Atomic Structure and Relative Masses

1. 1 Atomic Structure and Relative Masses 1.1 The Atomic Nature of Matter 1.2 The Experimental Evidence of Atomic Structure 1.3 Sub-atomic Particles 1.4 Atomic Number, Mass Number and Isotopes 1.5 Mass Spectrometer 1.6 Relative Isotopic, Atomic and Molecular Masses

2. 1.1 The Atomic Nature of Matter

3. 1.1 The atomic nature of matter (SB p.2) What is “atom”? The Greek philosopher Democritus

4. Continuous division Continuous division 1.1 The atomic nature of matter (SB p.2) These are iron atoms!! Iron

5. 1.1 The atomic nature of matter (SB p.2) Dalton’s atomic theory John Dalton proposed hisDalton’s atomic theory

6. Check Point 1-1 1.1 The atomic nature of matter (SB p.2) Main points of Dalton’s atomic theory 1. All elements are made up of atoms. 2. Atoms can neither be created nor destroyed. 3. Atoms of the same element are identical. They have the same mass and chemical properties. 4. Atoms of different elements are different. They have different masses and chemical properties. 5. Atoms of different elements combine to form a compound. The numbers of various atoms combined bear a simple whole number ratio to each other.

7. 1.2 The Experimental Evidence of Atomic Structure

8. 1.2 The experimental evidence of atomic structure (SB p.3) Discovery of electrons • A beam of rays came out from the cathode and hit the anode • He called the beamcathode rays

9. Deflected in the electric field Deflected in the magnetic field 1.2 The experimental evidence of atomic structure (SB p.4) The beam was composed ofnegatively charged fast-moving particles.

10. 1.2 The experimental evidence of atomic structure (SB p.4) The particles were constituents of all atoms!! He called the particles‘electrons’. Measure themass to charge ratio(m/e) of the particles produced Independent of the nature of the gasinside the discharge tube

11. No. of positively charged particles No. of negatively charged particles = Atom 1.2 The experimental evidence of atomic structure (SB p.4) Thomson’s atomic model An atom iselectrically neutral

12. + + + + + + Electron Positive charge 1.2 The experimental evidence of atomic structure (SB p.4) How are the particles distributed in an atom? • An atom was a positively charged sphere • Negatively charged electrons embedded in it like a‘raisin pudding’

13. 1.2 The experimental evidence of atomic structure (SB p.4) Gold foil scattering experiment • performed byErnest Rutherford

14. 1.2 The experimental evidence of atomic structure (SB p.4) • He bombarded a thin gold foil with a beam of fast-moving -particles (+ve charged) • Observation: • most -particles passed through the foil without deflection • very few -particles were scattered or rebounded back

15. 1.2 The experimental evidence of atomic structure (SB p.5) Interpretation of the experimental results • The condensed core is called‘nucleus’ • The positively charged particle is called‘proton’

16. Mass of atom > Total mass of protons 1.2 The experimental evidence of atomic structure (SB p.5) Rutherford’s atomic model Expectation: Mass of atom = Total mass of protons

17. 1.2 The experimental evidence of atomic structure (SB p.5) Chadwick’s atomic model • presence ofneutrons • proved by James Chadwick

18. Proton Electron Check Point 1-2 Neutron 1.2 The experimental evidence of atomic structure (SB p.5) Chadwick’s atomic model

19. 1.3 Sub-atomic Particles

20. Inside the condensed nucleus Let's Think 1 Moving around the nucleus 1.3 Sub-atomic particles (SB p.6) Sub-atomic particles • 3 kinds of sub-atomic particles: • Protons • Neutrons • Electrons

21. 1.3 Sub-atomic particles (SB p.7) A carbon-12 atom

22. 0 e -1 1 1 H n 1 0 1.3 Sub-atomic particles (SB p.6) Characteristics of sub-atomic particles

23. Let's Think 2 Check Point 1-3 1.3 Sub-atomic particles (SB p.6) Relative size of the atom and the nucleus

24. 1.4 Atomic Number, Mass Number and Isotopes

25. Atomic number Number of protons Number of electrons = = Reason:Atoms are electrically neutral. 1.4 Atomic number, mass number and isotopes (SB p.7) Atomic number Theatomic number (Z)of an element is thenumber of protonscontained in the nucleus of the atom. WHY?

26. Mass number Number of protons Number of neutrons = + 1.4 Atomic number, mass number and isotopes (SB p.8) Mass number Themass number (A)of an atom is thesum of the number of protons and neutronsin the nucleus.

27. 1.4 Atomic number, mass number and isotopes (SB p.8) Atomic numbers and mass numbers of some common atoms

28. Representation: Symbol of the element Mass number A X Let's Think 3 Z Atomic number 1.4 Atomic number, mass number and isotopes (SB p.8) Isotopes Isotopesare atoms of the same element withthesame number of protonsbutdifferent numbers of neutrons.

29. 35 37 Cl Cl 17 17 Check Point 1-4 1.4 Atomic number, mass number and isotopes (SB p.8) e.g. the two isotopes of chlorine are written as: OR labelled as Cl-35 andCl-37.

30. 1 2 H H 1 1 12 13 14 C C C 6 6 6 1.4 Atomic number, mass number and isotopes (SB p.9) Isotopes of some common elements

31. 1.5 Mass Spectrometer

32. 1.5 Mass spectrometer (SB p.10) Mass spectrometer A highly accurate instrument!

33. 1.5 Mass spectrometer (SB p.10) Mass spectrometer consists of 6 parts:

34. 1.8 Mass spectrometer (SB p.21) Mass spectrum of Cl2:

35. Check Point 1-5 1.8 Mass spectrometer (SB p.21) Mass spectrum of CH3Cl:

36. 1.6 Relative Isotopic, Atomic and Molecular Masses

37. 1.9 Relative isotopic, atomic and molecular masses (SB p.22) Relative isotopic mass Therelative isotopic massof a particular isotope of an element is the relative mass of one atom of that isotope on the carbon-12 scale. e.g. relative isotopic mass of Cl-35 = 35 relative isotopic mass of Cl-37 = 37

38. 1.9 Relative isotopic, atomic and molecular masses (SB p.22) What is carbon-12 scale? use carbon-12 as thereference standard Mg has the same mass as two C-12 atoms

39. 1.9 Relative isotopic, atomic and molecular masses (SB p.23) Relative atomic mass Therelative atomic massof an element is the weighted average of the relative isotopic masses of its natural isotopes on the carbon-12 scale.

40. Relative atomic mass of Cl = = 35.48 1.9 Relative isotopic, atomic and molecular masses (SB p.23) What is the relative atomic mass of Cl? The relative abundances of Cl-35and Cl-37 are75.77and24.23respectively

41. 1.9 Relative isotopic, atomic and molecular masses (SB p.23) Relative molecular mass Therelative molecular massis the relative mass of a molecule on the carbon-12 scale.

42. Relative molecular mass of CH3Cl = = 50.50 Example 1-6 Check Point 1-6 1.9 Relative isotopic, atomic and molecular masses (SB p.23) What is the relative molecular mass of CH3Cl?

43. The END

44. 1.1 The atomic nature of matter (SB p.3) Back Check Point 1-1 • What does the word “atom” literally mean? • Which point of Dalton’s atomic theory is based on the law of conservation of mass proposed by Lavoisier in 1774 which states that matter is neither created nor destroyed in the course of a chemical reaction? • Which point of Dalton’s atomic theory is based on the law of constant proportion proposed by Proust in 1799 which states that all pure samples of the same chemical compound contain the same elements combined together in the same proportions by mass? (a) Indivisible (b) Atoms can neither be created nor destroyed. Answer (c) Atoms of different elements combine to form a compound. The numbers of various atoms combined bear a simple whole number ratio to each other.

45. 1.2 The Experimental evidence of atomic structure (SB p.4) Back Check Point 1-2 • Atoms were found to be divisible. What names wer given to the particles found inside the atoms? • Give the most important point of the following experiments: • (i) E. Goldstein’s gas discharge tube experiment; • (ii) J. J. Thomson’s cathode ray tube experiment; • (iii) E. Rutherford’s gold foil scattering experiment. (a) Electron, proton and neutron Answer (b) (i) Discovery of cathode rays (ii) Discovery of electrons (iii) Discovery of nucleus in atoms

46. 1.3 Sub-atomic particles (SB p.6) Let's Think 1 The identity of an element is determined by the number of which sub-atomic particle? Answer The identity of an element is determined by the number of protons in its atomic nucleus. Back

47. 1.3 Sub-atomic Particles (SB p.7) Back Check Point 1-3 • Which part of the atom accounts for almost all the mass of that atom? • (b) The mass of which sub-atomic particle is often assumed to be zero? (a) Nucleus (b) Electron Answer

48. 1.3 Sub-atomic particles (SB p.7) Let's Think 2 Are there any sub-atomic particles other than protons, neutrons and electrons? Answer Other than the three common types of sub-atomic particles (proton, neutron and electron), there are also some sub-atomic particles called positron (anti-electron) and quark. Back

49. 1.3 Sub-atomic particles (SB p.7) Let's Think 3 If bromine has two isotopes, 79Br and 81Br, how many physically distinguishable combinations of Br atoms are there in Br2? Answer There are three physically distinguishable combinations of Br atoms (79Br—79Br, 79Br—81Br and 81Br—81Br) in Br2. Back

50. , 11 protons, 12 neutrons, 11 electrons. 1.4 Atomic number, mass number and isotopes (SB p.8) Back Check Point 1-4 Write the symbol for the atom that has an atomic number of 11 and a mass number of 23. How many protons, neutrons and electrons does this atom have? Answer