Rutherford's gold foil experiment, performed in conjunction with Geiger and
Marsden, provided evidence for the nucleus due to the scattering of alpha
particles. The repulsion of some alpha particles suggested that the nucleus
is positively charged, containing protons. Further work by Chadwick revealed
the existence of neutrons within the nucleus of the atom. The atomic number
describes the number of protons in the nucleus. For a neutral atom this is
also the number of electrons outside the nucleus. Subtracting the atomic
number from the atomic mass number gives the number of neutrons in the nucleus.
Isotopes are atoms of the same element (i.e., they have the same number of
protons, or the same atomic number) which have a different number of neutrons
in the nucleus. Isotopes of an element have similar chemical properties.
Radioactive isotopes are called radioisotopes. Most of the elements in the
periodic table have several isotopes, found in varying proportions for any
given element. The average atomic mass of an element takes into account the
relative proportions of its isotopes found in nature. A nuclear binding force
holds the nucleus of the atom together. The nuclear mass defect, a slightly
lower mass of the nucleus compared to the sum of the masses of its constituent
matter, is due to the nuclear binding energy holding the nucleus together.
The mass defect can be used to calculate the nuclear binding energy, with
E = mc2. The average binding energy
per nucleon is a measure of nuclear stability. The higher the average binding
energy, the more stable the nucleus.
The Bohr's model of the atom described the electrons as orbiting in discrete,
precisely defined circular orbits. Electrons can only occupy certain allowed
orbitals. For an electron to occupy an allowed orbit, a certain amount of
energy must be available. Each orbit is assigned a quantum number, with the
lowest quantum numbers being assigned to those orbitals closest to the nucleus.
Only a specified maximum number of electrons can occupy an orbital. Under
normal circumstances, electrons occupy the lowest energy level orbitals closest
to the nucleus. By absorbing additional energy, electrons can be promoted
to higher orbitals, and release that energy when they return back to lower
energy levels. The Bohr's model of the atom helped to offer one possible
explanation for the emission spectrum formed by hydrogen and other gases.
Photons are used to describe the wave-particle duality of light. The energy
of a photon depends upon its frequency. This helps to explain the photoelectric
effect; only photons having a sufficiently high energy are capable of dislodging
an electron from the illuminated surface.
E = hv where E is the photon
energy in J, v is the photon frequency
in Hz, and h is Planck's constant,
6.626 × 10-34 J / Hz.
Quantum theory offers a mathematical model to help explain the nature of the atom.
Quantum theory describes a region surrounding the nucleus which has the highest
probability of locating an electron. These orbital "clouds" have some unusual and