# What exactly does the “per mole” unit of heat of atomization mean?

I am facing a confusion in the definition of the standard enthalpy of atomization. The confusion arises because of the "per mole" in its units ($\pu{kJ mol^-1}$).

My question: When I say "per mole", do I mean "per mole" of the product or per mole of the reactant?

When I mean "per mole" of the product, my representative reaction for $\Delta H_\mathrm{a}^\circ$ is:

$$\ce{1/4 P4(s) -> P (g)}$$

And when I mean "per mole" of the reactant, my representative reaction for $\Delta H_\mathrm{a}^\circ$ is:

$$\ce{P4 (s) -> 4P (g)}$$

Using either of them leads to different calculations involving Hess's law or other chemical reactions, hence I feel there is a need to resolve this confusion.

To add to this confusion, NCERT Chemistry (Part 1 page 171) writes:

It is the enthalpy change on breaking one mole of bonds completely to obtain atoms in the gas phase.

but then gives a representative reaction as:

$$\ce{CH4 -> C(g) + 4H(g)}$$

which involves breaking 4 moles of bonds instead.

If someone could write the correct definition of the standard enthalpy of atomization, with more than one reputed reference to back their claim, it'd definitely resolve this confusion.

• Here per mole means per mole of product formed (as this is atomization). Standard enthalpy of atomization is the enthalpy change when 1 mol of gaseous atoms is formed from its element in its defined physical state under standard conditions (298.15K, 1 atm). – user38977 May 8 '17 at 4:18

According to Wikipedia,

Standard enthalpy of atomization is the enthalpy change when $1$ mole of gaseous atoms is formed from its element in its defined physical state under standard conditions $(\pu{298.15K}, \pu{1 bar})$.

$\Delta H_{\text{at}}^{\circ}$ focuses on the production of $1$ mole of atoms from the element. Hence it's defined for the reaction:

$\ce{\dfrac{1}{8}S8 ->S}$

but not for the reaction:

$\ce{S8 -> 8S}$

So whenever you see the term "per mole", it is, by definition, implicitly referring to the product and not to the reactant.