Bonding
Ionic and Covalent Bonds
Sooner or later, we have to distinguish between covalent and ionic bonding. In fact,
we can distinguish organic and inorganic (not - organic) from the types of bonding
in the molecule. Organic molecules are made up mostly of covalent bonds.
What is a bond?
In life we know many types of bonds: love between husband-wife, friendship: the
force that keeps people together. Well, for our purposes, we understand that molecules
are held together by pairs of electrons in orbitals. The nature of the bond varies
depending on atoms being held together. If the two atoms have similar electronic
properties, then the bond between the two atoms is shared. This is a covalent bond.
If the electronic properties of the two atoms in a bond are different, then the
electrons are localized on one of the atoms and the bond is ionic.
Electronegativity
The electronegativity of an atom has to do with the ratio and distance between the
protons/electrons. We use the periodic chart as a reference. F (fluorine) in the
upper-right corner has the highest electronegativity index=4.0 and Cs (cesium) in
the lower-left corner has the lowest electronegativity.
Ionic Bond Formation:
When Cs and F are brought together, both atoms are susceptible to ionization because
they are one row removed from the last column of the periodic table -- the octet
column. If F (9 protons + 9 electrons) gains an electron, it becomes F-
(9 protons + 10 electrons) and stabilizxed because now all of the orbitals are filled.
Similarly, Cs () loses an electron to form Cs+
which has a filled set of orbitals. CsF exists as a pair of charged paticles and
the attraction between them is the attraction between the positive and negative charges
on these ions. The greater electronegativity of F is enough to overpower
Covalent Bond Formation:
When atoms have similar eletronegativity index ratings, there is no great preference
for which atom gains electrons and which atom loses electrons. So, the electrons
are shared between the two atoms.
Orbitals
Orbitals are vessels for electron pairs. They come in different sizes and shapes.
Sometimes the orbitals are localized on one atom (non-bonding) and sometimes the
orbitals are between two atoms holding the bonding electrons.
We classify orbitals into 3 basic types:
Nonbonding
Sigma bonding
Pi bonding