Isotopes of the same element differ in their

number of electrons
number of protons
atomic number
mass number

Isotopes of the same elements differ in their mass number. Isotopes are those elements have the same protons but differs from their neutrons. That is why it differs in their mass number because the mass number is the sum of protons and neutrons.

Neurotransmitters are messengers responsible for communication between neurons. they are released from synaptic vesicles and are received by receptors.  the communication is referred to as neurotransmission. the following statement is true about neurotransmitters: a. they're stored in synaptic vesicles in the axon.