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3. Session Overview Relative masses of atoms and molecules.Relative masses of atoms and molecules. The mole, the Avogadro constant.The mole, the Avogadro constant. The determination of relative atomic masses, A r, and relative molecular masses, M r, from mass Spectra.The determination of relative atomic masses, A r, and relative molecular masses, M r, from mass Spectra.

4. Atomic Structure

5. States of Matter

6. Structure of Atom (Li-Atom)

7. Scale on which the mass of the 12 C isotope weighs exactly 12 units. What is carbon –12 scale?

8. A r is mass of an atom of an element expressed in atomic mass units (amu). A r is mass of an atom of an element expressed in atomic mass units (amu). Relative atomic mass (A r )‏

9. Relative atomic mass (A r ) 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.

10. A r of Chlorine

11. Relative isotopic mass (m a )‏ 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

12. Relative molecular mass (M r )‏ Sum of the relative atomic masses of all the constituent atoms present in the molecule. Example: In case of NaCl molecule M r

13. 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 (H 2 O).

14. FM in case of Glucose (C 6 H 12 O 6 ) The relative atomic mass of C = 12, H = 1 and that of O =16. Therefore C 6 H 12 O 6 Number of atom x Relative atomic mass = Total mass C 6 12.01 72.06 H 12 1.01 12.12 O 6 16.00 96.00 180.18

15. The mole The mole represents the number 6.023 x 10 23, 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 10 23 atoms One mole of molecules = 6.023 x 10 23 molecules One mole of electrons = 6.023 x 10 23 electrons One mole of protons = 6.023 x 10 23 protons One mole of neutrons = 6.023 x 10 23 neutrons

16. 1 mole

17. 1 mole of NaCl = 6.023 x 10 23 molecules of NaCl = 58.5g. So weight of one mole of NaCl = 58.5g

18. The Avogadro’s constant A constant number represented by symbol L or N A. In order to define mole it is assigned as 6.023 x 10 23. 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.

19. 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.

20. 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.

21. Mass Spectrometer

22. 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.

23. 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 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 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. The Y-axis of mass spectrum represents signal intensity of ions.

24. 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

25. 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 + A + + B Molecular ion Positive ion Free radical

26. Charged particle produce line in the spectra. Uncharged particles don’t produce any line in the spectra. 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. 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. Most of the ions have single charge so m/z values is equivalent to mass itself. NOTE:

27. Mass spectra of Carbon Dioxide CO 2 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). CO 2 CO 2 + CO 2 + CO + + O

28. Mass spectra of Propane Molecular ion m/z = 44. CH 3 CH 2 CH 3 + e -1 CH 3 CH 2 CH 3 + + 2e -1 C-C cleavage gives methyl and ethyl fragments, one of which is a carbocation and the other a radical. CH 3 CH 2 + + CH 3 CH 3 CH 2 CH 3 + CH 3 CH 2 + CH 3 + Both distributions are observed, but the larger ethyl cation (m/z=29) is the most abundant.

29. 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.

30. Mass spectra of 2-chloropropane M + for isotope 35 Cl m/z = 78 and M +2 for 37 Cl m/z = 80.

31. 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 Mass spectra of Bromoethane

32. Mass of an atom expressed in amu is termed A r. M r of a molecule is sum of A r of it’s constituent atoms. The mole is represented by a constant number (Avogadro’s number) is represented as 6.023 x 10 23. Mass Spectrometry (MS) provides information concerning the mass of the molecule, and sometimes about its structure. RECAP RECAP