Single and multiple covalent bonds
Octet rule - Matter always wants to be in the most stable form. For any
atom, stability is achieved by following the octet rule, which is to say all
atoms (with a few exceptions) want 8 electrons in their outermost electron
shell (just like noble gases). The electrons present in the outermost shell of
an atom are called valence electrons.
Exceptions to the octet rule include hydrogen (H) and helium (He) that
follow the duet rule instead. They are the first two elements of the periodic
table and have a single electron shell which accommodates only 2
electrons. Other exceptions include some group 3 elements like boron (B)
that contain three valence electrons. Theoretically, boron can
accommodate five more electrons according to the octet rule, but boron is
a very small atom and five non-metal atoms (like hydrogen) cannot pack
around the boron nucleus. Thus, boron commonly forms three bonds,
BH\text{}_{3}3start text, end text, start subscript, 3, end subscript, with a
total of six electrons in the outermost shell. This also results in some
anomalous properties for boron compounds because they are kind of
“short of electrons”. It should be thus noted that covalent bonding between
non-metals can occur to form compounds with less than an octet on each
atom.
BH3 atom
In general, achieving the octet configuration (i.e. 8 electrons in the
outermost shell) is the driving force for chemical bonding between atoms.
Take a look at the outer shell configuration (i.e. number of valence
electrons) of three atoms – sodium (Na), chlorine (Cl) and neon (Ne):
Outer shell configuration diagrams of sodium (Na), chlorine (Cl) and neon
(Ne)
Ionic and covalent bonds
Let’s look at the following two scenarios A and B. There are two kids,
Emily and Sarah. They both are very good friends.
Scenario A:
Scenario A relationship diagram
Scenario B:
Scenario B relationship diagram
Now let’s apply the above analogy to chemical bonding. Assume that
Emily and Sarah represent two atoms, and the blanket symbolizes their
valence electrons. In scenario A, atom Emily is willing to donate her
electrons (blanket) to atom Sarah because by doing so both achieve an
octet configuration of 8 electrons in their respective outer shells, making
them both happy and stable. This donation of electrons is called ionic
bonding.
Example of an ionic bond
Example of an ionic bond
In scenario B, both the atoms Emily and Sarah are equally
electronegative. So, neither Emily nor Sarah is ready to part with her
electrons (blanket), and they instead share their valence electrons with
each other. This is called a covalent bond. Electronegativity is a measure
of how strongly an atom attracts electrons from another atom in a chemical
bond and this value is governed by where the particular atom is located in
the periodic table (francium is the least electronegative element while
fluorine is the most electronegative).
Example of a covalent bond
Example of a covalent bond
Polar and Non-polar covalent bond
Let’s go back to Emily and Sarah:
Scenario C:
Scenario C relationship diagram
Scenario D:
Scenario D relationship diagram
Let’s apply the above analogy to a covalent bond formation. In scenario
C, both Emily and Sarah are equally cold (in our analogy this translates to
them having the same electronegativity). Because they have the same
electronegativity, they will share their valence electrons equally with each
other. This type of a covalent bond where electrons are shared equally
between two atoms is called a non-polar covalent bond.
Example of a non-polar covalent bond
Example of a Non-polar covalent bond
In scenario D, Emily is cold but Sarah is much colder (no doubt mild
hypothermia from playing outside in the rain too long)! Together they
share the blanket, but Sarah has a tendency to keep pulling the blanket
from Emily in order to warm up more. In the atomic world, one atom
(Sarah) is more electronegative than another atom (Emily), and naturally
pulls the shared electrons towards itself. This pulling of electrons creates
slight polarity in the bond. Covalent bonds where electrons are not shared
equally between two atoms are called polar covalent bond.
Example of a polar covalent bond
Example of a polar covalent bond
As shown above, the electrons in a covalent bond between two different
atoms (H and Cl in this case) are not equally shared by the atoms. This is
due to the electronegativity difference between the two atoms. The more
electronegative atom (Cl) has greater share of the electrons than the less
electronegative atom (H). Consequently, the atom that has the greater
share of the bonding electrons bears a partial negative charge (δ-) and the
other atom automatically bears a partial positive charge (δ+) of equal
magnitude.
Properties of non-polar covalent bonds:

often occurs between atoms that are the same
electronegativity difference between bonded atoms is small (<0.5 Pauling

units)
electrons are shared equally between atoms

Properties of polar covalent bond:

always occurs between different atoms

electronegativity difference between bonded atoms is moderate (0.5 and
1.9 Pauling units)

electrons are not shared equally between atoms
Methane (CH\text{}_{4}4start text, end text, start subscript, 4, end
subscript) is an example of a compound where non-polar covalent bonds
are formed between two different atoms. One carbon atom forms four
covalent bonds with four hydrogen atoms by sharing a pair of electrons
between itself and each hydrogen (H) atom. The electronegativity value
for carbon (C) and hydrogen (H) is 2.55 and 2.1 respectively, so the
difference in their electronegativity values is only 0.45 (<0.5 criteria); the
electrons are thus equally shared between carbon and hydrogen. So we can
conveniently say that a molecule of methane has a total of four non-polar
covalent bonds.
Single and Multiple Covalent Bonds
The number of pairs of electrons shared between two atoms determines the
type of the covalent bond formed between them.
Number of electron pairs shared
Type of covalent bond formed
1
Single
2
Double
3
Triple
Now let’s move on to a couple of examples and try to determine the type
of covalent bonds formed
Diagram of single covalent bond being formed
Nitrogen atom can attain an octet configuration by sharing three electrons
with another nitrogen atom, forming a triple bond (three pairs of electrons
shared)
Diagram of nitrogen bonding into octet configuration
Consider the molecule carbon dioxide (CO\text{}_{2}2start text, end text,
start subscript, 2, end subscript). Let’s determine the type of covalent
bonds it forms.