CHEMICAL BONDING AND MOLECULAR STRUCTURE 1 Flashcards
what is a chemical bond
The attractive force which holds various
constituents (atoms, ions, etc.) together in different
chemical species is called a chemical bond.
Every
system tends to be more stable and bonding is nature’s
way of lowering the energy of the system to attain stability.
what was kossel lewis approach to molecular bonding
He considered the nucleus and inner shell e-s to be the kernel.
Each e- was considered to be at the corner of a cube surrounding the kernel. When all 8e-s are filled, it is said to be stable.
Lewis postulated that atoms achieve the
stable octet when they are linked by
chemical bonds. In the case of sodium and
chlorine, this can happen by the transfer of
an electron from sodium to chlorine thereby
giving the Na+
and Cl–
ions. In the case of
other molecules like Cl2
, H2
, F2
, etc., the bond
is formed by the sharing of a pair of electrons
between the atoms. In the process each atom
attains a stable outer octet of electrons.
what are valence e-s?
what are lewis symbols?
In the formation of a
molecule, only the outer shell electrons take
part in chemical combination and they are
known as valence electrons. The inner shell
electrons are well protected and are generally
not involved in the combination process.
Lewis symbols are notations invented by American Chemist lewis to represent the no of valence e-s of that atom.
what is the significance of lewis symbols
The
number of dots around the symbol represents the number of valence electrons. This number
of valence electrons helps to calculate the
common or group valence of the element. The
group valence of the elements is generally
either equal to the number of dots in Lewis
symbols or 8 minus the number of dots or
valence electrons.
what were kossel’s postulates
- In the periodic table, the highly
electronegative halogens and the highly
electropositive alkali metals are separated
by the noble gases; - The formation of a negative ion from a
halogen atom and a positive ion from an
alkali metal atom is associated with the
gain and loss of an electron by the
respective atoms; - The negative and positive ions thus
formed attain stable noble gas electronic
configurations. The noble gases (with the
exception of helium which has a duplet
of electrons) have a particularly stable
outer shell configuration of eight (octet)
electrons, ns2np6
. - The negative and positive ions are
stabilized by electrostatic attraction.
what is ionic bond? what is electrovalence
The bond formed, as a result of the
electrostatic attraction between the
positive and negative ions was termed as the electrovalent bond. The electrovalence
is thus equal to the number of unit
charge(s) on the ion. Thus, calcium is
assigned a positive electrovalence of two,
while chlorine a negative electrovalence of
one.
state the octet rule
Kössel and Lewis in 1916 developed an
important theory of chemical combination
between atoms known as electronic theory
of chemical bonding. According to this,
atoms can combine either by transfer of
valence electrons from one atom to another
(gaining or losing) or by sharing of valence
electrons in order to have an octet in their
valence shells. This is known as octet rule.
what are langmuir’s rules
- Each bond is formed as a result of sharing
of an electron pair between the atoms. - Each combining atom contributes at least
one electron to the shared pair. - The combining atoms attain the outer-
shell noble gas configurations as a result
of the sharing of electrons.
what are typed of bonds
when a single pair of electrons are shared between two atoms, they are said to be bound by a single covalent bond
when two atoms shaire two pairs of electrons, the covalent bond between is a double covalent bond
when two atoms share three elctrons pairs, the bond formed between them is a triple covalent bond
how does ion possess charge
In case of polyatomic ions, the net charge is
possessed by the ion as a whole and not by a
particular atom. It is, however, feasible to
assign a formal charge on each atom.
define formal charge
The
formal charge of an atom in a polyatomic
molecule or ion may be defined as the
difference between the number of valence
electrons of that atom in an isolated or free
state and the number of electrons assigned
to that atom in the Lewis structure.
f.c= v-l - 1(s)/2
formal charges
do not indicate real charge separation within
the molecule. explain
Indicating the charges on the
atoms in the Lewis structure only helps in
keeping track of the valence electrons in the
molecule. Formal charges help in the
selection of the lowest energy structure from
a number of possible Lewis structures for a
given species. Generally the lowest energy
structure is the one with the smallest
formal charges on the atoms. lower energy more stability.The formal
charge is a factor based on a pure covalent
view of bonding in which electron pairs
are shared equally by neighbouring atoms.
