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Arsenic was known to the ancient
Egyptian, and is mentioned in one
papyrus as a ways of gilding metals.
The Greek philosopher Theophrastus
knew of two arsenic sulfide minerals:
orpiment (As2S3) and realgar (As4S4).
Pen Ts’ao Kan-Mu compiled his great
work on the natural world in the 1500s,
during the Ming dynasty. He noted the
toxicity associated with arsenic
compounds and mentioned their use as
pesticides in rice fields.
A more dangerous form of arsenic,
called white arsenic, has also been long
known. This was the trioxide, As2O3,
and was a by-product of copper refining.
When this was mixed with olive oil and
heated it yielded arsenic metal itself.
The discovery of the element arsenic
is attributed to Albertus Magnus in ~
AD 1251.
http://www.rsc.org/periodic-table/history
Atomic Structure
Three particles:
Proton - particles with a positive charge - located in
nucleus.
Neutrons - particles with a neutral charge - located
in nucleus.
Electrons - particles with a negative charge.
Located in a roughly spherical zone around the
nucleus.
Atomic number and symbol
Atomic number is the number of protons in
the nucleus.
All atoms of same element will have same number of
protons.
Atomic symbol is the letter designation for a given
element:
C - Carbon (6 protons)
O - Oxygen (8 protons)
Na - Sodium (natrium) (11 protons)
Cl - Chlorine (17 protons)
Fe - Iron (ferrum) (26 protons)
Atomic Weight
Atomic Weight is sum of weights of all atomic
particles.
proton - 1.6 x 10-27 kg
neutron - 1.6 x 10-27 kg
electron - 1 x 10-30 kg
Atomic Mass Number is based on
each proton and neutron having a
mass of 1.
Oxygen has 8 protons and 8 neutrons
- atomic mass number of 16.
Atomic mass number is the
total number of nucleons in the atom’s
nucleus.
Isotopes
Atomic mass number for Oxygen on the Periodic
Table of Elements?
Every oxygen atom has 8 protons.
The number of neutrons in the nucleus can
change.
O16 - 8 protons, 8 neutrons
O17 - 8 protons, 9 neutrons
O18 - 8 protons, 10 neutrons
O17 and O18 are isotopes of common oxygen O16.
Ionic bonding involves transfer of an electron from one
atom (which becomes a positively charged cation) to
another (which becomes a negatively charged anion).
The two ions attract strongly to form a crystal lattice.
Covalent bonding occurs between
atoms of non-metals.
When the atoms form a bond they
become lower in energy and the
system is more stable.
The energy saved by moving to a
more stable situation is released as
heat.
For this reason bond formation is
always exothermic, i.e. heat energy
is released.
Conversely, in order to break a
chemical bond energy must be used
- it is an endothermic process.
Metallic Bonding
Bonds are formed between metal atoms that have low electronegativities and so do not
attract their valence electrons strongly. This allows the outermost electrons to be shared
by all the surrounding atoms, resulting in positive ions (cations) surrounded by a sea of
electrons .
A solid (not molten) metal is crystalline with its metal cations packed together as
closely as possible. Several packing arrangements are possible:
FCC (face centered cubic) structures
are more likely to be ductile than
BCC, (body centered cubic)
or HCP (hexagonal close packed).
Limitations: This Drude (1900) – Lorence (1923) electrons-sea theory explains many
properties of metals such as malleability and thermal conductivity. But some questions of
quantitative aspect are still unsolved such as why does the resistance to electrical
conductance of some metals increase with rise of temperature while in some cases it
decreases exponentially; why do some metals exhibit almost no electrical conductance
in the solid state and must be classified as insulators.
Electronegativity is defined as the tendency of an
atom in a molecule to attract electrons to itself.
Linus Pauling
Van Der Waals bonds
are intermolecular
forces of attraction or
repulsion which act
between neighboring
particles (atoms,
molecules or ions).
They are weak compared to the intramolecular
forces (e.g., covalent and ionic bonding) that
keep a molecule together.
Some dipoles are permanent, and other temporary. Van der
Waals bonds arises due to temporary dipoles on atoms. The
positively charged region of an atom attracts electrons
(which have a negative charge) from the neighboring atom,
setting up a dipole in that atom. This sets off a domino-like
cascade of dipoles throughout the atoms of a substance.
Beryl (unit cell outlined)
The crystal structure of Quartz (SiO2).