The Dalton atomic model suspected no nucleus. It assumed that atoms were the fundamental building
blocks of nature and no particles exist that are smaller than atoms. However, Rutherford's
Gold-foil experiment led him to to suspect that atoms had nuclei where most of its mass was
located, and that the nucleus was positive.
Bombarding Nitrogen with alpha particles, he noticed that the disintegration of the nucleus
produced a positive particle equal to the mass of a hydrogen atom. He called this particle
the proton and postulated that they made up the nucleus.
But a lot of positive charge concentrated in a small region
would cause a lot of repulsion forces. So when Chadwick discovered
the neutron in 1932, Heisenberg created the proton-neutron
model of the nucleus. It also successfully explains isotopes.
The proton is the nucleon that has a positive unit chage. Its mass is one atomic mass unit
(amu, or 1.67 * 10^-27 kg, 1,836 times the mass of an electron). It is present in the
nucleus of every element. Protons are responsible for the nuclear charge, the positive charge of the
nucleus that deflected Rutherford's alpha particles. This charge also corresponds to the atomic
number, which determines what kind of element the atom belongs to, and where on the periodic table
the element belongs.
THe neutron is an electrically neutral particle slightly
heavier than the proton. They are present in numbers
greater than or equal to the number of protons in every element
except Hydrogen (whose nucleus is a single proton), and
variations in the number of neutrons of elements
accound for isotopes.
The neutron has a maganetic moment, which suggests that
its internal structure is made up of electrically charged
components whose net charge is zero. The neutron is
actually the fusion of a proton and an electron
(since in beta decay a neutron is turned into a proton
and an electron) and a neutrino.
Isotopes are atoms of the same element that have
differenet mass. This is explained by the proton-neutron
model of the nucleus. The number of protons account
for which element it is and thus how it behaves,
but the number of neutrons there are determines which
isotope it is, and there can be more neutrons than there
For example, elemental Hydrogen has one proton (H-1) and
a mass of one amu. But an isotope of Hydrogen, deuterium,
has a proton and a neutron, and a mass of two amu.
As another example, Carbon-12, Carbon-13 and Carbon-14
all have 6 protons (this determines that it is Carbon atom)
but have 6, 7 and 8 neutrons, respectively.
Sometimes there are too many neutrons in the nucleus.
When this happens, the neutron goes under beta decay,
a type of radioactive decay. The neutron splits into a
proton, an electron (beta particle) and a neutrino.
The proton stays in the nucleus, but the beta particle
and the neutrino(an electrically neutral, light or massless
particle) are emitted, and energy is released, carried
by the electron (when the electron doesn't carry all the
energy, the rest is carried by the neutrino).
Nucleons are generally knoen as a baryon.
Baryons are elementary particles consisting of three quarks and no antiquarks. They obey the
Fermi-Dirac statistics, which encompass all elementary particles that obey the Pauli
Exclusion Principle. The proton is the lightest baryon and is made of two up-quarks and
one down-quark. The neutron is made of two down-quarks
and one up-quark.
Nucleon's antiparticles are the antiproton and the antineutron,
which have the same mass as the corresponding nucleons but
opposite electric charge and magnetic moment.