Alveoli. I don't know what a nephron is but neurons and synapses are in the brain.
This is a homogenous mixture because you cannot differentiate between the members of the mixture. A technique might be heating them until one melts and evaporates due to different boiling temperatures, or mixing them together with fluids that would dissolve one, yet keep the other one whole.
Remember that the number of protons in the nucleus determines an element's identity. Chemical changes do not affect the nucleus, so chemical changes cannot change one type of atom into another. The number of protons in a nucleus does change sometimes, however. The identity of the atom, therefore, changes.
