Ionisation enthalpy means energy change when an electron is removed.

According to Wikipedia

A chemical property is any of a material's properties that becomes evident during, or after, a chemical reaction; that is, any quality that can be established only by changing a substance's chemical identity.

Chemical changes are defined as changes that involve breaking or formation of bonds.

Ionisation is not involving any breaking or formation of bond.

But when we are removing electron from any atom we are changing its chemical properties. Properties of Sodium cation are not as same as Sodium metal.

So my question is whether ionisation enthalpy physical or chemical property?

Also please tell me about whether electronegativity is physical or chemical property?

  • $\begingroup$ What if definitions of both property domains overlap? Similarly, there is no sharp boundary between chemistry and physics. $\endgroup$
    – Poutnik
    Sep 18, 2023 at 7:25
  • 1
    $\begingroup$ Like, the ionization enthalpy has aspects of both physical and chemical property. $\endgroup$
    – Poutnik
    Sep 18, 2023 at 7:41
  • 3
    $\begingroup$ The question is void. All chemical properties are physical properties, too. $\endgroup$ Sep 18, 2023 at 9:03
  • 2
    $\begingroup$ As Ivan says by other words, chemistry is the part of physics that physicists do not want to get their hands dirty with. $\endgroup$
    – Poutnik
    Sep 18, 2023 at 10:56

1 Answer 1


Ionization enthalpy is defined as the energy required to separate an outermost electron from the atom to completely ionize the atom.

Perhaps you are familiar with NCERT textbook class 11 Part-2 from which this is a table enter image description here

Here the heading says 'atomic' and physical properties of alkali metals. No need to mind 'atomic' as it is used to refer to metallic and ionic radii. That means the ionization enthalpy is referred to as a physical property here. So ionization enthalpy is indeed a physical property.

The reason being that a chemical property is based on how the substance behaves chemically in response to some other chemical. Sodium is a chemical and it can be conferred certain chemical properties toward other chemicals such as oxygen, nitrogen, water, etc. But when it becomes sodium ion, it is not a chemical in the way sodium is. $\ce {Na}^+$ doesn't have a really independent existence. It will be either as aqueous solution or molten state or as counter-ion in salts. $\ce {Na}^+$ has more to do with the standard potential which again is a physical property as per the above table.

  • $\begingroup$ Yeah but if we want to measure ionisation enthalpy we would need to chemically react our sodium and if we follow what Wikipedia says then it should be chemical property. $\endgroup$ Sep 18, 2023 at 7:44

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