HSC chemistry 1st paper; chapter-3: periodic properties

1. Chemistry 1st
paper
Chapter -3
Periodic properties of elements and chemical bond
Topic: Periodic properties of elements:
 Periodic property: A periodic property is a characteristic of atoms that varies regularly across
the periodic table and repeats with a regular interval of atomic number.
 Periodic properties of elements are as followings:
1. Atomic size/radius
2. Ionic radius
3. Ionization potential or energy
4. Electron affinity
5. Electro negativity
6. Melting point
7. Metallic character
 Atomic size/radius: The distance of electron of outermost shell of atom from the nucleus is
mainly the atomic radius although it is quite impossible to take, so the average distance
between two nuclei is called atomic radius.
Q. How it changes across a period?
Ans: Moving from left to right across a period, the atomic radius decreases. The nucleus of
the atom gains protons moving from left to right, increasing the positive charge of the nucleus
and increasing the attractive force of the nucleus upon the electrons. Across a period,
elements have same outermost energy level.
 Ionic radius: The average distance between two nuclei of cation and anion is called ionic
radius.
Cation is relatively small in size where anion is bigger.
N.B: যত (+) তত small এবং যত (–) তত large.
 Ionization potential or energy: The amount of energy required to remove one mole of
electron from one mole of gaseous atom is called ionization potential or energy.
Example: 1
4961 
 KjmolHeNaNa
For the 1st
electron it is called 1st
ionization energy and fro the 2nd
electron it is called 2nd
ionization energy. 2nd
ionization energy is always greater than 1st
ionization energy.
Jakir sir
01712653341

2. Trend of ionization energy across a period: Moving from left to right across a period,
the ionization energy increases. The nucleus of the atom gains protons moving from left to right,
increasing the positive charge of the nucleus and increasing the attractive force of the nucleus
upon the electrons. So the required energy to remove electron increasing across a period.
For example in the 2nd
period 1st
ionization energy of Li is minimum and Ne is maximum.
Moving down a group in the periodic table, the number of electrons and filled electron shells
increases, but the number of valence electrons remains the same. The outermost electrons in
a group are same but electrons are found farther from the nucleus as the number of filled
energy shells increases. Therefore, the attraction of nucleus upon the electrons decreases.
For example 1st
ionization energy of K is less than Na.
Explanation:
1. 1st
ionization energy of Be is greater than B.
Ans: Moving from left to right across a period, the ionization energy increases. But 1st
ionization energy of Be is greater than B. Because the electronic configuration of Be is
quite stable due to present completely filled 2s2
orbital. So that some extra energy needed
to overcome the stability of Be. On the other hand B gains electron in 2p1
.
22
21:)4( SSBe
122
221:)5( PSSB
2. 1st
ionization energy of N is greater than O.
Ans: Moving from left to right across a period, the ionization energy increases. But 1st
ionization energy of N is greater than O. Because the electronic configuration of N is quite
stable due to present half filled 2p3
orbital. So that some extra energy needed to overcome
the stability of N. On the other hand O gains electron in 2p4
.
11122
22221:)7( zyx PPPSSN
11222
22221:)8( zyx PPPSSO
 Electron affinity: The amount of energy released when 1 mole of electron added to 1 mole
of gaseous atom is called electron affinity.
Example: 1
3461 
 KjmolHCleCl
Although 2nd
ionization energy of atom is more than 1st
ionization energy but 2nd
electron
affinity is less than 1st
electron affinity. When 2nd
electron approach to a negative ion then a
repulsive force work over there so that attraction of nucleus to electron reduce. As a result 2nd
electron affinity is less than 1st
electron affinity.

3. Trend of electron affinity across a period: Moving from left to right across a period, the electron
affinity increases. The nucleus of the atom gains protons moving from left to right, increasing the
positive charge of the nucleus and increasing the attractive force of the nucleus upon the
electrons. So the released energy by accepting electron increasing across a period.
For example in the 2nd
period electron affinity of Li is minimum and F is maximum.
Electron affinity of inert gas is zero.
 Why the electron affinity of Fluorine is less than chlorine?
Ans: Moving down a group in the periodic table, the number of electrons and filled electron
shells increases, but the number of valence electrons remains the same. The outermost
electrons in a group are same but electrons are found farther from the nucleus as the number
of filled energy shells increases. Therefore, the attraction of nucleus upon the electrons
decreases. As a result moving down a group electron affinity decreases but electron affinity of
Fluorine is less than chlorine. There are 7 electrons present in the valence shell of both
atoms.
522
221:)9( PSSF
52622
33221:)17( PSPSSCl
In case of F it is 2nd
energy level where 8 electrons can be accommodated. As F is small in
size so electron density is high. So that the taken electron repulsed by the present electrons
in the valence shell. As a result attraction of nucleus upon electron decreases.
On the other hand in case of Cl electrons are located in 3rd
energy level where repulsive
force not worked.
 Electronegativity: The relative attraction of atoms in a covalent bond upon bond electrons is
called electronegativity.
Moving left to right in a period electronegativity increases and moving down in a group it is
decreases.
 Some important value of electronegativity:
Atom Electro negativity
F 4.0
Cl 3.1
O 3.5
N 3.0
C 2.5
H 2.2
Na 0.9

4.  Electronegativity and nature of bond:
Difference of electro
negativity(ΔEN)
Types of bond example
<0.5 Covalent CH4
0.5 – 1.7 Polar HCl
>1.7 Ionic NaCl
 Melting point:
•Trends in melting and boiling points in Period 3
The trends in melting points and boiling points going across Period 3 are not straightforward.
Melting points generally increase going from sodium to silicon, then decrease going to argon
Explanation of the trends
Melting point:
Sodium, magnesium and aluminium
Sodium, magnesium and aluminium are all metals. They have metallic bonding, in which positive
metal ions are attracted to delocalized electrons. Going from sodium to aluminium the charge on
the metal ions increases +1 ,+2,+3 , the number of delocalized electrons increases. So the
strength of the metallic bonding increases and the melting points and boiling points increase.
Silicon
Silicon is a metalloid , has giant covalent bonding. It has a giant lattice structure similar to that
of diamond, in which each silicon atom is covalently-bonded to four other silicon atoms in a
tetrahedral arrangement. This extends in three dimensions to form a giant molecule or
macromolecule.
Silicon has a very high melting point and boiling point because all the silicon atoms are held
together by strong covalent bonds which need a very large amount of energy to be broken.
Phosphorus, sulphur, chlorine:
These are all non-metals with strong covalent bonds between their atoms. Phosphorus exists as
P4 molecules, sulphur exists as S8 molecules , chlorine exists as Cl2 molecules. The strength of
the Van Der Waals’ forces decreases as the size of the molecule decreases.
So that the melting points decreases in the order of S8 > P4 > Cl2.

5.  Metallic character: nature of oxides of the elements of 3rd
period.
Element Na Mg Al Si P S Cl
Oxide ONa2
MgO 32OAl 2SiO 52OP 3SO 72OCl
ONa2 :
It is strongly basic. It is react with both acid and water.
NaClHClONa
NaOHOHONa


2
22
MgO :
It is also basic but weaker than ONa2
as it reacts with acid only .
OHMgClHClMgO 22 
32OAl :
It is amphoteric as it is reacts with both acid and base.
OHNaAlONaOHOAl
OHAlClHClOAl
2232
2332


2SiO :
It is slightly acidic.
OHSiONaNaOHSiO 2322 
52OP :
It is acidic.
OHPONaNaOHOP
POHOHOP
24352
43252


3SO :
It is acidic and produce strong acid.
OHSONaNaOHSO
SOHOHSO
2423
4223


72OCl :
It is acidic and produce strongest oxoacid.
OHNaClONaOHOCl
HClOOHOCl
2472
4272


From the above reaction we can say moving left to right in a period metallic character of
elements decreases.