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law of conservation of mass

- the law that states that mass cannot be created or destroyed in ordinary chemical and physical changes

law of definite proportions

the law that states that a chemical compound always contains the same elements in exactly the same proportions by weight or mass.

law of multiple proportions

the law that states that when two elements combine to form two or more compounds, the mass of one element that combines with a given mass of the other is in the ratio of small whole numbers


the smallest unit of an element that maintains the chemical properties of that element

nuclear forces

the interaction that binds protons and neutrons, protons and protons, and neutrons and neutrons together in a nucleus.

atomic number

the number of protons in the nucleus of an atom; the atomic number is the same for all atoms


an atom that has the same number of protons (or the same atomic number) as other atoms of the same element do but that has a different number of neutrons (and thus a different atomic mass)

mass number

the sum of the numbers of protons and neutrons that make up the nucleus of an atom


an atom that is identified by the number of protons and neutrons in its nucleus

atomic mass unit

a unit of mass that describes the mass of an atom or molecule; it is exactly 1/12 of the mass of carbon atom with mass number 12 (abbreviation, amu)

average atomic mass

the weighted average of the masses of all naturally occurring isotopes on an element


the SI base unit used to measure the amount of a substance whose number of particles is the same as the number of atoms of carbon in exactly 12 g of carbon-12

avagadro's number

6.02 x 10^23 the number of atoms or molecules in 1 mol

molar mass

- the mass in grams of 1 mol of a substance

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