|atom||the smallest particle of an element that retains its identity in a chemical reaction|
|atomic mass||the weighted average of the masses of the isotopes of an element|
|atomic mass unit (amu)||a unit of mass equal to one-twelfth the mass of a carbon-12 atom|
|atomic number||the number of protons in the nucleus of an atom of an element|
|cathode ray||a stream of electrons produced at the negative electrode (cathode) of a tube containing a gas at low pressure|
|Dalton's atomic theory||the first theory to relate chemical changes to events at the atomic level:|
1. All elements are composed of tiny indivisible particles called atoms.
2. Atoms of the same element are identical. The atoms of any one element are different from those of any other element.
3.Atoms of different elements can physically mix together or can chemically combine in simple whole-number ratios to form compounds.
4.Chemical reactions occur when atoms are separated, joined, or rearranged. Atoms of one element, however, are never changed into atoms of another element as a result of a chemical reaction.
|electron||a negatively charged subatomic particle|
|group||a vertical column of elements in the periodic table; the constituent elements of a group have similar chemical and physical properties|
|isotopes||atoms of the same element that have the same atomic number but different atomic masses due to a different number of neutrons|
|mass number||the total number of protons and neutrons in the nucleus of an atom|
|neutron||a subatomic particle with no charge and a mass of 1 amu; found in the nucleus of an atom|
|nucleus||the tiny, dense central portion of an atom, composed of protons and neutrons|
|period||a horizontal row of elements in the periodic table|
|periodic table||an arrangement of elements in which the elements are separated into groups based on a set of repeating properties|
|proton||a positively charged subatomic particle found in the nucleus of an atom|