Chemistry - Atomic Structure (Protons, Neutrons, Electrons, Valency, Isotopes)

An atom is the smallest unit of matter, consisting of a central nucleus and surrounding shells.

Protons ($p^+$) are positively charged particles located in the nucleus with a mass of $1 \text{ amu}$.

Neutrons ($n^0$) are neutral particles found in the nucleus with a mass of approximately $1 \text{ amu}$.

Electrons ($e^-$) are negatively charged particles that revolve around the nucleus in fixed orbits. Their mass is negligible ($\approx 1/1837 \text{ amu}$).

Atomic Number ($Z$) is defined as the number of protons in the nucleus of an atom. In a neutral atom, $Z = \text{number of protons} = \text{number of electrons}$.

Mass Number ($A$) is the sum of the number of protons and neutrons in the nucleus: $A = p + n$.

Electronic Configuration describes the distribution of electrons in different shells ($K, L, M, N...$). The maximum number of electrons in a shell is governed by the $2n^2$ rule.

Valency is the combining capacity of an atom. It is determined by the number of electrons in the outermost shell (valence electrons).

Isotopes are atoms of the same element that have the same Atomic Number ($Z$) but different Mass Numbers ($A$). They have the same chemical properties but different physical properties.