Atomic Structure - Periodic Table till the first 20 elements

An atom is the smallest unit of matter. It consists of a central nucleus containing protons and neutrons, with electrons revolving around it in specific orbits called shells.

The three subatomic particles are: Protons ($p^{+}$) which are positively charged, Neutrons ($n^{0}$) which are neutral, and Electrons ($e^{-}$) which are negatively charged.

Atomic Number ($Z$) represents the number of protons in the nucleus of an atom. In a neutral atom, $Z$ also equals the number of electrons.

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

Electronic Configuration is the distribution of electrons in different shells ($K, L, M, N$). The maximum number of electrons that can be accommodated in a shell is given by $2n^2$, where $n$ is the orbit number.

The first 20 elements of the periodic table range from Hydrogen ($Z=1$) to Calcium ($Z=20$).

Valency is the combining capacity of an atom. It is determined by the number of electrons in the outermost shell (valence electrons). For elements with $1, 2, 3, 4$ valence electrons, the valency is usually equal to the number of valence electrons. For $5, 6, 7$ valence electrons, valency is calculated as $8 - \text{valence electrons}$.