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

Atoms consist of a central nucleus containing protons (charge $+1$, relative mass $1$) and neutrons (charge $0$, relative mass $1$), surrounded by electrons (charge $-1$, relative mass $\frac{1}{1840}$) orbiting in shells.

The Atomic Number ($Z$) represents the number of protons in the nucleus. The Nucleon Number (Mass Number, $A$) represents the total number of protons and neutrons: $^A_Z X$.

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. They have the same chemical properties because they have the same number of electrons in their outer shell.

Electronic configuration describes the arrangement of electrons in shells. For the first 20 elements, shells fill in the pattern $2, 8, 8, 2$. The number of valence (outer) electrons determines the chemical properties and the Group number.

The Periodic Table is arranged by increasing atomic number. Rows are called Periods (representing the number of electron shells) and columns are called Groups (representing the number of valence electrons).

Group I (Alkali Metals) reactivity increases down the group as the outer electron is further from the nucleus and more easily lost. Group VII (Halogens) reactivity decreases down the group as it becomes harder to attract an electron into the outer shell.