The valence band charge densities of wide band gap zinc blende structures gallium nitride (GaN), InN and aluminium nitride (ALN) are studied using the empirical pseudo-potential method. Strain effects on these charge densities are elucidated. A correlation of these effects to the ionicity of these materials is given. The bowing parameter and the energy band gap versus alloy composition for AlGaN are obtained. The results obtained agree with experimental as well as theoretical results published elsewhere. The investigation will be useful for band gap engineering and the design of lattice mismatched heterostructure optoelectronic devices on these materials in the UV to orange light range application.