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
•For any chemical process in a closed system, the mass of the
reactants must equal the mass of the products.
•Very brief outline of the historical development of atomic theory
(outline principles only; mathematical treatment not required): Dalton:
•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
•discovery of protons in nuclei of various atoms;
•Bohr: model of the atom;
•Chadwick: discovery of the neutron.
HISTORY OF THE ATOM
• GREEKS – MATTER MADE OF TINY INDIVISIBLE
• ALL MATTER MADE OF SMALL PARTICLES
• ATOMS ARE INDIVISIBLE
• ATOMS CANNOT BE CREATED OR DESTROYED
DISCOVERY OF THE ELECTRON
• CROOKES CONDUCTED EXPERIMENTS WITH A
• 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
• INCREASED THE + CHARGE UNTIL THE DROPLET
• TOOK MEASUREMENTS AND CALCULATED THE
CHARGE ON THE ELECTRON.
• ALPHA PARTICLES SHOULD PASS THROUGH
WITH LITTLE DEFLECTION
• MOST PASS THROUGH UNDEFLECTED
• SOME BOUNCED RIGHT BACK!
• HARD DENSE CORE OF POSITIVE MATTER IN
THE CENTER OF EACH ATOM-NUCLEUS
• ATOMS ARE MOSTLY EMPTY SPACE.
• RUTHERFORD CONTINUED TO BOMBARD
DIFFERENT ELEMENTS SUCH AS NITROGEN
• SMALL POSITIVE PARTICLES WERE GIVEN OFF-- PROTONS
• JAMES CHADWICK BOMBARDED BERYLLIUM
WITH ALPHA PARTICLES.
• SMALL PARTICLES WERE GIVEN OFF WHICH
WERE NEUTRAL AND HAD THE SAME MASS AS
THE PROTON—THE NEUTRON.
• Properties of electrons,
protons and neutrons
(relative mass, relative
charge, location within
• Protons are
Atomic number (Z ), mass number (A),
isotopes; hydrogen and carbon as
examples of isotopes.
Relative atomic mass (A r). The
12C scale for relative atomic
• Also called
this is the
• Also called
this is the
The Number of Electrons
• Atoms must have equal numbers
of protons and electrons. In our
example, an atom of krypton
must contain 36 electrons since it
contains 36 protons.
•Mass Number =
(Number of Protons) +
(Number of Neutrons)
• Atoms that have the same
number of protons but different
numbers of neutrons are called
• The element hydrogen for
example, has three commonly
known isotopes: protium,
deuterium and tritium
•an atom of deuterium
consists of one proton
one neutron and one
• An atom of tritium consists
of one proton two
neutrons and one electrons
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
• 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-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.
• Use of the mass spectrometer in determining
relative atomic mass.
• Fundamental processes that occur in a mass
• 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
Stage 2: Acceleration
• The ions are accelerated so
that they all have the same
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