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Lesson № 2
• Theory of chemical bond
• Contemporary interpretation of
the periodic law of D.I.
Mendeleev on the basis of
electronic theory of atom.
Part 3:
THEORY OF
CHEMICAL BOND
IONIC BONDING
Ionic compounds are formed when
electrons are transferred from one
atom to another to form ions with
complete outer shells of electrons.
In an ionic compound the positive and
negative ions are attracted to each
other by strong electrostatic forces, and
build up into a strong lattice.
Ionic compounds have high melting
points as
considerable
energy
is
required to overcome these forces of
attraction.
The classic example of an ionic compound is
sodium chloride Na + Cl –,
formed when sodium metal burns in chlorine.
The crystal structure of sodium
chloride, NaCl, a typical ionic
compound.
The purple spheres
are sodium cations, Na+, and the
green spheres are chlorideanions,
Cl−.
Chlorine is a covalent molecule, so each atom
already has an inert gas configuration.
However, the energy given out when the
ionic lattice is formed is sufficient to break
the bond in the chloride molecule to give
atoms of chlorine.
Each sodium atom then transfers one
electron to a chlorine atom to form the ions.
The charge carried by an ion depends on the
number of electrons the atom needed to lose
or gain to achieve a full outer shell.
Thus magnesium chloride two chlorine
atoms each gain one electron from a
magnesium atom to form Mg2+ Cl-2.
Cations
Group 1
+1
Li+Na+K+
Group 2 Group 3
+2
+3
Mg2+Ca2+ Al3+
Anions
Group 4
-3
N3- P3-
Group 5
-2
O2- S2-
Group 6
-1
F- Cl- Br-
Formulas of ionic
compounds
• Lithium fluoride Li+F• Magnesium chloride Mg2+Cl-2
• Sodium oxide Na+2O2• Calcium sulfide Ca2+S2• Potassium nitride K+3N3• Calcium phosphide Ca2+3P3-2
• Aluminium bromide Al3+Br-3
• Iron (lll) oxide Fe3+2O2-3
• Iron (ll) oxide Fe2+O2-
Формулы ионных
соединений
• Фторид лития Li+F• Магния хлорид Mg2+, Cl-2
• Оксид натрия Na+2O2• Кальция сульфид Ca2+S2• Калия нитрид K+3N3• Кальций фосфористый Ca2+3P3-2
• Алюминий бромид Al3+Br-3
• Железо (III) оксид Fe3+2O2-3
• Железо (ll) оксид Fe2+O2-
COVALENT BONDING
Single covalent bonds
Covalent bonding involves the
sharing of one or more pairs of
electrons so that each atom in the
molecule achieves an inert gas
configuration.
The simples covalent molecule is hydrogen.
Each hydrogen atom has one electron in its outer shell.
The two electrons are shared and attracted
electrostatically by both positive nuclei resulting in a
directional bond between the two atoms to form a
molecule.
When one pair of electrons is shared the resulting bond is
known as a single covalent bond is chlorine, Cl2.
Covalent bonds are affected by the
electronegativity of the connected
atoms.
Two atoms with equal electronegativity
will make nonpolar covalent bonds such
as H–H and Сl–Сl.
An unequal relationship creates a polar
covalent bond such as with H−Cl.
Co-ordinate (dative) bonds
The electrons in the shared pair may originate
from the same atom. This is known as a
coordinate covalent bond.
Carbon monoxide (CO), ammonium ion
(NH4+), hydroxonium ion (H3O+).
Sulfur dioxide and sulfur trioxide are bond
sometimes shown as having a coordinate
bond between sulfur and oxygen or they are
show as having double between the sulfur
and the oxygen. Both are acceptable.
METALLIC BONDING
The valence electrons in metals become
detached from the individual atoms so
that metal consist of a close packed lattice
of positive ions in a sea of delocalized
electrons.
A metallic bonds is the attraction that
two neighbouring positive ions have
for the delocalized electron between
them.
Metals are malleable, that is, they can
be bent and reshaped under pressure.
They are also ductile, which means
they can be drawn into a wire.
Part 4:
PERIODIC TABLE
Periodic table
• The periodic table is a tabular
arrangement of the chemical
elements, organized on the basis of
their atomic numbers, electron
configurations (electron shell model),
and recurring chemical properties.
• Elements are presented in order of
increasing atomic number (the
number of protons in the nucleus).
• The table is a visual representation of the
periodic law which states that certain properties
of elements repeat periodically when arranged
by atomic number.
• In the standard periodic table, the elements are
listed in order of increasing atomic number (the
number of protons in the nucleus of an atom).
• The table arranges elements into vertical
columns (groups) and horizontal rows (periods)
to display these commonalities.
horizontal rows (periods)
The table arranges elements into
vertical columns (groups) and
horizontal rows (periods) to
display these commonalities.
The table arranges elements into
vertical columns (groups) and
horizontal rows (periods) to
display these commonalities.
Columns (groups)
Columns (groups)
Period
Columns (groups) are
determined by the electron
configuration of the atom.
A period is a horizontal row in
the periodic table.
Elements with the same
number of electrons in a
particular subshell fall into the
same columns (e.g. oxygen and
selenium are in the same
column because they both
have four electrons in the
outermost p-subshell )
Elements in the same period
show trends in atomic radius,
ionization energy, electron
affinity, and electronegativity.
The Periodic Table and physical
properties
In the Periodic Table elements are placed in order of
increasing atomic number.
Elements with the same number of valence electrons
are placed vertically in the same group.
The groups are numbered from 1 to 8 (or 0).
Some groups have their own name:
Group 1- alkali metals
Group 7- halogens
Group 8 or 0- noble gases
• Elements with the same outer shell of valence electrons are
placed horizontally in the same period. The transition
elements are located between groups 2 and 3.
Atomic radius
• The atomic radius is the distance from the nucleus to
the outermost electron.
Across a period electrons are being added to the same energy level, but the number of
protons in the nucleus increases. This attracts the energy level closer to the nucleus and
the atomic radius decreases across a period.
Ionic radius
• It is important to distinguish between positive ions
(cations) and negative ions (anions). Both cations and
anions increase in size down a group as the outer level
gets further from the nucleus.
Periodicity
• Elements in the same group tend to have similar
chemical and physical properties. There is a change
in chemical and physical properties across a period.
The repeating pattern of chemical and physical
properties shown by the different periods is known
as periodicity.
• These periodic trends can clearly be seen in atomic
radii, ionic radii, ionization energies,
electronegativities and melting points.
The Periodical Table and chemical
properties
Group 1- the alkali metals
Group 7 - the halogens
CHANGE FROM METALLIC TO NON-METALLIC NATURE OF
THE ELEMENTS ACROSS
PERIOD 3
Metals tend to be shiny and are good
conductors of heat and electricity.
Sodium, magnesium, and aluminum all
conduct electricity well.
Silicon is a semiconductor and is called
a metalloid as it
possesses some of the
properties of a metal
and some of a nonmetal.
Phosphorus, sulfur, chlorine, and argon
are non-metals and do not conduct
electricity.
• Sodium oxide and magnesium oxide are
both basic and react with water to form
hydroxides, e.g.
Na20 + H20
2NaOH
MgO + H20 Mg(OH)2
Metals can also be distinguished from nonmetals by their chemical properties.
Metal oxides tend to be basic, where as nonmetal oxides tend to be acidic.
Aluminum is a metal but its oxide is amphoteric, that is, it
can be either basic or acidic depending on whether it is
reacting with an acid or a base.
The remaining elements in period 3 have acidic oxides.
For example, sulfur trioxide reacts with water to form
sulfuric acid, and phosphorus pentoxide reacts with
water to form phosphoric (V) acid.
S03(g) + H20(l) H2S04(aq)
P4O10(s) +6H20(I) 4H3P04(aq)
Chlorine itself reacts with water to some extent to form
an acidic solution.
Cl2(aq) + H20(l) HCI(aq) + HCIO(aq)