−3rd ionization energy is the energy that enables the reaction X2+ ⟶ X3+ + e−The term ionization potential is an older and obsolete term for ionization energy, because the oldest method of measuring ionization energy was based on ionizing a sample and accelerating the electron removed using an electrostatic potential.
The most notable factors affecting the ionization energy include:

Electron configuration: this accounts for most element's IE, as all of their chemical and physical characteristics can be ascertained just by determining their respective electron configuration.
Nuclear charge: if the nuclear charge (atomic number) is greater, the electrons are held more tightly by the nucleus and hence the ionization energy will be greater.
Number of electron shells: if the size of the atom is greater due to the presence of more shells, the electrons are held less tightly by the nucleus and the ionization energy will be lesser.
Effective nuclear charge (Zeff): if the magnitude of electron shielding and penetration are greater, the electrons are held less tightly by the nucleus, the Zeff of the electron and the ionization energy is lesser.
Type of orbital ionized: an atom having a more stable electronic configuration has less tendency to lose electrons and consequently has higher ionization energy.
Electron occupancy: if the highest occupied orbital is doubly occupied, then it is easier to remove an electron.Other minor factors include:

Relativistic effects: heavier elements (especially those whose atomic number is greater than 70) are affected by these as their electrons are approaching the speed of light, and hence have a smaller atomic radius/higher IE.
Lanthanide and actinide contraction (and scandide contraction): the unprecedented shrinking of the elements affect the ionization energy, as the net charge of the nucleus is more strongly felt.
Electron pair energies and exchange energy: these would only account for fully filled and half-filled orbitals. A common misconception is that "symmetry" plays a part; albeit, none so far has concluded its evidence.

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  1. J

    Al has higher 2nd ionisation energy — why?

    Homework Statement The question's in the screenshot attached. Homework EquationsThe Attempt at a Solution I don't know why (B) would be right. I feel like since for Mg's second ionisation energy, it is going from Na to Ne, and Ne is another energy level, Mg's 2nd ionisation energy should be...
  2. K

    Solve Stellar Ionisation Homework: Type O Stars

    Homework Statement Hi, I am studying for a test on Monday and I was wondering if someone could clarify something in my notes on the ionisation rate of Type O stars. The line in my notes says; L = 105L0 ⇒ S* ~ 105 /13.6eV ⇒ S* ~ 1049 Basically, I don't have an actual formula for S* and I was...
  3. C

    A Iron ionisation energy value query

    I'm looking at fe ionisation. Wherever I look I get similar values. Probably because everybody just keeps reusing published values... I think the value for the 24 th level is incorrect but I'm not sure how to validate this. The published value is 195200 but I think it should be more like...
  4. G

    B How do beta particles remove electrons?

    Hello all, I'm just learning about beta decay and the emission of beta particles. I have come to an understanding that this is ionising radiation because it has the ability to remove electrons and turn the molecules it interacts with into ions. I've looked on the Internet for this information...
  5. G

    B Electron arrangement and ionisation energy

    Why is it easier to remove an electron from p orbital than s orbital. I thought P orbital have higher energy so more energy is require to remove a electron from P orbital.
  6. P

    Unravelling the Mystery of Rare Gases and Binding Energy

    Hi! I saw something on my lecture notes that I don't really understand. It reads "Rare gases have filled s and p-sub-shells, which leads to a spherically symmetric charge distribution. Since electrons are indistinguishable they take on a common wavefunction. The point is that this results in a...
  7. J

    Bethe Bloch: Calculating ionisation energy losses for protons, alpha and muons

    Hi, this is my first post here. The forum seems like a great idea! I've got finals coming up in a few days and can't seem to get my head around this example and my tutor seems unreachable at the moment. Anyway, here it is: Homework Statement " A beam of particles contains protons, α particles...
  8. K

    Classically predicted ionisation energy of H

    So, as a matter of interest, I tried predicting the ionisation energy of a Hydrogen atom according to classical electrostatics. I started from the point of escape velocity, with Ve=√(2kq1/r) The following bit I think is wrong, but fetches an interesting result. As E=1/2mv2, the analogous...
  9. L

    Does ionisation energy depend on electronegativity?

    Because, if a higher electronegativity means a higher attraction to electrons an atom has, wouldn't that mean that a greater energy would have to be put into ionise an atom with a higher electronegativity?
  10. N

    Ionization Energy: Exploring the Workfunction and Beyond

    Hello! Not sure if this is the right place to post this question. It concerns ionization energies. A certain amount of energy is needed to eject an electron, if the energy provided exactly matches the workfunction, then the electron has zero kinetic energy. If it is some amount greater than...
  11. C

    What is the Ionisation Energy of Hydrogen in KJ mol-1?

    Hi, I've a question that shows me a diagram for wavelength, there's 4 of them 11.1 x 106 10.5 x 106 9.7 x 106 8.4 x 106 then it asked me to determine the ionisation energy of hydrogen in KJ mol-1 by using the above spectrum. From what i know E = hf f = c/lambda lambda being the...
  12. M

    What is the Relationship Between Wavelength and Ionization Energy in Hydrogen?

    Homework Statement The wavelengths λ of spectral lines produced by the Hydrogen atom are given by the expression: . Calculate the Energy in eV required to ionise the atom Homework Equations p=h/\lambda The Attempt at a Solution n2 = 1 and n1 = infinity, therefore \lambda=1/R. E = p2/2m...
  13. C

    D orbitals first ionisation energy

    Hi, may i know why is the first row of the d orbitals (starting from scandium to zinc) should such a weird graph of ionization energy against number of protons? Here the proton number 22 element (titanium) has a higher ionization energy than the proton number 23 element (Vanadium). It is not...
  14. R

    Understanding Atomic Electron Binding Energy

    Why is the first ionisation energy of chlorine is higher than that of sulphur ? i am having trouble with the questions above please help thanks :smile: