Transcript chap-5.1 - Environmental-Chemistry

5.1-Organizing the elements:
• To find a way to organize the elements,
scientist studied the elements and their
properties.
• Dmitri Mendeleev was the first Russian
chemist to design a way to organize the
elements.
• In 1869, Mendeleev published the first
periodic table of the elements. He arranged
the elements in his P.T. in rows by increasing
atomic mass.
Characteristics of Mendeleev’s P.T.
Elements are arranged in rows by increasing
atomic masses. Each time, a new row is
started when chemical properties of the
elements repeated. Therefore elements in
the same column had similar properties.
Mendeleev left gaps in the list, and predicted
that new elements would be discovered that
would fill these gaps. He used each new
element’s position in the P.T. to predict some
of the properties of the elements.
Mendeleev is often considered father of the
P.T. The element “mendelevium” was named
in his honor.
Achievement & drawback
• Drawback of mendeleev’s P.T.- It was difficult
to find out accurate values of atomic masses
of the elements, therefore some elements did
not quite fit the pattern in Mendeleev’s P.T.
according to increasing atomic masses.
• Achievement of mendeleev’s P.T.- In 1886, a
new element germanium was discovered that
is very similar to those predicted by
Mendeleev. Then other two elements, gallium
and scandium also discovered that closely
matched Mendeleev’s prediction.
Periodic law and modern P.T.
• As mendeleev’ P.T. was found defective & as
scientists learned more about the structure of
atom, they improved Mendeleev’s table.
• The modern periodic table organizes
elements by increasing atomic number. This
new arrangement fixed the discrepancies
with elements such as tellurium and Iodine in
Mendeleev’s periodic table.
• Modern Periodic Law: The law that states
that the repeating chemical and physical
properties of elements change periodically
with the atomic numbers of the elements.
Modern Periodic table:
Characteristics of modern P.T.
• Elements are arranged with increasing atomic
number from left to right in horizontal rows.
• Horizontal rows are known as periods.
• Elements become less metallic across
each period.
• Each vertical column in the P.T. is called a
group. Elements in a same group have similar
properties.
• Atomic number of the elements in the groups
increase with definite interval of 2, 8, 8, 18, 18
and 32.
Properties of elements in modern P.T.
• There are total 18 groups and 7 periods.
• Group-1 is known as Alkali metals
Group-2 is known as Alkaline earth metals
Group-3 to12 are called Transition metals
Group-13→boron gr. elements (are nonmetal)
Group-14→carbon gr. elements, are nonmetal
Group-15→nitrogen gr. elements, nonmetals
Group-16→oxygen gr. elements are nonmetal
Group-17→Halogen gr. elements nonmetals.
• Group-18→are Noble gases, Inert gases, or
Zero group elements, and also nonmetals.
• Elements along zigzag line are called semi
metals or metalloids. (B, Si, As, Te, Ge, & Sb)
Properties of elements in modern P.T.
• Period-1 is shortest period with only two
elements, Periods-2 and 3 are shorter periods
with 8-elements.
• Periods - 4 and 5 are longer period with 18elements and period-6 is longest period with
total 32 elements. Period-7 is incomplete.
• Below the transition metals, there are two
rows known as actinide series and
lanthanide series elements.
• The properties of elements are periodic with
increase in atomic number of the elements in
groups and periods.
The role of electrons in formation of
positive and negative ions
• The periodic trends in the periodic table are
the result of electron arrangement.
• Valence electron: Electrons in outer most
energy level or outer most shell in an atom
are called valance electrons.
• Valence electrons account for similar
properties of elements in the same group.
• If number of valence electrons are 1, 2, or
3, then atom shows tendency to lose these
electrons and form positive ions losing
electrons. Metal atoms form positive ions only
Formation of ions: continued…..
• Li(+1) and Na(+1) ions
• These elements (Li and
Na) in group-1 each
have one valence
electron.
• One valence electron is
lost easily with lowest
ionization energy to form
mono positive(+1) ions.
• Ion: An atom, radical, or
molecule that has gained
or lost one or more
electrons and has a
negative or positive
charge.
Formation of ions:
• In lithium neutral
atom, one valence
electron is lost since
one proton is greater
than total electrons.
So that Lithium atom
achieves 1-positive
charge.
• On other hand,
Chlorine atom gains
one electron, so one
electron is greater
than proton, Cl-atom
achieves 1-negative
charge.
Classification of elements:
• Elements in each category have similar
properties.
Classification of elements in P.T.
• Generally all metals form positive ions by
losing electrons but nonmetals form negative
ions by gaining electrons.
• All elements in periodic table are classified
into three different categories: 1) metals,
2)nonmetals, or 3) semi conductors
• Groups 1 to 12 are different types of metal
elements.
• Group 13 to 18 are nonmetal elements.
• Properties of actinides and lanthanides
resemble to properties of transition elements.
•
Families of elements & their properties
• Elements in the same column or group is
known as elements of same family.
• The elements in a same family have the same
number of valence electrons since have
similar properties.
• Gr. # # of valence electron Name of family
Gr. 1
1
alkali metals
Gr. 2
2
alkaline earth metals
Gr. 3
3
transition metals
Gr. 17 varied
halogens
Gr. 18 8
noble gases
• Only in noble gas Helium, it has only 2-valence
electrons instead of 8-electrons exceptionally.
Properties of family-alkali metals
• Alkali metals are very reactive because of
only one valence electron.
• Soft, shiny and react violently with water.
• Alkali metals are stored in kerosene oil to
prevent from reaching with moisture in the air.
• Alkali metals form positive ions with +1
charge, and found in combined with other
elements in the form of compounds like NaCl.
• Many alkali metals have similar melting point,
boiling point, and densities.
Properties of alkaline-earth metals:
• Alkaline-earth metals form compounds that
are found in limestone and in the human body
• Alkaline-earth metals are harder, denser,
stronger, and have higher melting points than
alkali metals.
• They have two valence electrons since their
ions have +2 charge.
• Bones and teeth get strength from calcium
compounds.
• Magnesium is used to build airplanes,
activates enzymes, and in medicine like milk
of magnesia and Epsom salt.
Properties of transition metals:
• Located in the middle of the periodic table and
show variable oxidation states from +1 to +7
• With the exception of mercury (Hg is liquid),
transition metals are harder, more dense, and
have higher melting points than alkali,
alkaline-earth metals.
• Gold, silver, and platinum are noble metals
and often shaped to make jewelry.
• Copper is used in electrical wire, Tungsten is
used in light bulb-filament, mercury is used in
thermometer, iron ,cobalt and manganese
play important roles body chemistry.
Properties of synthetic elements:
• Some elements are made in laboratory known
as synthetic elements. Ex. Technetium, and
promethium are synthetic and radioactive.
• The most stable isotope technetium-99, is
used by doctors to create brain scan to detect
cancer. Also medical problems that occur in
soft tissue of body can be detected.
• Promethium-147 is used as an ingredient in
some glow-in-the-dark paints.
• All elements have atomic number greater than
92 are also synthetic. Americium-241 are
found in most household detectors.
Properties of nonmetals Carbon gr.
• Nonmetals and their compounds are plentiful on
earth. Examples: oxygen, nitrogen, sulfur
• Carbon is also nonmetal and can form many
compounds. Diamonds, and graphite are pure form
of carbon. The “fullerenes” is third form of carbon
was confirmed in 1990. The most famous fullerene,
a cluster of 60 carbon atoms, is called a
buckministerfullerene invested by R. Buckminister
Fuller.
• Carbon can also combine with other elements to
form millions of carbon containing compounds.
Sugar, glucose, all organic compounds are carbon
compounds.
Properties of nonmetals-halogens
• Gr. 17 most reactive nonmetals are known as
halogens. They have 7 valence electrons, adding 1electron, they complete stable system of 8-electrons
forming -1negative ions.
• Chlorine has strong smell and used to kill bacteria in
pool water, drinking water too.
• Cl combine with sodium metal to form compound
common salt or table salt-NaCl.
• Fluorine, bromine, and iodine are other halogens.
Fluorine is used in tooth paste to prevent tooth
decay.
• Iodine used in iodized salt responsible for proper
thyroid gland function in human body.
Properties of nonmetal- elements:
• Except hydrogen all nonmetals are found on
the right side of P.T. Nonmetals include some
elements from gr. 13-16 and all elements of
gr. 17 (halogens) and 18 (noble gases).
• Outer most shell in noble gases is completely
filled, so they are chemically inert with other
elements, exist as single atom. They neither
lose, nor gain electron since their oxidation
state is zero.
• Helium is used to give lift to blimps & balloons
Argon is used to fill light bulb since it is inert.
Neon gives bright reddish-orange light.
Properties of semiconductors:
• The six elements: B, Si, As, Te, Ge, and Sb along
zigzag line in P.T. are referred as semiconductors or
metalloids.
• Metalloids have half of the properties like metals
and remaining half like a nonmetal.
• Metalloids are able to conduct heat and they are
known as semiconductors.
• Silicon, 28 % mass of earth’s crust. The common
compound is silicon dioxide-SiO2.
• Boron is extremely hard used to make heat
resistance glass.
• Arsenic is a shiny solid and tarnishes when
exposed to air. Antimony is a bluish, white and
shines like metal. Tellurium silvery white solid.