To determine which compound is an exception to the octet rule, we need to understand the concept of the octet rule. The octet rule states that atoms tend to form compounds in ways that give them eight valence electrons, achieving a noble gas electron configuration. However, there are certain compounds where this rule does not strictly apply, often because the central atom can have more than eight electrons in its valence shell.
Let's consider each of the given compounds:
1. [tex]\( H_2O \)[/tex] (Water):
- The oxygen atom in water has 6 valence electrons, and it shares electrons with two hydrogen atoms, gaining a total of 8 valence electrons in its shell (2 from hydrogen's [tex]\(\times\)[/tex] 2 hydrogen atoms).
- Therefore, [tex]\( H_2O \)[/tex] follows the octet rule.
2. [tex]\( HCl \)[/tex] (Hydrochloric acid):
- The chlorine atom in hydrochloric acid has 7 valence electrons. It shares one electron with the hydrogen atom, completing its octet.
- Therefore, [tex]\( HCl \)[/tex] follows the octet rule.
3. [tex]\( CCl_4 \)[/tex] (Carbon tetrachloride):
- The carbon atom in carbon tetrachloride has 4 valence electrons. It shares electrons with four chlorine atoms, gaining a total of 8 valence electrons in its shell.
- Therefore, [tex]\( CCl_4 \)[/tex] follows the octet rule.
4. [tex]\( ClF_3 \)[/tex] (Chlorine trifluoride):
- The chlorine atom in chlorine trifluoride has 7 valence electrons. It is bonded to three fluorine atoms (each fluorine atoms contributes one electron). If we count total electrons around chlorine, it now has 10 valence electrons (3 from bonding with fluorine plus 7 of its own).
- Therefore, [tex]\( ClF_3 \)[/tex] does not follow the octet rule. It exceeds the eight electrons limit, making it an exception to the octet rule.
Based on this detailed analysis, the compound that is an exception to the octet rule is:
[tex]\[ \boxed{ClF_3} \][/tex]
