Lewis Structures and Valence Electrons

Lewis Structures

PCI3 has one phosphorus atom and three chlorine atoms. Phosphorus has five valence electrons and each chlorine atom has seven valence electrons. Therefore, the total number of valence electrons in PCI3 is 26. The Lewis structure of PCI3 shows phosphorus with three single bonds to chlorine atoms and one lone pair of electrons.
OCI- has one oxygen atom, one chlorine atom, and one negative charge. Oxygen has six valence electrons and chlorine has seven valence electrons. The negative charge adds an additional electron. Therefore, the total number of valence electrons in OCI- is 14. The Lewis structure of OCI- shows chlorine with one double bond to oxygen and two lone pairs of electrons on oxygen, one lone pair of electrons on chlorine.
NH3 has one nitrogen atom and three hydrogen atoms. Nitrogen has five valence electrons and each hydrogen atom has one valence electron. Therefore, the total number of valence electrons in NH3 is eight. The Lewis structure of NH3 shows nitrogen with three single bonds to hydrogen atoms and one lone pair of electrons on nitrogen.
HF has one hydrogen atom and one fluorine atom. Hydrogen has one valence electron and fluorine has seven valence electrons. Therefore, the total number of valence electrons in HF is eight. The Lewis structure of HF shows hydrogen with one single bond to fluorine and three lone pairs of electrons on fluorine.
CF4 has one carbon atom and four fluorine atoms. Carbon has four valence electrons and each fluorine atom has seven valence electrons. Therefore, the total number of valence electrons in CF4 is 32. The Lewis structure of CF4 shows carbon with a single bond to each fluorine atom and three lone pairs of electrons on each fluorine atom.

Deficiency

N2 has two nitrogen atoms. Each nitrogen atom has five valence electrons, resulting in 10 total. The Lewis structure of N2 shows a triple bond between the two nitrogen atoms.
NO+ has one nitrogen and one oxygen atom, and a positive charge. Oxygen has six valence electrons and nitrogen has five, and the positive charge subtracts an electron. The total number of valence electrons in NO+ is 10. The Lewis structure of NO+ shows a triple bond between nitrogen and oxygen.

Octet deficient

BF3 has one boron atom and three fluorine atoms, resulting in 24 valence electrons. Boron has three valence electrons and each fluorine atom has seven. The Lewis structure of BF3 shows boron with a single bond to each fluorine atom. Boron has an incomplete octet with only six electrons, but it is stable.
BeH2 has one beryllium atom and two hydrogen atoms. Beryllium has two valence electrons and each hydrogen atom has one, resulting in four total valence electrons. The Lewis structure of BeH2 shows beryllium with a single bond to each hydrogen atom. Beryllium has an incomplete octet with only four electrons, but is stable.

Expanded octet

SF6 has one sulfur atom and six fluorine atoms. Each fluorine atom has seven valence electrons and sulfur has six, resulting in 48 total. The Lewis structure of SF6 has sulfur with six single bonds to fluorine atoms.
XeF2 has one xenon atom and two fluorine atoms. Xenon has eight valence electrons and each fluorine atom has seven, resulting in 22 total. The Lewis structure of XeF2 has xenon with two single bonds to fluorine atoms and three pairs of lone pairs of electrons around it.
CIF3 has one chlorine atom and three fluorine atoms. Chlorine has seven valence electrons and each fluorine atom has seven, resulting in 28 total. The Lewis structure of CIF3 shows chlorine with a single bond to each fluorine atom, two lone pairs on chlorine, and one fluorine with four lone pairs.
ICl4 has one iodine atom and four chlorine atoms. Iodine has seven valence electrons and each chlorine atom has seven, resulting in 35 total valence electrons. The Lewis structure of ICl4 shows iodine with a single bond to each chlorine atom, two lone pairs on iodine, and two chlorine atoms with three lone pairs each.

Resonance forms

NO3 has one nitrogen atom and three oxygen atoms. Nitrogen has five valence electrons and oxygen has six, resulting in 23 total valence electrons. The Lewis structure of NO3 shows nitrogen with one double bond and two single bonds to each oxygen atom. The double bond can be distributed among the three oxygen atoms, leading to three resonance structures.
O3 has three atoms, all oxygen atoms. The Lewis structure of O3 shows one double bond and one single bond between two oxygen atoms. The double bond can be shifted to the other oxygen atom, leading to two resonance structures.
NO2 has one nitrogen atom and two oxygen atoms. The Lewis structure of NO2 shows nitrogen with a double bond to one oxygen and a single bond to the other. The double bond can be shifted to the other oxygen atom, leading to two resonance structures.

Formal Charge

NCO- has one nitrogen atom, one carbon atom and one oxygen atom. Nitrogen has five valence electrons, carbon has four and oxygen has six. There is also a negative charge. The Lewis structure of NCO- shows nitrogen with one double bond to carbon and one single bond to oxygen. The negative charge is placed on the oxygen atom. The carbon has an incomplete octet, but is stable because it has a positive formal charge.
CO2 has one carbon atom and two oxygen atoms. Carbon has four valence electrons and oxygen has six. The Lewis structure of CO2 shows carbon with a double bond to each oxygen atom. Carbon has a formal charge of 0. Each oxygen atom has a formal charge of -1.
OCS has one oxygen atom, one carbon atom and one sulfur atom. The Lewis structure of OCS shows carbon with a double bond to oxygen and one single bond to sulfur. Carbon has a formal charge of 0 and sulfur has a formal charge of +1 while oxygen has a formal charge of +1.

MCQs

a) The total number of lone pairs in NCI3 is 9.
b) The best Lewis structure for CS2 is S=C=S.
c) In the Lewis structure of ICl2, there are 3 nonbonding pairs (lone pairs) of electrons around the central iodine atom.
d) The number of resonance structures for sulfur dioxide SO2 that satisfy the octet rule is 2.
e) The most useful guideline for the application of formal charges in neutral molecules is A Lewis structure with no formal charges on most of its atoms is preferred.
f) The formal charge on sulfur in the best Lewis structure for the SCN- (thiocyanate) ion is 0.
g) The molecules that violate the octet rule are BF3, XeCl2, and SF4.

Challenging Questions

a) The most favorable Lewis structure of SO₄¯² has sulfur with two double bonds and two single bonds to oxygen. The formal charge is distributed on the oxygen atoms.
b) The best Lewis structure for chlorate ion ClO3¯ has 2 single bonds, 1 double bond, and 10 lone pairs.