At ordinary room temperature, which of the following is a metal that exists as a liquid rather than as a solid?

Introduction / Context:
Most metals are solids at room temperature, but there are a few well known exceptions that are either liquids or have very low melting points. Recognising which elements are metals and which are nonmetals, and knowing their physical states at room conditions, is basic general knowledge in chemistry. This question asks you to choose the element that is both a metal and a liquid at ordinary room temperature.

Given Data / Assumptions:

• Room temperature is taken to be around 25 °C.
• The options are bromine, chlorine, mercury, and cesium.
• We must identify which of these is a metal and is liquid at room temperature.
• We assume standard atmospheric pressure.

Concept / Approach:
Mercury is widely known as the only metal that is liquid at room temperature under normal pressure. It is used in thermometers, barometers, and some electrical switches. Bromine is also a liquid at room temperature, but bromine is a nonmetal halogen, not a metal. Chlorine is a diatomic gas at room temperature, and cesium is a very soft alkali metal that melts at a relatively low temperature but still remains solid at typical room conditions unless the temperature is somewhat higher. Therefore, the only element in the list that is both metallic and liquid at standard room temperature is mercury.

Step-by-Step Solution:
Step 1: Identify bromine as a halogen (nonmetal) that is a reddish brown liquid at room temperature. Step 2: Identify chlorine as a greenish yellow gas at room temperature and also a nonmetal. Step 3: Recall that mercury is a metal with a melting point of about -39 °C, which means it is liquid well below and above room temperature. Step 4: Recognise that cesium is an alkali metal with a low melting point (around 28 °C), but at standard room temperature it is usually still a soft solid, not a definite liquid. Step 5: Conclude that the only element that meets both conditions, being a metal and being liquid at ordinary room temperature, is mercury.

Verification / Alternative check:
Periodic table references and basic chemistry texts highlight mercury as the only liquid metal at room temperature and bromine as the only liquid nonmetal at room temperature. Cesium and gallium melt slightly above typical room temperature, which is why they are often demonstrated in laboratories by melting in the hand or in warm water. Chlorine is always described as a gas under normal conditions. These widely known facts support the conclusion that mercury is the only metal in the given list that is liquid at room temperature.

Why Other Options Are Wrong:
Bromine, although liquid at room temperature, is a halogen nonmetal and therefore does not satisfy the requirement of being a metal. Chlorine is a gas at room temperature and is also a halogen nonmetal, not a metal. Cesium is indeed a metal and has a very low melting point, but under typical room conditions it is still a soft solid rather than a liquid, so it does not match the description in the question. These elements are either in the wrong physical state or of the wrong type (nonmetal rather than metal), making them incorrect choices.

Common Pitfalls:
Students may be tempted by bromine simply because they remember it is a liquid at room temperature but may forget that it is not a metal. Others might choose cesium because it is famous for having a low melting point and being very soft, without carefully considering that it still needs to be above its melting point to be liquid. To avoid these mistakes, always check both conditions: type of element (metal or nonmetal) and physical state at the given temperature. Mercury is the classic example that fits both conditions simultaneously.

Final Answer:
The metal that exists as a liquid at ordinary room temperature is mercury.