• By the end of 18th century, 23 elements were known
  • , by 1870, 65,
  • by 1925, 88,
  • today there are 109.

In 1864, John Newland, an English chemist arranged 24 elements in order of increasing atomic masses. He noticed that every eighth element, starting from any point, has similar properties. Few rows of his arrangement are shown below:

H Li Be B C N O
F Na Mg Al Si P S
Cl K Ca Cr Ti Mn Fe


In 1869, Mendeleev, a Russian chemist developed a classification scheme of elements. He recognized that if elements were placed in order of increasing atomic masses, the properties of elements repeated at regular intervals. He arranged 65 elements in periods and groups.



ANS: “ a table of elements obtained by arranging them in order of increasing atomic number in which elements having similar properties are placed in the same group is called periodic table.”

It is an arrangement of element with similar properties placed together in the same group. Thus knowing properties of one element, the properties of other elements placed in the same group can be known.




Periodic table is divided into seven horizontal rows of elements. Each row of elements is called   “PERIODS”. All the elements of a period are different from each other. Period number of an element indicates number of orbits in the atom of that elements.

  • There are seven periods in periodic table
  • 1st period has 2 elements
  • 2nd and 3rd periods both have 8 elements in each
  • 4th& 5th periods both have 18 elements in each
  • 6th& 7th periods have 32 elements and elements in these periods are termed as lanthanides and actinides.
  • First three periods are known as short periods while other known as long periods
  • From left to right properties of elements gradually change with in a period but when move to next period the pattern of properties repeats.



  • Modern periodic table is divided into eight vertical columns of elements.
  • Each vertical column of    elements is called a “GROUP”.
  • The groups of the periodic table are further divided into two sub-groups    or families “A and B”.
  1. Elements of sub-group “A” are known as Normal elements or Representative While the elements of sub-group “B” are known as Transition elements.
  • Group number of an    element is represented by roman numerals such as IA, IIA, VIA etc.
  • Total number of electrons in the valence shell represents the group number


  • Group 1A elements are called alkali metals
  • Group IIA elements are called alkaline earth metals
  • Group VIIA elements are known as halogens
  • Group VIIIA elements are known as noble gases
Example 3.1: Identifying the group and

 period of an element


Identify the group and period of   on the basis of electronic configuration.

Solution: =                                                                    Valence shells is M

As  n = 3, Al is present in the 3rd period.

Since total number of electrons in the           valence sub-shells are 2+1=3, it must be present in Group IIIA.


Valence shell is L

So  n = 2, B is present in the 2nd period. Since total number of electrons in the       valence shell are 2+1=3, it must be present in Group IIIA.


Valence shell is M

So n = 3, Mg is present in the 3rd period. Since total number of electrons in the     valence shell are 2, it must be present in Group IIA.

SELF assessment EXERCISE 3.2

Identify the group and period of the following elements on the basis of electronic configurations.


  1. Si= 1s22s22p63s23p2

Group :IV-A

Period : 3rd

  1. S= 1s22s22p63s23p4

Group :VI-A

Period : 3rd

  1. F 1s22s22p5

Group :VII-A

Period : 2nd

  1. Ar: 1s22s22p63s23p6

Group :VIII-A

Period : 3rd

s and p blocks in the periodic table

  • On the basis of valence sub shell, elements in the periodic table can also be classified into four blocks.
  • Elements of Group IA and Group IIA contain their valence electrons in s sub-shell. Therefore, these elements are called s-block
  • The elements of Group IIIA to VIIIA (except He) are known as p – block
  • Periodic table is divided into s,p,d,f blocks

Q: what is meant by periodicity of the roperties?

Ans:     Periodicity of properties:

‘’The repetition of similar properties after regular intervals in the periodic table is called periodicity of properties”

  • The physical and the chemical properties of the elements change periodically with the atomic number. There are some other properties of atoms which are periodic properties.
  • These properties are atomic radius (atomic size), ionization energy, electron affinity, electronegativity and valency. These atomic properties depend upon the location of the element in the periodic table. These properties show periodic variations.

1 – Shielding Effect:

“The reduction of force of attraction between nucleus and valence electrons by the electrons present in the inner sub-shells is called shielding effect”

Trends in periodic table:

Trend in period: when we move from left to right in period,the shielding effect does not change in period.

Reason: this is because in period from L→R , the number of inner shells remain same.



Trend in group:

The shielding effect increases in moving from top to bottom in a group.

Reason: this is because the number of inner shells increases in going from top to bottom in a group.




Atomic Size:

The size of an atom depend upon its electronic configuration

The size of an atom is the average distance between the nucleus of an atom and outer electronic shell”.

Trend in period:As you move across a period, atomic radius decreases.( L→R)

Reason:As you go across a period, electrons are added to the same energy level.  At the same time, protons are being added to the nucleus.  The concentration of more protons in the nucleus creates a “higher effective nuclear charge.”  In other words, there is a stronger force of attraction pulling the electrons closer to the nucleus resulting in a smaller atomic radius.

Trend in group: As you move down a group, atomic radius increases.

Reason:The number of energy levels increases as you move down a group as the number of electrons increases.  Each subsequent energy level is further from the nucleus than the last.  Therefore, the atomic radius increases as the group and energy levels increase.

Ionization Energy (I.E):

Ionization energy is the minimum amount of energy required to remove an electron from valence shell an isolated gaseous atom or ion.

First ionization energy: the energy required to remove the first electron from an atom is called the first ionization energy.

Unit: it is measured in KJ/mol or in electron –volt (eV).

For example: the ionization energy of sodium atom is +496 kJmol-1

Mg                               Mg+      + e–                                        EI.E= +735KJ/mol

Trend in Periodic table:

In group: ionization energy decreases in going from top to bottom in a group


  1. Nuclear charge increases and atomic size also increases due to addition of new shells at each element
  2. The number of inner electrons increases , this increase the shielding effect on outer electrons.

In period: I.E increases as we go from left to right in a period


  1. The nuclear charge increase by one unit at each element.
  2. the added electrons enter to the same shell.
  3. The shielding effect remains the same.

Due to these facts size of atoms decrease and valence electrons held strongly by the electrostatic force of nucleus


The amount of energy released when an electron adds up in the valence shell of an atom to form a uninegative gaseous ion.

X    + e                                     Xˉ

Example: Whaen an electron is added to the outermost shell of Cl, 349 KJ/mol energy is released.

Cl         + e–                                                        Clˉ

Trend in group:  the value of electron affinity decreases in going from top to bottom in a group.

Reason:  decreases due to shielding effect increase added electrons are less tightly binds to the nucleus

 Trend in a Period:  the value of electron affinity increases across a period from left to right

Reason:  In a period atomic size decreases due to increase in nuclear charge. When electron is added to the outer shell it binds tightly but shielding effect remain same. So it will release higher amount of energy.


Electronegativity is the ability of an atom to attract the electrons towards itself in a chemical bond.


Tendency of an atom to attract the shared pair of electrons towards itself is called Electronegativity.

Example: in HCl molecule the shared pair of electrons lies towards Cl atom. So chlorine is more electronegative than Hydrogen atom.

H   : Cl

Scale for Measuring Electronegativity: The most commonly used scale for E.N was devised by Pauling and Mullikan

 Pauling assigned a value of 4.0 to the Electronegativity of Flourine(F) and calculated the value of other elements.

Trend in periodic table:

In Periods: E.N value increase in period from left to right because nucleus has more strong attractions towards valence shell electrons.

In Group: E.N value decreases along group from top to bottom because increase in atomic size reduce tendency of nucleus to attract electrons.

Elements E.N Elements E.N
Li 1.00 F 4.00
Na 0.9 Cl 3.2
K 0.8 Br 2.9

Q.2:     Give short answers

  1. Write the valence shell electronic configuration of an element present in the 3rd period and Group IIIA.

Ans: 3s23p1

  1. Write two ways in which isotops of an element differ?

Ans:    i. Number of neutrons in it

  1. Physical properties of isotopes

iii. Mass number of atoms

  • Which atom has higher shielding effect, Li or Na?

Ans: Na>Li

Na has higher shielding effect due to presence of more inner electrons as compared to Li


  1. Explain why, Na has higher ionization energy than K?

Ans: i. as we move down the group I.E decreases due to increased atomic size and shielding effect. K has large atomic size as compared to Na.

  1. Alkali metals belong to S-block in the periodic table, why?

Ans: Elements of Group IA and Group IIA contain their valence electrons in s sub-shell. Therefore, Alkali metals are placed called s-block elements.

Q.3:     Arrange the elements in each of the following groups in order of increasing ionization energy:

(a)  Li, Na, K                 (b) Cl, Br, I

K<Na<Li                       I<Br<Cl

Q.4:     Arrange the elements in each of the following in order of decreasing shielding effect.

(a)  Li, Na, K                 (b) Cl, Br, I

K>Na>Li                       I>Br>Cl

(c)        Cl, Br               Br>Cl

Q.5:     Specify which of the following elements you would expect to have the greatest electron affinity.

S, P, Cl                                     Ans: Cl

Q.6:     Electronic configuration of some elements are given below, group the elements in pairs that would represent similar chemical properties.

A = 1s22s2

B = 1s22s22p6

C = 1s22s22p3

D = 1s2

E = 1s22s22p63s23p3

F = 1s22s1

G = 1s22s22p63s1

H = 1s22s22p63s2


Q.7:     Arrange the elements in groups and periods in Q. No. 6.

            IA                                                                                     VIIIA



Q.8:     For normal elements, the number of valence electrons of an element is equal to the        group number. Find the group number of the following elements.

, , ,


Al: 1s22s22p63s23p1                 G= IIIA             P= 3rd

S : 1s22s22p63s23p4                       G= VIA             P= 3rd

K: 1s22s22p63s23p6 4s1                 G= IVA             P= 4th

O: 1s22s22p4                                     G= VIA             P= 2nd

Q.9:     Write the valence shell electronic configuration for the following groups:

  1. Alkali metals I-A
  2. Alkaline earth metals II-A
  3. Halogens VII-A
  4. Noble gases VIII-A


Q.10:   Write electron dot symbols for an atom of the following elements

(a)        Be        (b)        K          (c)        N          (d)        I


Q.11:   Write the valence shell electronic configuration of the atoms of the following elements.

(a)        An element present in period 3 of Group VA                         Ans: 3s23p3

(b)        An element present in period 2 of Group VIA                        Ans: 2s22p4

Q.12:   Copy and complete the following table:


Atomic number Mass number No. of protons No. of neutrons No. of electrons
11 23 11 12 11
14 29 14 15 14
22 47 22 25 22
13 27 13 14 13


Q.13:   Imagine you are standing on the top of Neon-20 nucleus. How many kinds of sub-atomic particles you would see looking down into the nucleus and those you would see looking out from the nucleus.

In the nucleus : Proton & Neutron

Out of nucleus : Electron

Q.14:   Chlorine is a reactive element used to disinfect swimming pools. It is made up of two isotopes Cl-35 and Cl-37. Because Cl-35 is more than Cl-37, the atomic mass of chlorine is 35.5amu. is closer to 35 than 37. Write electronic configuration of each isotope of chlorine. Also write symbol for these isotopes (atomic number for chlorine is 17).

Ans: 1s22s22p63s23p5

both has same electronic configuration

Q.15:   In which block, group and period in the periodic table where would you place each of the following elements with the following electronic configurations?


  1. 1s22s1 Block : S                 P: 2nd  G: IA
  2. 1s22s22p5 Block : P           P: 2nd  G: VIIA
  3. 1s22s22p63s2 Block : S      P: 3rd   G: IIA
  4. 1s2 Block : S                      P: 1st    G: VIIIA



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