Atoms, Elements and Compounds - Atomic structure and the Periodic Table

Atoms consist of a central nucleus containing protons ($p^+$) and neutrons ($n^0$), surrounded by electrons ($e^-$) orbiting in shells or energy levels.

The Atomic Number ($Z$) is the number of protons in the nucleus. The Nucleon Number ($A$), or mass number, is the total number of protons and neutrons.

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons (e.g., $^{35}_{17}Cl$ and $^{37}_{17}Cl$). They have identical chemical properties but different physical properties.

Electronic configuration for the first 20 elements follows the pattern $2, 8, 8, 2$. The number of outer-shell electrons determines the chemical properties and the Group number in the Periodic Table.

The Periodic Table is arranged by increasing atomic number ($Z$). Periods (rows) indicate the number of shells, and Groups (columns) indicate the number of valence electrons.

Group I (Alkali Metals) become more reactive down the group as the outer electron is further from the nucleus and more easily lost ($M \rightarrow M^+ + e^-$).

Group VII (Halogens) become less reactive down the group as the attraction for an incoming electron decreases ($X + e^- \rightarrow X^-$).

Group VIII (Noble Gases) have a full outer shell, making them chemically inert (unreactive) and monatomic.