The question is incomplete, the complete question is;
“Consider the unbalanced chemical equation. Al(s) + Cl2(g) ------> AlCl3(s) A student tries to balance the equation by changing the subscript 2 on Cl to a 3. Explain why this is not correct. What is the correct balanced equation?”
Answer:
2Al(s) + 3Cl2(g) ------> 2AlCl3(s)
Step-by-step explanation:
If the student changes the subscript of Cl from 2 to 3, he/she has written a symbol for a molecule that does not exist. Halogens exist as diatomic and not triatomic gases.
The balanced equation is; 2Al(s) + 3Cl2(g) ------> 2AlCl3(s).
Notice that the basic principles of valency are not violated in any of the species in the equation above. Cl3 violates the octet rule but Cl2 obeys the octet rule.