2. The concept of Atoms and Molecules
• Relative atomic mass of an element .Average mass per atom of the natural
isotopic composition /1 /12 th mass of an atom of nuclide C-12
3. JEE MANTRA
and•Relative Atomic Mass Calculation BROMINE
bromine consists of 50% 79Br and 50% 81Br, calculate the Ar of bromine.
Ar = [ (50 x 79) + (50 x 81) ] /100 = 80
So the relative atomic mass of bromine is 80 or RAM or Ar(Br) = 80
Note the full working shown. Yes, ok, you can do it in your head BUT many students ignore
the %'s and just average all the isotopic masses (mass numbers) given, in this case bromine -79
and bromine-81.
4. Relative molecular mass of the compound
• The ratio of average mass per molecule of the natural isotope to 1/12th mass
of c-12,
• RMM (relative molecular mass)= sum of relative atomic masses of the atoms
of molecule
5. Calculate the relative molecular mass of
calcium hydroxide into the expression to find
Mr( Ca3(PO4)3
• (Ca2+)3(PO4
3-)2 or Ca3(PO4)3, but it makes no difference to the calculation
of relative formula mass or relative molecular mass.
• atomic masses: Ca = 40, P = 31, O =16
• RFM or Mr = (3 x 40) + 3 x {31 + (4 x 16)} = (120) + (3 x 95) = 405
6. Atomic mass unit
• The quantity 1/12th mass of an atom C-12 is known as atomic mass unit
• 1 amu=1.66 *10^-24 gramsgrams1.660 538 782(83) × 10−24g 1 gram of C-
12 = 6.02 x 10^23 amu The precise definition is that the atomic mass unit (u)
is one twelfth of the mass of an isolated atom of carbon-12 (12C) at rest
and in its ground state.[1] In other words, "A single atom of carbon-12 has a
mass of 12 u exactly, by definition."
7. ATOMIC MASS
• The average mass per atom of natural isotopic composition is called atomic
mass
• Atomic mass= Relative atomic mass x atomic mass unit
8. Molecular mass
• The average mass per molecule of natural isotopic composition of a
compound is known as molecular mass
Molecular mass= Relative molecular mass x atomic mass unit
