When the number of is the same for an atom the atom is neutral.
a. protons and neutrons
b. protons and electrons
c. neutrons and electrons
d. it depends on what the element is.?

They did not study the continental drift

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).