The correct formula for a covalent bond that obeys the octet rule is: HCI, PCI2, NCI CCI2, NaCI. Why?
A covalent bond is one in which atoms share electrons (instead of donating or accepting them, as is the case with ionic bonds) in such a way, that octet configuration is achieved. That is, the outer shell is filled with 8 electrons. Lets examine each compound one by one.
HCl is a covalent compound, with hydrogen and chlorine sharing an electron each. This way, hydrogen achieves a fully filled 1s orbital and chlorine also achieves the octet. Hence HCl is a covalent compound that fulfills the octet rule.
PCl2 is a covalent compound, but does not obey the octet rule. Phosphorus has a valency of +3 and shares two electrons, one each with a chlorine atom. However, there is still a positive charge leftover (although the two chlorine atoms achieve octet). In fact, `PCl_2^-` is the correct chemical formula.
NCl is a covalent compound. However, Nitrogen has a valency of +3 and requires 2 more chlorine atoms to achieve an octet configuration. In this case, one chlorine atom achieves the octet.
CCl2 is a covalent compound, yet carbon has 2 leftover electrons and does not have a fully filled configuration in this compound. The two chlorine atoms achieve octet configuration, but two more are required for a covalent compound (CCl4).
NaCl or sodium chloride is an ionic compound, where sodium donates an electron, which is accepted by chlorine (to complete its octet).
Thus, among the given options, only HCl is a covalent compound which satisfies the octet rule. In all other cases, except for NaCl, chlorine atoms achieve the octet configuration through covalent bonding.
Hope this helps.
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