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•Dobereiner's Law of Triads :`=>` By `1829` Dobereiner noted a similarity among the physical and chemical properties of several groups of three elements (Triads).

`=>` In each case, he noticed that the middle element of each of the Triads had an atomic weight about half way between the atomic weights of the other two.
•Chancourtois's Classification of Elements : Chancourtois arranged the then known elements in order of increasing atomic weights and made a cylindrical table of elements to display the periodic recurrence of properties.
•Newlands's Law of Octaves :Newlands arranged the elements in increasing order of their atomic weights and noted that every eighth element had properties similar to the first element.
•Mandeleev's Periodic Table :The properties of the elements are a periodic function of their atomic weights.
•Modern Periodic Law: The physical and chemical properties of the elements are periodic functions of their atomic numbers.
•Electronic Configuration: The distribution of electrons into orbitals of an atom is called its electronic configuration.
•Groups:These are the vertical columns in the periodic table.
•Periods:The horizontal rows in the periodic table.
•Valence electrons:The valence electrons are the electrons in the outer shell of an atom. The valence electrons are involved in forming bonds to adjacent atoms.
•The s-Block Elements :The elements of Group 1 (alkali metals) and Group 2 (alkaline earth metals) which have `ns^1` and `ns^2` outermost electronic configuration belong to the `s`-Block Elements.
•The p-Block Elements :The `p`-Block Elements comprise those belonging to Group 13 to 18 and these together with the `s`-Block Elements are called the `text(Representative Elements)` or `text(Main Group Elements)`.
•The d-Block Elements (Transition Elements) :`=>` These are the elements of Group 3 to 12 in the centre of the Periodic Table.

`=>` These are characterised by the filling of inner `d` orbitals by electrons and are therefore referred to as `text(d-Block Elements)`.
•The f-Block Elements (Inner-Transition Elements) : The two rows of elements at the bottom of the Periodic Table, called the `text(Lanthanoids)`, `Ce(Z = 58) – Lu(Z = 71)` and `text(Actinoids)`, `Th(Z = 90) – Lr (Z = 103)` are characterised by the outer electronic configuration `(n-2)f^(1-14) (n-1)d^(0–1)ns^2`.

•Atomic radius:The distance from the centre of nucleus to the point upto which the density of electron cloud is maximum.
•Covalent radius:It is defined as one-half the distance between the nuclei of two covalently bonded atoms of the same element in a molecule.
•Vander waals radius:It is defined as one-half the distance between the nuclei of two identical non-bonded isolated atoms or two adjacent identical atoms belonging to two neighbouring molecules of an element in the solid state.
•Metallic radius:It is defined as one-half the internuclear distance between the two adjacent metal ions in the metallic lattice.
•Ionic Radius :It may be defined as the effective distance from the centre of the nucleus of the ion upto which it exerts its influence on its electronic cloud.
•Ionization Enthalpy : The energy required to remove an electron from an isolated gaseous atom (`X`) in its ground state is called ionization enthalpy. In other words, the first ionization enthalpy for an element `X` is the enthalpy change (`Δ_i H`) for the reaction depicted in equation 3.1.

`X(g) → X^+ (g) + e^-`
•Electron Gain Enthalpy :`=>`When an electron is added to a neutral gaseous atom (`X`) to convert it into a negative ion, the enthalpy change accompanying the process is defined as the Electron Gain Enthalpy `(Δ_(eg)H)`.
`=>`Electron gain enthalpy provides a measure of the ease with which an atom adds an electron to form anion as represented

`X(g) + e^(-) → X^(-) (g) `
•Electronegativity : A qualitative measure of the ability of an atom in a chemical compound to attract shared electrons to itself is called electronegativity.
•Diagonal relationship:The behaviour of lithium and beryllium is more similar with the second element of the following group i.e., magnesium and aluminium, respectively. This sort of similarity is commonly referred to as diagonal relationship in the periodic properties.