Represent each of the following molecules using Lewis notation:

(a) bromine gas (Br₂)

(b) Calcium chloride (CaCl₂)

(c) carbon dioxide (CO₂)

(d) Which of the three molecules listed above contains a double bond?

(a)

Bromine is a halogen element. It has 7 electrons in its outermost orbit. It needs one more electron to obtain octet and become stable. Hence, in the case of bromine gas, 2 bromine atoms combine to form a molecule (Br₂). Each bromine atom contributes one electron for covalent bond. So, the 2 electrons involved in the bond are shared by both bromine atoms, hence octet is obtained.

(b)

Calcium has 2 electrons in outermost shell. It tends to lose them to get octet structure. Chlorine being halogen element has 7 electrons in the outermost orbit. It needs one more electron to obtain octet and become stable. So, calcium donates its 2 electrons to 2 different chlorine atoms. Hence, calcium cation, chlorine anions are formed and bond forms.

Thus, CaCl₂ is formed.

(c)

Oxygen has 6 electrons in its outermost orbit. Carbon has 4 electrons in its outermost orbit. So, oxygen tends to gain 2 electrons to obtain octet. Carbon shares electrons to obtain octet. So, in this case, one carbon and 2 oxygen atoms share electrons. Each oxygen contributes its 2 electrons and carbon contributes 4 electrons. 2 pairs of electrons between each oxygen and carbon form bonds. These electrons are shared by both oxygen and carbon. Both carbon and oxygen obtain octet. Thus, 2 double bonds are formed and CO₂ is formed.

(d) Carbon dioxide –