Electronegativity tends to increase from left to right across a row, because

Electronegativity tends to increase from left to right across a row, because the attraction between the nucleus and the valence electrons increases. 

It is a mixture. simply put, a compound must be pure, that is, of only one chemical structure and composition. a mixture is made of different chemical substances (i.e. elements, molecules, compounds) that are not chemically bonded to each other, but instead physically grouped together, as in the case of gasoline (a homogeneous solution).