If a substance has an excess number of electrons on its surface, what type of charge does it have?

A negative charge

Explanation:

If there an an excess of electrons, there'll be more electrons in the substance than the number of protons, so the object will carry a negative charge.                                                                                                                                                                                                                                                                                                                                                                              

negative charge because electron is negatively charged sub -atomic particle.

answer; both surfaces of the membrane are nonpermeable