`=>` The aufbau (build up) principle and the electronic configuration of atoms provide a theoretical foundation for the periodic classification.

`=>` We can classify the elements into four blocks viz., `s`-block, `p`-block, `d`-block and `f`-block depending on the type of atomic orbitals that are being filled with electrons.

`=>` This is illustrated in Fig. 3.3.

`=>` We notice two exceptions to this categorisation.

● Strictly, helium belongs to the `s`-block but its positioning in the `p`-block along with other group `18` elements is justified because it has a completely filled valence shell `(1s^2)` and as a result, exhibits properties characteristic of other noble gases.

● The other exception is hydrogen. It has a lone `s`-electron and hence can be placed in group 1 (alkali metals). It can also gain an electron to achieve a noble gas arrangement and hence it can behave similar to a group 17 (halogen family) elements.

● Because it is a special case, we shall place hydrogen separately at the top of the Periodic Table as shown in Fig. 3.2 and Fig. 3.3.

`=>` 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.

`=>` They are all reactive metals with low ionization enthalpies.

● They lose the outermost electron(s) readily to form `1+` ion (in the case of alkali metals) or `2+` ion (in the case of alkaline earth metals).

`=>` The metallic character and the reactivity increase as we go down the group.

`=>` Because of high reactivity they are never found pure in nature.

`=>` The compounds of the `s`-block elements, with the exception of those of lithium and beryllium are predominantly ionic.

`=>` 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 outermost electronic configuration varies from `ns^2 np^1` to `ns^2 np^6` in each period.

`=>` At the end of each period is a noble gas element with a closed valence shell `ns^2 np^6` configuration.

● All the orbitals in the valence shell of the noble gases are completely filled by electrons and it is very difficult to alter this stable arrangement by the addition or removal of electrons.

● The noble gases thus exhibit very low chemical reactivity.

`=>` Preceding the noble gas family are two chemically important groups of non-metals.

● They are the halogens (Group 17) and the chalcogens (Group 16).

● These two groups of elements have high negative electron gain enthalpies and readily add one or two electrons respectively to attain the stable noble gas configuration.

`=>` The non-metallic character increases as we move from left to right across a period and metallic character increases as we go down the group.

`=>` 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)`.

`=>` These elements have the general outer electronic configuration `(n-1)d^(1-10) ns^(0-2)`.

`=>` They are all metals.

`=>` They mostly form coloured ions, exhibit variable valence (oxidation states), paramagnetism and oftenly used as catalysts.

`=>` However, `Zn`, `Cd` and `Hg` which have the electronic configuration, `(n-1) d^(10)ns^2` do not show most of the properties of transition elements.

`=>` In a way, transition metals form a bridge between the chemically active metals of `s`-block elements and the less active elements of Groups 13 and 14 and thus take their familiar name “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`.

● The last electron added to each element is filled in `f`-orbital.

● These two series of elements are hence called the `text(Inner- Transition Elements)` (`f`-Block Elements).

`=>` They are all metals.

`=>` Within each series, the properties of the elements are quite similar.

`=>` The chemistry of the early actinoids is more complicated than the corresponding lanthanoids, due to the large number of oxidation states possible for these actinoid elements.

`=>` Actinoid elements are radioactive.

`=>` Many of the actinoid elements have been made only in nanogram quantities or even less by nuclear reactions and their chemistry is not fully studied.

`=>` The elements after uranium are called Transuranium Elements.

In addition to displaying the classification of elements into s-, p-, d-, and f-blocks, Fig. 3.3 shows another broad classification of elements based on their properties.

`=>` The elements can be divided into Metals and Non-Metals.

`=>` Metals comprise more than `78%` of all known elements and appear on the left side of the Periodic Table.

● Metals are usually solids at room temperature [mercury is an exception; gallium and caesium also have very low melting points (`303K` and `302K`, respectively)].

● Metals usually have high melting and boiling points.

● They are good conductors of heat and electricity.

● They are malleable (can be flattened into thin sheets by hammering) and ductile (can be drawn into wires).

`=>` Non-metals are located at the top right hand side of the Periodic Table.

● In fact, in a horizontal row, the property of elements change from metallic on the left to non-metallic on the right.

● Non-metals are usually solids or gases at room temperature with low melting and boiling points (boron and carbon are exceptions).

● They are poor conductors of heat and electricity.

● Most nonmetallic solids are brittle and are neither malleable nor ductile.

● The elements become more metallic as we go down a group; the nonmetallic character increases as one goes from left to right across the Periodic Table.

● The change from metallic to non-metallic character is not abrupt as shown by the thick zig-zag line in Fig. 3.3.

● The elements (e.g., silicon, germanium, arsenic, antimony and tellurium) bordering this line and running diagonally across the Periodic Table show properties that are characteristic of both metals and nonmetals. These elements are called Semi-metals or Metalloids.