The basic problem of weak interaction between odd electrons in graphene is considered within the framework of broken spin-symmetry single-determinant approach. The modern implementations of the approach in the form of either unrestricted Hartree- Fock scheme (UBS HF) or spin-polarized DFT (UBS DFT) were discussed with particular attention to the applicability of spin-contaminated solutions of both techniques for the description of electronic properties of graphene. The UBS DFT applications generally reveal the open-shell character of the singlet state of the object and manifest an extra spin density concentrated on zigzag edge atoms. The UBS HF approach supports these findings but exhibits the extra spin density not only on zigzag edge atoms but on all atoms of the sheet. This very peculiarity permits to quantitatively describe the odd electron behavior via both enhanced chemical reactivity and magnetism. The former is presented in terms of a quantified atomic chemical susceptibility. The magnetic response of graphene sheets is provided by a collective action of all odd electrons and is size dependent. The relative magnetic coupling constant J decreases when the sheet size increases and J approaches the limit value of 10-2 to 10-3 kcal/mol when the sheet size is of a few nanometers. © 2009 Wiley Periodicals, Inc.