Final answer:
Bromine does not exist as a diatomic gas unlike nitrogen, oxygen, and chlorine; it is a diatomic liquid at room temperature due to stronger intermolecular London forces.
Step-by-step explanation:
The element that does not exist as a diatomic gas among nitrogen, oxygen, chlorine, and bromine is bromine. While nitrogen (N₂), oxygen (O₂), and chlorine (Cl₂) are all diatomic gases at room temperature, bromine (Br₂) is unique because it exists as a diatomic liquid under normal conditions.
All of the halogens, which include fluorine (F₂), chlorine (Cl₂), bromine (Br₂), and iodine (I₂), are known to form diatomic molecules. However, their states of matter at room temperature vary; as one moves down the group in the periodic table, the intermolecular London forces increase, leading to a higher likelihood of these elements being found in liquid or solid form under standard conditions. Bromine, being a liquid at room temperature, therefore does not qualify as a diatomic gas.