Neutron

Statement 10.1.A.1.iii gives the defining property in four words: a neutron has no electric charge. Everything distinctive follows from that zero.

• No electric force. Coulomb's law is proportional to the product of the charges, so a neutron neither attracts nor repels a proton, an electron or another neutron electrically.
• No magnetic force from motion. Put $q = 0$ into $F_B = qvB\sin\theta$ and a moving neutron passes through a magnetic field undeflected.
• No barrier on approach to a nucleus, since there is no repulsion to overcome. Beyond AP Physics 2, that is why neutrons penetrate matter so readily, though penetrating power appears in none of the four current CEDs.

Mass is the other half. The AP Physics 2 and C: Electricity and Magnetism tables print $m_n = 1.67 \times 10^{-27}$ kg, the same figure to three significant figures as the proton mass, so nucleon counting works without weighting the two types differently.

What neutrons decide is the [isotope](/glossary/isotope), not the element. Statement 15.2.A.2 makes the proton count the element and 15.2.A.2.ii makes the total nucleon count the isotope, so the neutron number $A - Z$ is what distinguishes two nuclei of the same substance. It is also what beta decay changes: a neutron becoming a proton lowers the neutron count by one and raises $Z$ by one, leaving $A$ alone.

Neutron emission as a decay mode is excluded from the framework by the Topic 15.8 boundary statement, which names it alongside electron capture.