Notesale: Turn your study into money

Document Preview

Extracts from the notes are below, to see the PDF you'll receive please use the links above

---

CHEMICAL COMBINATION OR BONDING
A chemical bond is formed when atoms are held together by attractive forces
...
The sharing or exchange of electrons takes place so that the
outer energy levels of the atoms involved are filled and the atoms are more stable
...

If an electron is exchanged it means that it is transferred from one atom to another, in other
words one atom gains an electron while the other loses an electron
...

The type of bond that is formed depends on the elements that are involved
...

You need to remember that it is the valence electrons that are involved in bonding and that
atoms will try to fill their outer energy levels so that they are more stable
...
This means when the individual atoms
combine to form molecules through a bond, the potential energy of the combining atoms
decreases and the resulting molecules are more stable than the free atoms
...
It follows from this that the process of bond formation between the atoms decreases the
energy of the molecule formed from these atoms and forms a system of lower energy and
greater stability
...
Hence, they stay in
atomic form
...
This low energy of noble gas atoms is associated with their outer shell electronic
configuration, i
...
the stable arrangement of eight electrons (called octet)
...

The chemical stability of the octet of noble gases led chemists to assume that when atoms of
other elements combine to form a molecule, the electrons in their outer shells are arranged
between themselves in such a way that they achieve a stable octet of electrons (noble gas
configuration) and thus a chemical bond is established between the atoms
...
This octet rule was later called “Electronic Theory of
Valence”
...
This means an atom with less than eight electrons in the outer shell is chemically
active and has a tendency to combine with other atoms
...
Therefore, they do so by forming different types of chemical bonding which are as
follows:
a
...

b
...
Co-ordinate covalent or dative bonding
d
...
Hydrogen bonding
f
...

Each atom contributes equal numbers of electrons so that the atomic orbital overlap
...
Covalent bonds are usually formed between nonmetal atoms due to their high electronegativity
...
g
...


Polar and Non-polar covalent compounds
When a covalent bond is formed between atoms of different electronegativity, the more
electronegative atom attracts bonding electrons towards itself leading to slight charge separation
...
g
...

Because the two atoms have the same electronegativity, the electron pair in the covalent bond is
shared equally between them
...
As a
result, a polar covalent bond is formed where one atom will have a slightly negative charge and
the other a slightly positive charge
...
So, in a molecule such as hydrogen chloride (HCl), hydrogen is Hδ+
and chlorine is Clδ−
...

Trichloromethane the bonds are polar but the molecule is unsymmetrical and therefore it is a
polar covalent compound
...
g Sulphur, carbon dioxide and giant atomic structure e
...
, Silicon (IV)oxide,
Diamond
...
They are soluble in non-polar solvents e
...
, benzene
2
...
They have low melting and boiling point
4
...
The atom with lone pairs of electrons donates an electron pair therefore it is called
donor Atom and the atom receiving the electrons is acceptor
...
Ammonia molecule (NH3) combines with hydrogen proton
(H+) to form ammonium radical (NH4+)
...
The electropositive atom loses electrons to become positively charged
while the electronegative atom gains the electrons to become negatively charged
...
For example,
sodium combines with chlorine to form sodium chloride
...
1
...
Because the
electronegativity of chlorine is higher than the electronegativity of sodium, chlorine will attract
the valence electron in the sodium atom very strongly
...
Sodium has lost an electron and forms a Na+ ion
...
The attractive force between the positive and negative ion is what holds the
molecule together
...
Magnesium has two valence electrons and an electronegativity of 1
...
5, it attracts the two valence
electrons from the magnesium atom and these electrons are transferred from the magnesium
atom to the oxygen atom
...
The attractive force between the oppositely charged ions is what holds the
molecule together
...
5
...
Magnesium loses two electrons to form Mg2+, and oxygen gains two electrons to form O2−
...


The balanced equation for the reaction is:
2Mg + O2 → 2MgO
Because oxygen is a diatomic molecule, two magnesium atoms will be needed to combine with
two oxygen atoms to produce two molecules of magnesium oxide (MgO)
...

The arrangement of ions in a regular, geometric structure is called a crystal lattice
...
The structure of a crystal lattice is shown

The strength of the ionic bond depends on the magnitude of the forces which depends on:
Charge of the ion
...

Ionic radius
...

Properties of Ionic Compounds
Ionic compounds have a number of properties:
1
...

2
...

3
...
This means that ionic compounds are
often hard and have high melting and boiling points
...
Ionic compounds are brittle, and bonds are broken along planes when the compound is
Stressed
...
Solid crystals don’t conduct electricity, but ionic solutions do
...
The strength of the metallic bond increases with increase in the number of
delocalized electrons
...
In a crystal lattice, the valence electrons of metal tend to separate from its orbital and
move at random forming an electron cloud
...


This force of attraction is referred to as metallic bond
...
This force of
attraction is referred to as metallic bond
...
For instance, the decreasing order
of metallic bond in the crystal lattice of sodium, magnesium and aluminum is as follows:
The Properties of Metals
Metals have several unique properties as a result of this arrangement:
1
...
Because the electrons are loosely bound and are able to move, they can
transport heat energy from one part of the material to another
...
Electrical conductors
Metals are good conductors of electricity, and are therefore used in electrical conducting
wires
...

3
...
The
loosely bound electrons are able to absorb and reflect light at all frequencies, making metals
look polished and shiny
...
Malleable and ductile
This means that they can be bent into shape without breaking (malleable) and can be
stretched into thin wires (ductile) such as copper, which can then be used to conduct

electricity
...

5
...
The high
melting point is due to the high strength of metallic bonds
...
Density
Metals have a high density because their atoms are packed closely together
...
When a hydrogen
atom is bonded to a very electronegative atom, including fluorine, oxygen, or nitrogen, a very
polar bond is formed
...
These partial charges are similar to what happens in every polar molecule
...
These
molecules will be attracted to other molecules which also have partial charges
...
Compounds of Nitrogen, Oxygen and Fluorine form hydrogen bonds
...

2
...

The effects of hydrogen bonding on boiling point depend on
(a) Number of hydrogen bond formed
(b) Electronegativity of the atom
3
...
g
...
Each
oxygen atom is tetrahedrally attached to four hydrogen atoms i
...
This arrangement gives ice a more Open structure hence
an increased volume therefore a fixed mass for water, the density Decreases and it freezes
...
The magnitude of
these forces is maximum for solids and decreases for liquids and minimum for gases
...
These forces arise from the electrostatic attraction of the nuclei of one
molecule for the electrons of a different molecule
...
Van der Waals’ forces are short
range forces
...
(all gases)
...
Larger the dipole moment of a molecule, greater is the dipole-dipole attractive force
between the molecules
...


(ii) Ion-dipole interaction
If a charged particle such as an ion is introduced into the neighbourhood of an uncharged
nonpolar molecule (e
...
, an atom of a noble gas), it will distort the electron cloud of the atom
or molecule in the same way as a cation does on a large anion (polarisation)
...
Such interactions occur only in solution of ionic compounds in non-polar
solvents
...
Then there occurs dipole induced dipole interaction
...


(iv) London or dispersion forces (instantaneous dipole-induced dipole interactions)
These interactions are present in non-polar molecules like N2, O2, Cl2, CH4 etc
...
These molecules do not have permanent dipole
moment
...
This causes a momentary distortion of the electronic
charge cloud which results in an instantaneous temporary polarity
...
These two dipoles attract
each other by electrostatic forces called London forces
...




---