Atomic Theory Vocabulary

Tiniest particle of an element that has all the properties of the element. Atoms are composed of protons, neutrons and electrons.
Atomic mass
Average of the masses of naturally occurring isotopes of an element, weighted by abundance.
Atomic mass units (AMU)
Unit to describe the mass of atoms, molecules and subatomic particles. The new name for this unit is Daltons (D).
Atomic number
Number of protons in an atom.
Electron cloud
The space around the nucleus of the atom through which the electrons move.
Electrons (e-)
Negatively-charged subatomic particle found in the electron cloud. Its mass is insignificant.
Electron shell/orbital
Energy level of an electron that we model as an orbit the e- will travel around as it circles the nucleus. If the orbit is farther from the nucleus, we say the e- has more energy.
Atoms of the same element having different numbers of neutrons. Isotopes have the same chemical properties.
Mass number
Mass of an atom in atom mass units (AMU). It is the sum of the number of protons and neutrons.
Neutrons (no)
Neutral subatomic particle found in the nucleus. Its mass is one AMU.
Central part of an atom made up of the protons and neutrons.
Protons (p+)
Positively-charged subatomic particle found in the nucleus. Its mass is one AMU.