Chemistry: Atomic Structure, Bonding, and Periodicity - Atomic Number, Mass Number, Isotopes, and Electronic Configuration

The atom consists of three primary subatomic particles: protons ($p^+$), neutrons ($n^0$), and electrons ($e^-$). Protons and neutrons are located in the central nucleus, while electrons orbit in shells.

The Atomic Number ($Z$) represents the number of protons in an atom. Since atoms are electrically neutral, the number of protons equals the number of electrons.

The Mass Number ($A$) is the total number of protons and neutrons in the nucleus of an atom.

Isotopes are atoms of the same element that have the same number of protons (Atomic Number) but different numbers of neutrons (Mass Number). For example, $^{12}_{6}C$ and $^{14}_{6}C$ are isotopes of Carbon.

Electronic Configuration describes the distribution of electrons in specific energy levels or shells. For the first 20 elements, the shells follow a maximum capacity of $2, 8, 8, 2$.

The Valence Shell is the outermost electron shell. The number of electrons in this shell determines the chemical properties and reactivity of the element.

Standard Atomic Notation is written as $^{A}_{Z}X$, where $X$ is the chemical symbol, $A$ is the mass number, and $Z$ is the atomic number.