To write the Lewis formula of CH 3 F, first of all, we have to find the total
number of valence electrons of all the atoms involved in this structure, i.e.
C, H and F, having four, one and seven valence electrons, respectively.
4 þ 3ð1Þ þ7 ¼ 14
C 3H
H
The carbon atom bonds with three hydrogen atoms and one fluorine atom,
and it requires four pairs of electrons. The remaining six valence electrons
are with the fluorine atom in the three nonbonding pairs.
C
H
H
F
H
:
..
..
In the periodic table, the period 2 elements C, N, O, and F have valence
electrons that belong to the second shell (2s and three 2p). The shell can be
completely filled with eight electrons. In period 3, elements Si, P, S and Cl
have the valence electrons that belong to the third shell (3s, three 3p and
five 3d ). The shell is only partially filled with eight electrons in 3s and
three 3p, and the five 3d orbitals can accommodate an additional ten
electrons. For these differences in valence shell orbitals available to
elements of the second and third periods, we see significant differences
in the covalent bonding of oxygen and sulphur, and of nitrogen and
phosphorus. Although oxygen and nitrogen can accommodate no more
than eight electrons in their valence shells, many phosphorus-containing
compounds have 10 electrons in the valence shell of phosphorus, and many
sulphur-containing compounds have 10 and even 12 electrons in the
valence shell of sulphur.
So, to derive Lewis structures for most molecules the following sequence
should be followed.
(a) Draw a tentative structure. The element with the least number of atoms is
usually the central element.
(b) Calculate the number of valence electrons for all atoms in the compound.
(c) Put a pair of electrons between each symbol.
(d) Place pairs of electrons around atoms beginning with the outer atom until
each has eight electrons, except for hydrogen. If an atom other than
hydrogen has fewer than eight electrons then move unshared pairs to
form multiple bonds.
2.3 CHEMICAL BONDING THEORIES: FORMATION OF CHEMICAL BONDS
23
number of valence electrons of all the atoms involved in this structure, i.e.
C, H and F, having four, one and seven valence electrons, respectively.
4 þ 3ð1Þ þ7 ¼ 14
C 3H
H
The carbon atom bonds with three hydrogen atoms and one fluorine atom,
and it requires four pairs of electrons. The remaining six valence electrons
are with the fluorine atom in the three nonbonding pairs.
C
H
H
F
H
:
..
..
In the periodic table, the period 2 elements C, N, O, and F have valence
electrons that belong to the second shell (2s and three 2p). The shell can be
completely filled with eight electrons. In period 3, elements Si, P, S and Cl
have the valence electrons that belong to the third shell (3s, three 3p and
five 3d ). The shell is only partially filled with eight electrons in 3s and
three 3p, and the five 3d orbitals can accommodate an additional ten
electrons. For these differences in valence shell orbitals available to
elements of the second and third periods, we see significant differences
in the covalent bonding of oxygen and sulphur, and of nitrogen and
phosphorus. Although oxygen and nitrogen can accommodate no more
than eight electrons in their valence shells, many phosphorus-containing
compounds have 10 electrons in the valence shell of phosphorus, and many
sulphur-containing compounds have 10 and even 12 electrons in the
valence shell of sulphur.
So, to derive Lewis structures for most molecules the following sequence
should be followed.
(a) Draw a tentative structure. The element with the least number of atoms is
usually the central element.
(b) Calculate the number of valence electrons for all atoms in the compound.
(c) Put a pair of electrons between each symbol.
(d) Place pairs of electrons around atoms beginning with the outer atom until
each has eight electrons, except for hydrogen. If an atom other than
hydrogen has fewer than eight electrons then move unshared pairs to
form multiple bonds.
2.3 CHEMICAL BONDING THEORIES: FORMATION OF CHEMICAL BONDS
23