The explanation of the acidity of a molecule HA via the polarity of an element‐hydrogen‐bond (“concept of polarity”) does not hold a critical view. An analysis of the reaction of various acids HA with the base H2O in a thermodynamic cycle revealed three terms to be essential for differences in the acidity: The standard free bonding enthalpy, ΔBG0(H−A), the standard free enthalpy of hydratisation of the corresponding base A−, ΔhydG0(A−), and the energy of ionisation of A−, IE(A−). In many cases, the stronger bond corresponds to a more polar bond, but hence to a weaker acid, thus contradicting the concept of polarity. On the other side, the bigger IE(A−) is, the more stable is A− and thus the more acidic is HA. The hypothesis is proposed, that an increase in IE(A−) is based on an extended delocalization of the electrons and the negative charge in A−. An analysis of the orbitals of CH3−, NH2−, OH− and F− shows, that delocalisation of the valence electrons and hence that of the negative charge increases in this series. It is this effect, instead of the polarity of the H−A bond, that causes the acidity to increase in the series CH4−NH3−H2O−HF. Delocalisation of the negative charge can be demonstrated to secondary school students using the Kimball free cloud model. Thus, a “concept of delocalisation” is proposed to replace the “concept of polarity” in order to explain different acidities.