Which of the following is not a property of hydrogen? (a) It is a colourless, odourless gas. (b) It is highly combustible. (c) It is highly poisonous gas. (d) It is lighter than air.

The periodic table is a systematic arrangement of elements based on their chemical properties, electron configurations, and atomic number. Hydrogen is positioned at the top of the periodic table, indicating it is the lightest and simplest of all elements with an atomic number of 1. It's the only element in Group 1 that is not a metal; instead, it exhibits some qualities similar to halogens and some to alkali metals.

Understanding where hydrogen is placed in the periodic table helps students recognize its unique positioning and predict its behavior in chemical reactions. Although hydrogen's placement can be a topic of debate among scientists, its location provides a starting point for studying elemental properties and their periodic trends.

The chemical properties of an element describe how it interacts with other substances. Hydrogen, identified by its symbol H, has a valence of 1, meaning it can bond with many elements to form compounds. One of hydrogen's most notable chemical properties is its ability to react with oxygen to form water (H₂O), a reaction that releases substantial energy.

Hydrogen also forms a wide range of compounds, including acids, bases, and hydrogenated organic compounds. Being highly reactive, especially at elevated temperatures or when a catalyst is present, understanding hydrogen's chemical properties is crucial for grasping its role in chemical reactions, energy production, and the environment.

Reactivity is a measure of how readily an element will undergo chemical reactions. Hydrogen is known for being highly reactive. This is particularly evident when it combusts in the presence of oxygen to form water, a reaction that also forms the basis for potential energy storage in fuel cells.

H₂ is the molecular form which hydrogen typically exists in nature, and because of its low molecular weight, it requires careful handling as it can easily escape containment and combust. Hydrogen's reactivity also makes it a reducing agent in many chemical processes and a key player in the synthesis of ammonia through the Haber process, crucial for producing fertilizers. Students must exercise caution and understand the conditions under which hydrogen is reactive to safely conduct experiments involving this element.

Hydrogen is known for its low density when in gas form. At standard conditions of temperature and pressure, its density is approximately 0.08988 g/L, making it less dense than air. This physical property explains why hydrogen, when not contained, will rise rapidly in the atmosphere.

The low density of hydrogen has practical applications in industries such as buoyant craft where lifting power is required. It's also why hydrogen is a sought-after alternative fuel: despite its energy density by mass being high, its energy density by volume is low compared to traditional fuels. For students, grasping the concept of hydrogen's density is crucial when considering its storage and transport, which are challenges for its use as a sustainable energy carrier.