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126.96.36.199.1 - definitions
Physical chemistry definitions of key words
Terms in this set (14)
Atomic number (proton number)
...of an element is the number of protons in the nucleus of an atom of the element.
Mass number (nucleon number)
...of an element is the number of protons plus the number of neutrons in the nucleus of an atom of the element.
...is any atomic species of which the proton number and nucleon number are specified.
...are atoms of the same element having the same number of protons but a different number of neutrons. Thus they have the same atomic name but different mass numbers. (This does not affect the electron number or the electronic structure so does not affect the chemistry of the element.)
Relative isotopic mass
...is defined as the mass of one atom of that isotope compared to one twelfth of the mass of an atom of carbon-12.
Relative atomic mass, Ar
...is defined as the weighted average of one atom of the element compared to one twelfth of the mass of a C-12 atom.
Relative molecular mass, Mr
...of a compound is the sum of all the relative atomic masses of all the atoms which make up the compounds formula. (NB many compounds do not consist of molecules so this may be better termed the relative formula mass)
...is defined as the amount of substance which contains the same number of particles (atoms, ions, molecules, formula units, electrons) as there are carbon atoms in 12g of carbon-12.
The Avogadro constant, 6.02 x 10^23 mol^-1
One mole of any substance is 6.02 x 10^23 particles of it (these particles could be atoms, ions, molecules or formula units).
For example, the Avogadro constant is the number of atoms in 1 mole (12g) of atoms of C-12 or the number of molecules in 1 mole of molecules of H2.
Molar mass, M, of an element
...of an element is the relative atomic mass in grams per mole.
Molar mass, M, of a compound
...of a compound is the relative molecular mass in grams per mole.
One mole of any substance is...
Its relative atomic mass or relative molecular mass expressed in grams i.e. Molar mass.
...of a compound shows the ratio of atoms present in their lowest terms (smallest numbers).
The molecular formula
...is a simple multiple of the empirical formula and shows the actual numbers of atoms present in one molecule of the compound.
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