A neutron is one of two particles found inside the nucleus (central part) of an atom. The other particle is called a proton. Electrons are particles that move around an atom outside the nucleus.
British physicist Ernest Rutherford discovered the atom in 1911. He constructed a model showing an atom with a nucleus containing protons and electrons. Scientists studying the model knew that something must be missing from it. Rutherford suggested that some sort of neutral particle might exist in the nucleus. He and a graduate student working with him, James Chadwick, could not prove his theory, mainly because neutrons cannot be detected by any standard tools such as cloud chambers or Geiger counters.
Axon: The projection of a neuron that carries an impulse away from the cell body of the neuron.
Central nervous system: The portion of the nervous system in a higher organism that consists of the brain and spinal cord.
Cytoplasm: The fluid inside a cell that surrounds the nucleus and other membrane-enclosed compartments.
Dendrite: A portion of a nerve cell that carries nerve impulses toward the cell body.
Ion: A molecule or atom that has lost one or more electrons and is, therefore, electrically charged.
Myelin sheath: A white, fatty covering on nerve axons.
Neurotransmitter: A chemical used to send information between nerve cells or nerve and muscle cells.
Peripheral nervous system: The portion of the nervous system in an organism that consists of all the neurons outside the central nervous system.
Receptors: Locations on cell surfaces that act as signal receivers and allow communication between cells.
Stimulus: Something that causes a response.
Synapse: The space between two neurons through which neurotransmitters travel.
Finally, Chadwick tried directing a beam of radiation at a piece of paraffin (a waxy mixture used to make candles). He observed that protons were ejected from the paraffin. Chadwick concluded that the radiation must consist of particles with no charge and a mass about equal to that of the proton. That particle was the neutron.
In the early 1960s, the American physicist Robert Hofstadter discovered that both protons and neutrons contain a central core of positively charged matter that is surrounded by two shells. In the neutron, one shell is negatively charged, just balancing the positive charge in the particle's core.