Lewis Structures defining

G N Lewis first recognized that the most stable covalently-bonded species have an even number of electrons. He made a major contribution to the understanding of bonding by proposing that

a covalent bond is a shared electron pair.

covalently bonded atoms share sufficient electrons to complete a noble gas configuration

Although more sophisticated bonding theories have developed, Lewis structures are still used by practicing chemists to show the distribution of bonding and nonbonding electrons when predicting structure or reactivity. The Lewis structure of H₂O shows that this molecule has two bonds (four bonding electrons) and four non-bonding electrons.

Because the Lewis structure shows the number of bonding and non-bonding domains about the central atom, it can be used to predict the orientation of the bonds about the central atom (shape).

To construct the Lewis structure for a given molecular formula:

Step 1: Count the total number of valence electrons on the group of atoms. For H₂O: 2(2 H) + 6(1 O from Group 16) = 8 For PCl_3:5(P from Group 15) + 3 × 7 (3 Cl from Group 17) = 26 Subtract electrons if overall positive; Add electrons if overall negative.
Step 2: Join atoms by single bonds. Subtract the total number of electrons used from the Step 1 total. For H₂O: Subtract 4, two for each of two bonds (this leaves 4). For PCl_3: Subtract 6, two for each of three bonds (20 remain)
Step 3: Complete octets at outer atoms other than H. Subtract electrons used from Step 2 total. An octet is complete if there are eight electrons around the atom. These may be bonding and non-bonding. Each single bond to an atom counts as two electrons. Cl \| For: Cl – P – Cl Six additional electrons are required at each chlorine. These will be non-bonding. (Subtract 18 from 20 to give 2 remaining) For H-O-H: No change because the outer atoms are hydrogen (4 electrons remain).
Step 4: Place remaining valence electrons at the central atom. For H₂O: 4 electrons remain; place two nonbonding pairs at O For PCl₃: 2 electrons remaining: place one non-bonding pair at P Central atoms from the second row of the periodic table have a maximum of eight electrons. Central atoms from other rows have a minimum of eight electrons.

Step 1:
Count the total number of valence electrons on the group of atoms.

For H₂O: 2(2 H) + 6(1 O from Group 16) = 8
For PCl_3:5(P from Group 15) + 3 × 7 (3 Cl from Group 17) = 26

Subtract electrons if overall positive; Add electrons if overall negative.

Step 2:
Join atoms by single bonds.
Subtract the total number of electrons used from the Step 1 total.

For H₂O:
Subtract 4, two for each of two bonds (this leaves 4).
For PCl_3:
Subtract 6, two for each of three bonds (20 remain)

Step 3:
Complete octets at outer atoms other than H. Subtract electrons used from Step 2 total.
An octet is complete if there are eight electrons around the atom.
These may be bonding and non-bonding.
Each single bond to an atom counts as two electrons.

Cl
|
For: Cl – P – Cl
Six additional electrons are required at each chlorine.
These will be non-bonding. (Subtract 18 from 20 to give 2 remaining)

For H-O-H:
No change because the outer atoms are hydrogen (4 electrons remain).

Step 4:
Place remaining valence electrons at the central atom.

For H₂O: 4 electrons remain; place two nonbonding pairs at O
For PCl₃: 2 electrons remaining: place one non-bonding pair at P

Central atoms from the second row of the periodic table have a maximum of eight electrons.
Central atoms from other rows have a minimum of eight electrons.