Question

__________ is the smallest unit of matter which may or may not have an independent existence, but always takes part in a chemical reaction.
(A) An atom
(B) A compound
(C) A crystal
(D) All of the above

Answer 1

Hint: Of the given options, consider if the species in each option can be broken down into smaller species. If it can be broken down, then it is not the smallest unit of matter.

Complete answer:
> A compound is not the smallest unit of matter. A compound can be broken down into smaller atoms. For example, a water molecule can be broken down into one oxygen atom and two hydrogen atoms.
> A crystal is not a smallest unit of matter. A crystal is an arrangement containing several atoms, molecules or ions.
> Noble gases are monatomic in nature. Thus, a molecule of a noble gas contains one atom. So, atoms may have an independent existence in helium, neon, argon, krypton and xenon. On the other hand, certain elements such as hydrogen, oxygen and halogens (chlorine, fluorine, bromine and iodine) are diatomic in nature. One molecule of these elements contain two atoms. There are examples of triatomic molecules such as ozone. This suggests that atoms may not have independent existence as they are present in combined states in the form of molecules.
> When different atoms combine, they form molecules. Thus, atoms take part in a chemical reaction. Thus, one carbon atom combines with one oxygen atom to form carbon monoxide molecules. Similarly, one nitrogen atom combines with three hydrogen atoms to form ammonia molecules.
> An atom is the smallest unit of matter which may or may not have an independent existence, but always takes part in a chemical reaction.

Hence, the correct option is the option (A).

Note: Even-though an atom contains electrons, protons and neutrons, it cannot be divided into smaller units. You can say that an atom is indivisible.