In general chemistry, which of the following is a non-metal that exists as a liquid at room temperature under normal atmospheric pressure?

Introduction / Context:
Most elements are solids at room temperature, with some being gases and very few being liquids. Among the non-metals, only a very small number are liquid at ordinary room temperature and normal atmospheric pressure. This question tests whether you can correctly identify bromine as the non-metal that is a liquid under these conditions, distinguishing it from other common non-metals like chlorine, phosphorus, and helium.

Given Data / Assumptions:
- We focus on non-metallic elements and their physical state at room temperature. - Room temperature is assumed to be about 25 degrees Celsius under normal atmospheric pressure. - Options include chlorine, phosphorus, bromine, and helium. - We assume standard textbook information on physical states of elements.

Concept / Approach:
At room temperature, chlorine (Cl2) is a greenish-yellow gas, phosphorus in its common forms (such as white and red phosphorus) is a solid, and helium (He) is a colourless noble gas. Bromine (Br2), however, is a reddish-brown liquid and is one of the very few elements that exist as a liquid at room temperature, along with mercury (a metal). The approach is to recall the physical states of these elements and select the one that is a liquid non-metal under normal conditions.

Step-by-Step Solution:
Step 1: Recall that chlorine is a halogen gas at room temperature and is stored in pressurised cylinders due to its gaseous nature. Step 2: Remember that phosphorus exists in forms like white, red, and black phosphorus, all of which are solids at room temperature. Step 3: Bromine is another halogen, but unlike chlorine and fluorine, it is a volatile liquid at room temperature, giving off reddish-brown fumes. Step 4: Helium is a noble gas that remains gaseous at room temperature and is used in balloons and as a cooling medium at very low temperatures. Step 5: Among all four options, only bromine matches the description of a non-metal that is liquid at room temperature.

Verification / Alternative check:
You can verify this fact from the periodic table and standard chemistry references, which note that bromine is one of the only elements that is liquid at room temperature, the other common example being mercury, which is a metal. Bromine's melting point is just below room temperature and its boiling point is above, so it remains liquid in the typical classroom or laboratory environment. Routine safety instructions also emphasise that bromine is a corrosive liquid, reinforcing its liquid state under normal conditions.

Why Other Options Are Wrong:
Chlorine is a gas at room temperature and standard pressure, so option a is incorrect. Phosphorus, in its various allotropes, is a solid and must be heated significantly to become liquid, so option b is also wrong. Helium is a noble gas and remains a gas at room temperature, becoming liquid only at extremely low temperatures near absolute zero, so option d does not satisfy the condition of being liquid at ordinary room temperature. Only bromine satisfies both conditions of being a non-metal and liquid at room temperature.

Common Pitfalls:
A frequent mistake is to confuse bromine with chlorine or to think that all halogens must be gases at room temperature. Another pitfall is to overlook that helium, although often discussed in the context of low-temperature liquids, is not liquid at normal room conditions. To avoid errors, remember the specific list: mercury (a metal) and bromine (a non-metal) are the standard textbook examples of elements that are liquid at room temperature.

Final Answer:
The non-metal that remains liquid at room temperature is bromine, a reddish-brown liquid.