Ch. 18 Vocabulary
Atomic mass – the average mass of all the known isotopes of an element. Video on
Atomic Mass
Atomic mass unit – defined as the mass of 1/12 of a carbon-12 atom (6 protons and 6
neutrons in the nucleus plus 6 electrons outside the nucleus).
Atomic number – the number of protons that an atom contains. Example of Atomic
Number, Video on Atomic Number
Atomic theory – a theory that states that all matter is composed of tiny particles called
atoms.
Chemical symbol – an abbreviation that represents the name of an element; used in
chemical formulas.
Electron – a subatomic particle in an atom that is negatively charged and that occupies
the energy levels in an atom; electrons are involved in chemical bonds and reactions.
Energy level – a region around the nucleus of an atom where electrons are most likely to
be found; only a certain number of electrons can be found in each energy level of an
atom. Example of Energy Level,
Group of elements – elements that exhibit similar chemical properties; arranged in
columns on the periodic table.
Isotopes – forms of the same element that have different numbers of neutrons and
different mass numbers. Example of Isotopes, Video on Isotopes
Mass number – the total number of protons and neutrons in the nucleus of an atom.
Example of Mass Number
Neutron – an uncharged particle found in the nucleus of an atom.
Nucleus – the center core of an atom that contains protons and neutrons. Video on
Nucleus
Periodic table of elements – a table that visually organizes the similarities between all
known elements. 3D Periodic Table, Table with all info, Example of Periodic Table,
Video on Periodic Table
Proton – a subatomic particle identical with the nucleus of the hydrogen atom; found
with neutrons in all atomic nuclei; carries a positive charge.
Strong nuclear force – the force that holds protons together when they are very close
together (only 10-15 meters apart).
Subatomic particles – particles that are smaller than an atom; protons, neutrons, and
electrons are subatomic particles.
Valence electrons – the electrons in an atom that are involved in the formation of
chemical bonds. Periodic Table w/ Valence Electrons