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Session Overview
• Relative masses of atoms and molecules.
• The mole, the Avogadro constant.
• The determination of relative atomic masses, Ar,
and relative molecular masses, Mr, from mass
Spectra.
Atomic Structure
States of Matter
Structure of Atom (Li-Atom)
What is carbon –12 scale?
Scale on which the mass of the 12C
isotope weighs exactly 12 units.
Relative atomic mass (Ar)
Ar is mass of an atom of an element
expressed in atomic mass units (amu).
Relative atomic mass (Ar) of elements
having isotopes
Average of the atomic masses of all the isotopes of a
chemical element weighted by isotopic abundance.
For example, naturally occurring chlorine consists of atoms
of relative isotopic masses 35 (75%) and 37 (25%). Its
relative atomic mass is 35.5.
Ar of Chlorine
æ 75
ö
ç
÷
Ar = çç
´ 35÷
+
÷
÷
è100
ø
æ 25
ö
÷
çç
÷
´
37
= 35.5
÷
çè100
÷
ø
Relative isotopic mass (ma)
Mass of a particular isotope of an element, on the scale carbon-12 = 12
exactly. For example in case of isotopes of hydrogen the relative isotopic
mass will be
Relative molecular mass (Mr)
Sum of the relative atomic masses of all the constituent
atoms present in the molecule.
Example: In case of NaCl molecule Mr
Relative Formula Mass (FM)
Sum of the atomic masses (atomic weights) of the
atomic species as given in the formula of the
compound. Examples:
FM in case of water (H2O).
FM in case of Glucose (C6H12O6)
The relative atomic mass of C = 12, H = 1 and that of O =16. Therefore
C6H12O6
C
H
O
Number of atom
6
12
6
x
Relative atomic mass
12.01
1.01
16.00
= Total mass
72.06
12.12
96.00
180.18
The mole
The mole represents the number 6.023 x 1023, just as K represents
thousand or dozen represent twelve. This is also referred to as
Avogadro’s number.
One mole of atoms = 6.023 x 1023 atoms
One mole of molecules = 6.023 x 1023 molecules
One mole of electrons = 6.023 x 1023 electrons
One mole of protons = 6.023 x 1023 protons
One mole of neutrons = 6.023 x 1023 neutrons
1 mole
1 mole of NaCl = 6.023 x 1023 molecules of NaCl = 58.5g.
So weight of one mole of NaCl = 58.5g
The Avogadro’s constant
•
A constant number represented by symbol L or NA. In order to
define mole it is assigned as 6.023 x 1023.
•
This number is approximately equal to the number of protons in a
gram of pure protons.
•
This number was introduced by Famous Scientist Lorenzo
Romano Amedo carlo Avogadro, known as Amedo Avogadro, an
italian lawyer born in 1776.
Mass Spectroscopy
Principle:
Ions are generated from either inorganic or organic
compounds by any suitable method, separated by
their mass to charge ratio (m/z) and detected
qualitatively and quantitatively by their respective
m/z abundance.
Mass Spectrometer
The mass spectrometer is an instrument which can measure the
masses and relative concentrations of atoms and molecules.
It converts molecules into ions so that they can easily move about and
get manipulated by external electric and magnetic fields.
This technique measures the mass to charge ratio of charged
particles.
A mass spectrum is generated which represents masses of sample
components, thus tells composition of a physical sample.
Mass Spectrometer
Mass spectroscopy is technique which enables one
• to identify unknown compounds by the mass of the compound
molecules or their fragments.
• to determine the isotopic composition of elements in a
compound.
• to determine the structure of a compound by observing its
fragments.
• to determine other important physical, chemical and even
biological properties of a compounds with variety of
approaches.
Mass Spectrum
• The
mass
spectrum
is
two
dimensional
representation of signal intensity (ordinate) versus
m/z (abscissa). Where m/z is mass to charge ratio.
Mass spectrum are different for different chemical
compounds.
• The X-axis of a mass spectrum represents a
relationship between the mass of a given ion and the
number of varying charges that it carries.
• The Y-axis of mass spectrum represents signal
intensity of ions.
Fragmentation patterns in the mass
spectra of organic compounds.
Sample is bombarded with stream of electrons which
knock out an electron off an organic compound to form
positive ion.
This ion is called molecular ion or the the parent ion.
It is represented by symbol M+ or M+ - the dot in this
representations shows the presence of an unpaired
electron.
M
M+ + e-1
Fragmentation of molecular ion
The molecular ions are not very stable and so they
break up into smaller pieces.
The simplest of these cases is the breaking of the
molecular ion into two fragments – one of which is
charged positive ion and the other is an uncharged
free radical.
M+
Molecular ion
A+
Positive ion
+
B
Free radical
NOTE:
• Charged particle produce line in the spectra.
Uncharged particles don’t produce any line
in the spectra.
• The most intense peak in the spectra is
termed base peak.
• Most of the ions have single charge so m/z
values is equivalent to mass itself.
Mass spectra of Carbon Dioxide
CO2 is composed of only three atoms,
its mass spectrum id very simple. It has
mass 44amu.
In this molecule the molecular ion is
also the base peak. The only fragments
of molecular ion are CO (m/z=28) and O
(m/z=16).
CO2
CO2+
CO2+
CO
+
+ O
Mass spectra of Propane
Molecular ion m/z = 44.
CH3CH2CH3 + e-1
CH3CH2CH3+ + 2e-1
C-C cleavage gives methyl and ethyl
fragments, one of which is a carbocation
and the other a radical.
CH3CH2+ + CH3
CH3CH2CH3+
CH3CH2 + CH3+
Both distributions are observed, but the
larger ethyl cation (m/z=29) is the most
abundant.
Mass spectra of Boron
Two peaks in the mass spectrum show that there
are two isotopes of Boron:
Boron – 10 (23) and Boron – 11 (100)
23 and 100 are relative abundances.
Mass spectra of 2-chloropropane
M+ for isotope 35Cl m/z = 78 and M+2 for 37Cl m/z = 80.
Mass spectra of Bromoethane
The carbons and hydrogen add up to 29. The M+
and M+2 peaks are therefore at m/z values given
by:
29 + 79 = 108
29 + 81 = 110
RECAP
•
Mass of an atom expressed in amu is termed Ar.
•
Mr of a molecule is sum of Ar of it’s constituent
atoms.
•
The mole is represented by a constant number
(Avogadro’s number) is represented as 6.023 x 1023.
•
Mass Spectrometry (MS) provides information
concerning the mass of the molecule, and
sometimes about its structure.