The electron
The electron is an elementary particle
possessing negative electric charge. It has a rest mass of 0.511 MeV.
In the theory of elementary particles, it is classified
as a lepton. Such particles are subject to electromagnetic
and weak interactions - as well as to gravity - but not to strong
interactions. Although the electron is structureless (as far as is
presently known), it has
spin and a magnetic moment.
The proton
The proton is a particle possessing positive
electric charge, equal in absolute value to that of the electron.
It has a rest mass of 938 MeV. Being subject to strong interactions -
as well as to the other kinds of interactions - it is classified as
a hadron. It is not an elementary particle, but is composed of more
fundamental particles called
quarks.
It has a
spin and a magnetic moment.
The proton is very similar in structure and properties to the
neutron, the main difference being the electric charge.
The neutron
The neutron is an electrically neutral
particle of rest mass equal to 940 MeV. Being subject to strong interactions -
as well as to the other kinds of interactions - is is classified as
a hadron. It is not an elementary particle, but is composed of more
fundamental particles called
quarks. It has a
spin and a magnetic moment.
The neutron is very similar in structure and properties to the
proton, the main difference being the electric charge.
Spin
The intrinsic angular momentum of a particle
is called spin. In classical mechanics, angular momentum is a vector
whose direction specifies the rotation axis and the sense of the
rotation. In quantum mechanics, the spin state of a particle is
specified by giving one component, which can assume
2s+1 discrete
values, where s
is an integer or half-integer number, characteristic
of the particle. For
electrons,
protons,
neutrons
and
quarks,
s=1/2
and
therefore these particles can be in two different spin states,
commonly refered to as spin up and spin down.
By analogy with the
classical case, we may think of these states as differing by the
sense of the intrinsic rotation, as shown in the figures.