The ion neutral chemistry of the lower Martian atmosphere is more complex than any other ionospheric region and similar, in some aspects, to the terrestrial D region. Two ionization sources operate at these altitude levels: solar radiation, which is the most important one below around 70 km, and galactic cosmic radiation. The interaction of positive ions and electrons produced by these ionization sources with the Martian atmosphere, which is abundant in species able to be charged negatively, produces a high variety of positive and negative ions. We present here a new model of the ion chemistry developed to calculate the concentration of electrons and most abundant ions in the atmosphere of Mars below 100 km. The present knowledge of Mars' atmosphere allows us to include new neutral and ionic species, such as nitrogen derivated, in a more detailed ion-chemistry calculation than previous lower ionopheric models of Mars. We obtain that the electron density profile has a low-altitude secondary peak below the main photon-induced maximum. This peak is placed at around 35 km and is due to the high efficiency of the electron attachment to Ox molecules, which entails that the concentration of negative ions is higher than that of electrons below around 30 km. The most abundant ionic species predicted are positive ions like hydronimum hydrates H3O+(H2O)n and O2+ and negative ions like CO3-(H2O)n and NO3-(H2O)n.