Which of the following substances is a pure chemical element rather than a mixture or compound: glass, cement or sodium?

Introduction / Context:
This chemistry question checks your basic understanding of what is meant by a pure element, a compound and a mixture. Distinguishing between these types of substances is fundamental for topics such as chemical reactions, stoichiometry and material science.

Given Data / Assumptions:

• Options include glass, cement, sodium, none of these and concrete.
• We are asked to identify which option represents a pure chemical element.
• We assume normal everyday meanings of these materials as used in school and competitive exam chemistry.
• Pure element means a substance made up of only one kind of atom.

Concept / Approach:
A chemical element is a substance that cannot be broken down into simpler substances by ordinary chemical processes and whose atoms all have the same number of protons. Compounds are formed when atoms of different elements chemically combine in fixed ratios. Mixtures consist of two or more substances physically combined, not necessarily in fixed proportions. Therefore, we identify whether each option is an element, a mixture or a compound, and then choose the pure element from the list.

Step-by-Step Solution:
1) Glass is usually a mixture of silica (silicon dioxide) and other metal oxides; it is not a single pure compound or element.2) Cement is a complex mixture of compounds, mainly calcium silicates and aluminates, and again is not a pure element.3) Sodium is a metallic chemical element with symbol Na and atomic number 11 in the periodic table.4) Concrete is a mixture of cement, sand, gravel and water, so it is clearly not a pure element.5) Since sodium consists of only one type of atom, it is the only pure element among the given choices.

Verification / Alternative check:
We can check with periodic table knowledge. Sodium appears as an alkali metal in Group 1 and is a basic building block for many compounds such as sodium chloride. Glass, cement and concrete are not present as single entries in the periodic table because they are not pure elements. This confirms that sodium is the correct answer and satisfies the definition of a pure chemical element.

Why Other Options Are Wrong:
Glass: It is an amorphous mixture, often containing silica, lime and other oxides, so it does not qualify as a pure element.
Cement: It is a complex mixture of many compounds such as calcium silicates, not a single pure substance or element.
None of these: This would be correct only if no option were a pure element, but sodium clearly is one, so this choice is incorrect.
Concrete: It is made by mixing cement, sand and aggregates with water and is a classic example of a heterogeneous mixture, not an element.

Common Pitfalls:
A common mistake is to confuse common construction materials like glass or cement with pure substances because they may appear uniform to the naked eye. However, uniform appearance does not guarantee chemical purity. Another trap is to choose none of these when unsure, instead of recalling that metals like sodium are listed individually in the periodic table as elements. Remember that any familiar metal such as sodium, copper or iron in its pure form is an element, whereas everyday building materials are usually mixtures or complex compounds.

Final Answer:
The pure chemical element among the given options is sodium.