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Atom

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

scanning tunneling microscope

an instrument used to generate images of individual atoms

John Dalton

english chemist and schoolteacher who formulated a theory to describe 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

Electrons

negatively charged particles found outside the nucleus

Cathode ray

a stream of electrons produced at the negative electrode (cathode) of a tube containing a gas at low pressure

Protons

positively charged particles located in the nucleus

Neutrons

subatomic particles with no charge

nucleus

the center of the atom which contains the protons and neutrons

atomic number

the number of protons in the nucleus of an atom

periodic table

an arrangement of elements according to similarities in their properties

mass number

the sum of the numbers of protons and neutrons in the nucleus of an atom

group

a vertical column of elements in the periodic table

isotopes

atoms with the same number of protons but different numbers of neutrons

atomic mass unit (amu)

1/12th of the mass of a carbon 12 atom

atomic mass

weighted average mass of the atoms in a naturally occurring sample of an element

period

a horizontal row of the periodic table

quantum

the amount of energy needed to move an electron from one energy level to another

atomic orbitals

a region in space around the nucleus of an atoms where an electron is likely to be moving

energy level

the regions around the nucleus within which the electrons have the highest probability of being found

quantum mechanical model

the modern description of the behavior of electrons in atoms

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