Atomic Structure - Atoms and Molecules

Atoms are the basic building blocks of matter and are the smallest unit of an element that can take part in a chemical reaction.

A molecule is the smallest unit of an element or a compound that can exist independently and shows all the properties of that substance (e.g., $O_2$, $H_2O$).

Dalton's Atomic Theory states that atoms are indivisible (later disproven) and that atoms of the same element are identical in mass and properties.

An atom consists of three subatomic particles: protons ($p^+$), neutrons ($n^0$), and electrons ($e^-$).

The nucleus is the central part of the atom containing protons and neutrons, while electrons revolve around the nucleus in fixed paths called orbits or shells.

Atomic Number ($Z$) is the number of protons present in the nucleus of an atom. In a neutral atom, number of protons = number of electrons.

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

The maximum number of electrons that can be accommodated in a shell is given by the formula $2n^2$, where $n$ is the shell number.

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

Radicals are atoms or groups of atoms that carry a charge, such as the Ammonium ion ($NH_4^+$) or Sulfate ion ($SO_4^{2-}$).