The key difference between an atom and a monoatomic ion lies in their electrical charge. Here's a breakdown of each:
Atom: An atom is the fundamental unit of matter, consisting of a nucleus at the center and one or more electrons orbiting around it. Atoms are electrically neutral because they have an equal number of protons (positively charged) in the nucleus and electrons (negatively charged) outside the nucleus. The number of protons determines the atomic number of an element and defines its unique identity. For example, all hydrogen atoms have one proton, all carbon atoms have six protons, and so on.
Monoatomic Ion: A monoatomic ion is an atom that has gained or lost electrons, resulting in a net positive or negative charge. When an atom loses one or more electrons, it becomes a positively charged ion called a cation. Cations are formed by elements on the left side of the periodic table, as they have fewer electrons in their outermost shells and tend to lose electrons to achieve a stable electron configuration. For example, when a sodium (Na) atom loses one electron, it forms a sodium cation (Na+).
Conversely, when an atom gains one or more electrons, it becomes a negatively charged ion called an anion. Anions are formed by elements on the right side of the periodic table because they have more electrons in their outermost shells and have a tendency to gain electrons to attain a stable electron configuration. For instance, when a chlorine (Cl) atom gains one electron, it forms a chloride anion (Cl-).
In summary, atoms are electrically neutral, while monoatomic ions carry a positive or negative charge due to the gain or loss of electrons. The electrical charge of a monoatomic ion is denoted by a superscript next to its chemical symbol, indicating the number of charges it carries.