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atom

the smallest particle of an element that retains its identity in a chemical reaction

Dalton's atomic theory

the first theory to releate chemical changes to events at the atomic level; described the structure and chemical reactivity of matter in terms of atoms

Democritus

Greek philosopher who was among the first to suggest the existence of atoms

Scanning Tunneling microscope

an instrument used to generate images of individual atoms

electrons

negatively charged subatomic particles

cathode ray

stream of electons produced at the negative electrode of a tube containing a gas at low pressure

protons

positively charged subatomic particles

neutrons

subatomic particles with no charge

nucleus

the central core of the atom which is made up of protons and neutrons

atomic number

the number of protons in the nucleus of an atom of an element

mass number

the total numbe rof protons and neutrons in the nucleus of an atom

isotopes

atoms of the same element have the same atomic number of different atomic masses due to a different number of neutrons

atomic mass unit (amu)

a unit of mass equal to one-twelfth the mass of a carbon-12 atom

atomic mass

the weighted average of the masses of the isotopes of an element

periodic table

an arrangement of elements in which the elemtns are separated into groups based on a set of repeating properties

period

a horizontal row of elements in the periodic table

group

a vertical column of elements in the periodic table; the constiuent elements of a group have similar chemical and physical properties

Atomic number

a number equal to the number of protons as well as the number of electrons in an atom

Number of neutrons

Equal to the mass number minus the atomic number

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