The energy levels of the helium atom and isoelectronic ions in two dimensions are considered. The difficulties encountered in the analytical evaluation of the perturbative and variational expressions for the ground state, promote an interesting factorization of the inter-electronic interaction, leading to simple expressions for the energy. This expression provides an insight into the general structure and screening effect in the ground state. Some of the considerations are extended to excited states. The dimensionality properties of the energy spectrum of a helium atom, the screening effect and their implications are of significant pedagogical interest in the study of perturbative and variational approaches to analyse the properties of helium atoms, in quantum mechanics and atomic physics, for general graduate students and specialists in atomic physics.