Atomic Mass Unit
The mass of a single atom is so small that it is impossible to weigh it directly
Atomic masses are therefore defined in terms of a standard atom which is called
the unified atomic mass unit
This unified atomic mass is defined as one-twelfth of the mass of a carbon-12
isotope
The symbol for the unified atomic mass is u (often Da, Dalton, is used as well)
1 u = 1.66 x 10-27 kg
Relative atomic mass, Ar
The relative atomic mass (Ar) of an element is the ratio of the average mass of the
atoms of an element to the unified atomic mass unit
The relative atomic mass is determined by using the average mass of
the isotopes of a particular element
The Ar has no units as it is a ratio and the units cancel each other out
Relative isotopic mass
The relative isotopic mass is the mass of a particular atom of
an isotope compared to the value of the unified atomic mass unit
Atoms of the same element with a different number of neutrons are called isotopes
Isotopes are represented by writing the mass number as 20Ne, or neon-20 or Ne-20
o To calculate the average atomic mass of an element the percentage
abundance is taken into account
o Multiply the atomic mass by the percentage abundance for each isotope and
add them all together
o Divide by 100 to get average relative atomic mass
o This is known as the weighted average of the masses of the isotopes
,Relative molecular mass, Mr
The relative molecular mass (Mr) is the ratio of weighted average mass of a
molecule of a molecular compound to the unified atomic mass unit
The Mr has no units
The Mr can be found by adding up the relative atomic masses of all atoms present
in one molecule
When calculating the Mr the simplest formula for the compound is used, also known
as the formula unit
o Eg. silicon dioxide has a giant covalent structure, however the simplest
formula (the formula unit) is SiO2
, Relative formula mass, Mr
The relative formula mass (Mr) is used for compounds containing ions
It has the same units and is calculated in the same way as
the relative molecular mass
In the table above, the Mr for potassium carbonate, calcium hydroxide and
ammonium sulfates are relative formula masses
Mole & Avogadro Constant
The mass of a single atom is so small that it is impossible to weigh it directly
Atomic masses are therefore defined in terms of a standard atom which is called
the unified atomic mass unit
This unified atomic mass is defined as one-twelfth of the mass of a carbon-12
isotope
The symbol for the unified atomic mass is u (often Da, Dalton, is used as well)
1 u = 1.66 x 10-27 kg
Relative atomic mass, Ar
The relative atomic mass (Ar) of an element is the ratio of the average mass of the
atoms of an element to the unified atomic mass unit
The relative atomic mass is determined by using the average mass of
the isotopes of a particular element
The Ar has no units as it is a ratio and the units cancel each other out
Relative isotopic mass
The relative isotopic mass is the mass of a particular atom of
an isotope compared to the value of the unified atomic mass unit
Atoms of the same element with a different number of neutrons are called isotopes
Isotopes are represented by writing the mass number as 20Ne, or neon-20 or Ne-20
o To calculate the average atomic mass of an element the percentage
abundance is taken into account
o Multiply the atomic mass by the percentage abundance for each isotope and
add them all together
o Divide by 100 to get average relative atomic mass
o This is known as the weighted average of the masses of the isotopes
,Relative molecular mass, Mr
The relative molecular mass (Mr) is the ratio of weighted average mass of a
molecule of a molecular compound to the unified atomic mass unit
The Mr has no units
The Mr can be found by adding up the relative atomic masses of all atoms present
in one molecule
When calculating the Mr the simplest formula for the compound is used, also known
as the formula unit
o Eg. silicon dioxide has a giant covalent structure, however the simplest
formula (the formula unit) is SiO2
, Relative formula mass, Mr
The relative formula mass (Mr) is used for compounds containing ions
It has the same units and is calculated in the same way as
the relative molecular mass
In the table above, the Mr for potassium carbonate, calcium hydroxide and
ammonium sulfates are relative formula masses
Mole & Avogadro Constant
