The change Delta R-x of bond length R-x, for atom X in a molecule upon electronic transition can be derived from the intensities I-i of the vibrational stretching progression v = 0 -> i of the electronic absorption or emission spectrum. In many cases, a simple model is sufficient for a reasonable estimate of Delta R-x. For symmetric molecules, however, conceptual problems in the literature of many decades are evident. The breathing modes of various types of symmetric molecules X-n and AX (A at the center) are here discussed. In the simplest case of a harmonic vibration of the same mode in the initial and final electronic states, we obtain AR, [2S/(com,)]1/2/w1/2 (all quantities in atomic units). o.) and S are respectively the observed vibrational quanta and the Huang-Rhys factor (corresponding, e.g., to the vibrational intensity ratio li/I0 S), nix is the mass of vibrating atom X, and is is a topological factor for molecule X or AX. The factor 1/EI/2 in the expression for AR, must not be neglected. The spectra and bond length changes of several symmetric molecules AX and X are discussed. The experimental bond length changes correctly derived with factor 1/v1/2 are verified by reliable quantum chemical calculations.