Element Definition

An element is a pure chemical substance consisting of a single type of atom distinguished by its atomic number (number of protons). Elements cannot be broken down into simpler substances by chemical means.

Element Option Analysis

We need to identify which of the given options is a pure element:

• Air: This is a mixture of gases, primarily nitrogen and oxygen, along with other trace gases. It is not a single element.
• Sodium chloride: Commonly known as table salt (chemical formula NaCl), this is a compound made of sodium (Na) and chlorine (Cl) atoms chemically bonded together. It is not an element.
• Oxygen: This is a pure substance (chemical symbol O) composed only of oxygen atoms. It is listed on the periodic table and is a fundamental element.
• Water: This is a compound with the chemical formula H₂O, made of hydrogen (H) and oxygen (O) atoms chemically bonded. It is not an element.

Conclusion

Based on the analysis, Oxygen is the only substance listed that fits the definition of an element.