ATOMIC NATURE OF MATTER
DOE-HDBK-1019/1-93
Atomic and Nuclear Physics
Atomic Nature of Matter Summary
Atoms consist of three basic subatomic particles. These particles are the proton, the
neutron, and the electron.
Protons are particles that have a positive charge, have about the same mass as a
hydrogen atom, and exist in the nucleus of an atom.
Neutrons are particles that have no electrical charge, have about the same mass as a
hydrogen atom, and exist in the nucleus of an atom.
Electrons are particles that have a negative charge, have a mass about eighteen
hundred times smaller than the mass of a hydrogen atom, and exist in orbital shells
around the nucleus of an atom.
The Bohr model of the atom consists of a dense nucleus of protons and neutrons
(nucleons) surrounded by electrons traveling in discrete orbits at fixed distances
from the nucleus.
Nuclides are atoms that contain a particular number of protons and neutrons.
Isotopes are nuclides that have the same atomic number and are therefore the same
element, but differ in the number of neutrons.
The atomic number of an atom is the number of protons in the nucleus.
The mass number of an atom is the total number of nucleons (protons and neutrons) in
the nucleus.
The notation AZ X is used to identify a specific nuclide. "Z" represents the atomic
number, which is equal to the number of protons. "A" represents the mass
number, which is equal to the number of nucleons. "X" represents the chemical
symbol of the element.
Number of protons
= Z
Number of electrons = Z
Number of neutrons = A - Z
The stability of a nucleus is determined by the different forces interacting within
it. The electrostatic force is a relatively long-range, strong, repulsive force that
acts between the positively charged protons. The nuclear force is a relatively
short-range attractive force between all nucleons. The gravitational force the
long range, relatively weak attraction between masses, is negligible compared to
the other forces.
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