Correct Option
The correct option is 1 and 3 only.
Explanation
Matter is classified into elements, compounds, and mixtures based on chemical composition. An element is a pure substance consisting of only one type of atom, whereas a compound is formed when two or more elements combine chemically in a fixed ratio.
Statement-wise Analysis
- Statement 1 is Correct: While many elements exist as single atoms (monoatomic), several non-metallic elements exist as diatomic molecules under ordinary conditions to achieve stability. Examples include Hydrogen ($H_2$), Nitrogen ($N_2$), and Oxygen ($O_2$).
- Statement 2 is Incorrect: When elements combine to form a compound, they lose their individual properties, and the resulting compound acquires entirely new chemical and physical properties. For example, Sodium ($Na$) is a reactive metal and Chlorine ($Cl$) is a toxic gas, but they combine to form Sodium Chloride ($NaCl$), which is common table salt. Similarly, water ($H_2O$) extinguishes fire, whereas its constituents, Hydrogen and Oxygen, are combustible and supporters of combustion, respectively.
- Statement 3 is Correct: By definition, an element is a pure substance composed of identical atoms that have the same atomic number (number of protons). It cannot be broken down into simpler substances by ordinary chemical means.
Key Takeaway
Key Takeaway: Elements consist of a single type of atom and may exist as monoatomic or polyatomic molecules (e.g., $O_2$, $P_4$). Compounds are chemically bonded combinations of different elements with properties distinct from their constituents.