Chemistry - Atoms, Elements and Compounds

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

Atomic number ($Z$) is the number of protons in the nucleus of an atom. The Mass number ($A$) 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, resulting in different mass numbers (e.g., $^{35}_{17}Cl$ and $^{37}_{17}Cl$).

Relative Atomic Mass ($A_r$) is the average mass of naturally occurring atoms of an element on a scale where the $^{12}C$ atom has a mass of exactly $12$ units.

Elements are substances made of only one type of atom. Compounds are substances made of two or more elements chemically combined in fixed proportions, such as $CO_2$ or $NaCl$.

Mixtures contain two or more substances (elements or compounds) that are not chemically bonded and can be separated by physical means.

Electronic configuration follows a specific pattern: the first shell holds up to $2$ electrons, the second holds up to $8$, and the third holds up to $8$ (for the first 20 elements). For example, Sodium ($Na$) is $2, 8, 1$.

Noble gases (Group VIII/0) have a full outer shell of electrons (a stable octet), making them chemically unreactive.

Ionic bonding occurs through the transfer of electrons from a metal atom to a non-metal atom, resulting in the formation of positive cations ($Mg^{2+}$) and negative anions ($O^{2-}$).