Explanation:
The octet rule states that atoms tend to gain, lose or share electrons in order to have eight electrons in their outermost shell. However, there are exceptions to this rule, and some molecules do not follow the octet rule.

Molecules that do not follow the octet rule usually have an odd number of valence electrons or have atoms with less than eight valence electrons. Let's look at each option given in the question to understand which molecules do not follow the octet rule.

Option A:
CCl4 - Carbon has four valence electrons, and each chlorine atom has seven valence electrons. In this molecule, carbon shares its electrons with each chlorine atom to form covalent bonds. Carbon ends up with eight electrons, but each chlorine atom has ten electrons in its outermost shell, which is more than the octet rule. Therefore, this molecule follows the octet rule.
N2O4 and N2O - These molecules have an odd number of valence electrons, so they do not follow the octet rule.

Option B:
BF3 - Boron has three valence electrons, and each fluorine atom has seven valence electrons. In this molecule, boron shares its electrons with each fluorine atom to form covalent bonds. Boron ends up with only six electrons in its outermost shell, which is less than the octet rule. Therefore, this molecule does not follow the octet rule.
BeCl2 - Beryllium has two valence electrons, and each chlorine atom has seven valence electrons. In this molecule, beryllium shares its electrons with each chlorine atom to form covalent bonds. Beryllium ends up with only four electrons in its outermost shell, which is less than the octet rule. Therefore, this molecule does not follow the octet rule.
NO2 - Nitrogen has five valence electrons, and each oxygen atom has six valence electrons. In this molecule, nitrogen shares its electrons with each oxygen atom to form covalent bonds. Nitrogen ends up with only seven electrons in its outermost shell, which is less than the octet rule. Therefore, this molecule does not follow the octet rule.

Option C:
NaCl and MgCl2 - These molecules follow the octet rule because they form ionic bonds, and the ions have a complete outermost shell.
Tl2O - Thallium has three valence electrons, and oxygen has six valence electrons. In this molecule, thallium shares its electrons with each oxygen atom to form covalent bonds. Thallium ends up with only six electrons in its outermost shell, which is less than the octet rule. Therefore, this molecule does not follow the octet rule.

Option D:
PCl3 - Phosphorus has five valence electrons, and each chlorine atom has seven valence electrons. In this molecule, phosphorus shares its electrons with each chlorine atom to form covalent bonds. Phosphorus ends up with ten electrons in its outermost shell, which is more than the octet rule. Therefore, this molecule does not follow the octet rule.
NH3 and H2O - These molecules follow the octet rule because the central atom shares its electrons with the surrounding atoms to form covalent bonds, and each atom ends up with eight electrons in its outermost shell.

In conclusion, molecules that do not follow