An extension of the Bergia, Fubini, Stanghellini and Villi model for the form factors of nucleons is proposed for all mesons and baryons, according to the following assumptions; 1) 4-dimensional rotational invariance in isospace for the K interactions, ordinary charge independence of π interactions for all baryons doublets; 2) the coupling between the photon and the elementary particles considered occurs only through a bipion (T=1,J=1) and a tripion (T=0,J=1) intermediate states; 3) invariance underG transformation is requested for all bipion and tripion interactions with the elementary particles considered. With these assumptions, Clementel-Villi type formulae for the form factors are obtained, whose constants are normalized on the minimum necessary number of the experimental charge values. The static limit of the formulae may then be used to predict all other charge states for pions, K-mesons and baryons, yielding thus a partial justification of the Gell-Mann relation between charge, isospin and hypercharge. Some further consequences of the model and comparison with an « eight-fold way » model are discussed.