Final answer:
A single atom can indeed exist by itself, and when atoms of the same type bond together, they form what is known as a pure substance. When two or more different atoms bond, they form a compound, which is characterized by a fixed ratio of the atoms involved.
Step-by-step explanation:
Yes, one atom can exist by itself, and this is often seen in noble gases like helium and neon which naturally exist as individual atoms. When atoms of the same kind group together, it is called an element, or more specifically, a pure substance. Conversely, when two or more different atoms bond together, the result is called a compound. These compounds are formed in fixed simple whole number ratios. For instance, water (H2O) consists of two hydrogen atoms for every one oxygen atom.
Atoms of the same element can vary in mass, such as the isotopes of chlorine, which affect their properties. Additionally, atoms are not indivisible; they can be broken down into subatomic particles, namely protons, neutrons, and electrons. However, in a chemical reaction context, the atom is the smallest unit that participates.
All matter consists of atoms, and it's the way these atoms bond together that forms the diverse array of substances we observe in the world. Atoms forming covalent bonds, like the one nitrogen atom bonding with three hydrogen atoms to form ammonia (NH3), exemplify the chemical diversity possible through the combination of these fundamental particles.