`=>` The elements in which the last electron enters the s-orbital of their outermost energy level are called s- block elements and hence these elements are called representative elements.

`=>` The s-block elements are present on the extreme left in the periodic table.

`=>` It consists of elements of group IA and IIA.

`=>` Elements of IA group are called alkali metals while that of IIA group are called alkaline earth metals.

`=>` They all are metals, malleable, ductile and good conductors of heat and electricity.

`=>` They show low ionisation potential.

`=>` They are highly electropositive. They are powerful reducing agent, hence cannot be prepared by reduction. These are prepared by electrolysis of their salts in fused or molten state.

`=>` They are soft metals (due to weak metallic bond) and have low melting and boiling points.

`=>` Due to high reactivity, alkali metals are stored under liquid paraffins.

`=>` Except `Li` and `Be` they form ionic compounds (because of small size of `Li` and `Be` form covalent compounds according to Fajans rule).

`=>` They do not show variable oxidation state.

`=>` The ionic nature of compounds increase from top to bottom.

`=>` Elements of IA form monovalent cation while that of IIA form divalent cation. By the loss of electrons from their outermost shell.

`=>` They form diamagnetic (no unpaired electrons) and colourless salts except dichromates and permanganates which are coloured.

`=>` They form hydrides with hydrogen.

`=>` Of all these elements, only hydrogen is a non-metallic gas.

The elements in which the last electron enters the p-orbital of the outermost energy levels are called p-block elements. These elements are present in the right portion of periodic table. The elements of group 13 to 18 are in p-block.

`=>` p-block elements include metals, non-metals as well as metalloids.

`=>` These elements have smaller atomic radii than s-block elements. The atomic radius decreases from left to right in a period due to increase in nuclear charge.

`=>` They have high value of ionisation energy.

`=>` They have high electronegativities. It is due to their small atomic size, p-block elements possess higher electron affinities than s-block elements.

`=>` They usually form covalent compounds.

`=>` Halogens, oxygen, sulphur and phosphorus are reactive elements of p-block elements.

`=>` Some of these elements show variable valency and exist in more than one oxidation state in their compounds.

`=>` Few elements, viz oxygen, sulphur, phosphorus etc. exhibit allotropy.

`=>` Their oxides are acidic in nature.

The middle block of periodic table (groups 3 to 12) contains transition elements. Their two outermost shells are incomplete. Since these elements represent a transition (change) from the most electropositive element to the most electronegative element, they are named as transition elements. The d-block comprises of three series which are

(i) First transition series scandium ( Z = 21) to zinc (Z=30).

(ii) Second transition series yttrium (Z = 39) to cadmium (Z = 48).

(iii) Third transition series lanthanum (Z =57); hafnium (Z = 72) to mercury (Z = 80).

`=>` They are metals having high melting and boiling points (strong metallic bond).

`=>` Almost all of them show variable valence and exist in several oxidation state in their compounds.

`=>` They form both ionic and covalent compounds.

`=>` They are good conductor of heat and electricity due to free and mobile electrons. Silver is the best conductor of heat and lead is the poorest.

`=>` They form complex compounds.

`=>` Transition elements and their compounds act as catalysts.

`=>` Density of d-block elements are very high as compared to s-block elements.

`=>` Properties of transition elements on moving across a period from left to right do not change gradually as those of s- and p- block elements because the last two orbits are incomplete.

`=>` Most of these ions contains unpaired electrons hence they are paramagnetic and coloured (due to d-d transition).

`=>` Zinc, cadmium, mercury having `d^(10)` configuration do not form coloured salts.

`=>` The elements in which the last electron enter the f-orbitals of their atom are called f-block elements (penultimate orbit).

`=>` The first series called lanthanides consists of elements having atomic number 58 to 71 (Ce to Lu). They all are placed along with the element, lanthanum (La) in the same position (group 3, period 6) because of very close resemblance between them. This is also known as 4f inner-transition series.

`=>` The second series of 14 rare-earth elements is called actinoids. It consists of elements 90 to 103 (Th to Lr) and they are all placed along with the element, actinium (Ac) in the same position (group 3, period 7). This is also known as 5f inner-transition series.

`=>` All f-block elements are metals and are highly reactive.
`=>` These are highly electropositive metals due to low ionisation energies.
`=>` These have high density, high melting and boiling points.
`=>` They show variable valency. Their ions are coloured and paramagnetic in nature.
`=>` They are radioactive.
`=>` They form complex compounds.
`=>` They generally form ionic compounds.

In a period as well as in a group there is a regular gradation (gradual increase or decrease in a particular property) in physical and chemical properties of elements with the change in atomic number. This regular gradation in properties is called periodicity. The reason of periodicity in properties is the repetition of similar configuration at regular intervals.

`=>` Half of the distance between the centers of two atoms of that element that are just touching each other.

`=>` In case of covalent bond, the radius is covalent radius; in ionic bond the radius is ionic radius and in absence of bond the radius is van der Waal's radius. In general, van der Waal's radius > covalent radius

`=>` Atomic radii of elements increases on moving down the group due to increase in number of shells by a factor of one which reduces effective nuclear charge.

`=>` Atomic radii of elements decreases on moving left to right in a period due to increase in effective nuclear charge as the electrons enter in the same shell throughout the period.

`=>` Radius of cation is always smaller than its neutral atom because of increase in effective nuclear charge per electron e.g. `Sn > Sn^(2+) > Sn^(4+) `.

`=>` Radius of an anion is always larger than its neutral atom because of decrease in effective nuclear charge per electron. e.g. `O^(2-) > O^- > O`

`=>` Isoelectronic species : These species have same number of electrons. In case of isoelectronic species, the ionic radii decreases with increase in atomic number. e.g. Ion `[underset(8)O^2 > underset (9) F^- > underset(11) Na^+ > underset(12) Mg^(2+) ]`

Energy required to completely remove an electron from a gaseous atom or ion. IP measures tendency of cation formation.

`=>` IP increases from left to right due to increase in effective nuclear charge while it decreases from top to bottom.

`=>` `IP prop 1/text(size of atom)`

`=>` `IP_1` of group 2 elements is greater than the corresponding elements of group 13. e.g. `IP_1` of `Mg > IP ` of `Al`

`=>` It is due to the stable configuration of group 2 elements `( ns^2).` Similarly `IP_1` of group 15 elements is greater than the corresponding elements of group 16. e. g. `IP_1` of `N > IP_1` of `O`.

`=>` Zero group elements on account of the stable configuration exhibit exceptionally high value of IP (highest in its period).

`=>` Within the same orbit, IP decrease in order `s > p > d > f`

Energy change that occurs when an electron is added to a gaseous atom. It measures the tendency of anion formation.

`=>` EA increases in a period from left to right and decreases in a group from top to bottom.

`=>` EA of zero group elements is extremely low.

`=>` Fluorine has less EA than chlorine because of its small size `F` has more electron density and hence greater electron electron repulsion.

`=>` Order of EA of halogens `Cl > F > Br > I.`

`=>` Measure of the attraction of an atom for the electrons in a chemical bond. `=>` Decreasing order of electronegativity is `underset(4)F > underset(3.5)O > underset(3)(Cl) = underset(3)N > underset(2.8) (Br) > underset(2.5) C apropx underset(2.5)I > underset(2.1)H`. ` \ \ \ \ \ \ \ \ \ \ \ text(Trends in Periodic Properties)`

Periodic property	In a period from left to right	In a group from top to bottom
Ionisation energy	Increase	Decrease
Electron affinity	Increase	Decrease
Electronegativity	Increase	Decrease
Non-metallic character	Increase	Decrease
Oxidising character	Increase	Decrease
Acidic nature of oxides	Increase	Decrease
Atomic size	Decrease	Increase
Electropositivity	Decrease	Increase
Metallic character	Decrease	Increase
Basic nature of oxides	Decrease	Increase