law of conservation of mass/matter The law of conservation of mass/matter, also known as law of mass/matter conservation says that the mass of a closed system will remain constant, regardless of the processes acting inside the system. Matter cannot be created/destroyed, although it may be rearranged. For any chemical process in a closed system, the mass of the reactants must equal the mass of the products.
Learning Outcomes Very brief outline of the historical development of atomic theory (outline principles only; mathematical treatment not required): Dalton: atomic theory; Crookes: vacuum tubes, cathode rays; Stoney: naming of the electron; Thomson: negative charge of the electron; e/m for electrons (experimental details not required); Millikan: magnitude of charge of electrons as shown by oil drop experiment (experimental details not required); Rutherford: discovery of the nucleus as shown by the α−particle scattering experiment; discovery of protons in nuclei of various atoms; Bohr: model of the atom; Chadwick: discovery of the neutron.
Models of the Atom Dalton’s model (1803) Thomson’s plum-pudding model (1897) Rutherford’s model (1909) Bohr’s model (1913) Charge-cloud model (present) Dorin, Demmin, Gabel, Chemistry The Study of Matter, 3 rd Edition, 1990, page 125 Greek model (400 B.C.) + - - - - - e e e + + + + + + + + e ee e e e e "In science, a wrong theory can be valuable and better than no theory at all." - Sir William L. Bragg
HISTORY OF THE ATOM GREEKS – MATTER MADE OF TINY INDIVISIBLE PARTICLES
DALTON 1766-1844 ALL MATTER MADE OF SMALL PARTICLES CALLED ATOMS ATOMS ARE INDIVISIBLE ATOMS CANNOT BE CREATED OR DESTROYED
ROBERT MILLIKAN Famous oil-drop experiment It measured the charge on the electron X-rays ionised air molecules by striping electrons off their atoms. Oil droplets picked up electrons became negative Increased the + charge until the droplet hovered. Took measurements and calculated the charge on the electron.
Relative Atomic Mass The relative atomic mass of an element the mass of one of the element's atoms -- relative to the mass of an atom of Carbon 12,
Learning Outcomes Calculation of approximate relative atomic masses from abundance of isotopes of given mass number (e.g. Calculation of approximate relative atomic mass of chlorine).
Chlorine Chlorine-35 and Chlorine-37 are both isotopes of chlorine
Relative mass of chlorine Chlorine consists of roughly 75% Chlorine-35 and roughly 25% Chlorine-37. We take an average of the two figures The relative atomic mass of chlorine is usually quoted as 35.5.
Learning outcomes Use of the mass spectrometer in determining relative atomic mass. Fundamental processes that occur in a mass spectrometer: vaporisation of substance, production of positive ions, acceleration, separation, detection (mathematical treatment excluded).
THE MASS SPECTROMETER Atoms can be deflected by magnetic fields - provided the atom is first turned into an ion.
Stage 1: Ionisation The atom is ionised by knocking one or more electrons off to give a positive ion.
Stage 2: Acceleration The ions are accelerated so that they all have the same kinetic energy.
Stage 3: Deflection The ions are then deflected by a magnetic field according to their masses. The lighter they are, the more they are deflected.
Stage 4: Detection The beam of ions passing through the machine is detected electrically.