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Notes on Periodic table

203 / May-30-2021 12:18:18 By- srijan /

Periodic Table: The systematic arrangement of all the known elements in the form of table is called periodic table.
There are mainly two types of periodic table, they are:
1. Mendeleev's Periodic Table
2. Modern Periodic Table.

Mendeleev's Periodic Table:

The physical and chemical properties of the elements are periodic funtions of their atomic weights.
This means that the physical and chemical properties of the elements are repeated when they are arranged according to increase in atomic weight.

# Merits of Mendeleev's Periodic Table:
i) Systematic study of elements:- Mendeleev's periodic table has made the study of elements simpler.
It is quite difficult to study all elements separately.
ii) Prediction of new elements or undiscovered elements:- At the time of Mendeleev's, only 63 elements were discovered.
Mendeleev left gaps for undiscovered elements in his periodic table and predicted their existence. Later, these elements were discovered and it was concluded
that the prediction of Mendeleev was correct.
iii) Correction of doubtful or wrong atomic weights:- Mendeleev corrected the atomic weight of some elements on the basis of their position in the periodic table.
For eg: The atomic weight of Be is corrected as from 13.5.

# Demerits of Mendeleev's Periodic Table:
The main demerits of Mendeleev's Periodic Table are:
i) Position of hydrogen: The position of hydrogen is controversial because its properties are similar to those of alkali metals as well as halogens.
ii) Position of isotopes: In this periodic table, elements had been arranged according to the increase in atomic weight.
Isotopes of an element have different atomic weights. Hence, they should be placed separately in the periodic table but they are not placed separately.
iii) Anomalous pairs of elements: In this periodic table, elements have been arranged according to increase in atomic weight
but there are some pairs of elements which do not follow general trends.

Modern Periodic Table:

The physical and chemical properties of the elements are periodic functions of their atomic number.
* Groups: Vertical column of the periodic table is called group. There are 18 groups in the modern periodic table.
* Periods: The horizontal rew of the periodic table are called periods. There are 7 periods in the modern periodic table.
* Blocks: In modern periodic table elements have been classified into four blocks. They are s-block, p-block, d-block and f-block.

# Properties or characteristics of s-block elements:

i) The last electron of these elements goes into s-subshell.
ii) The outermost electronic configuration of these elements is ns1-2, where n= 1,2,3,4,5,6 or 7.
iii) These elements are strong reducing agents.
iv) The ionization energy of these elements are low.
v) The elements are very reactive and form ionic compounds.
vi) These elements are highly electropositive in nature.
vii) They are soft-metals.
viii) These elements show +1 or +2 oxidation states.

# Properties or characteristics of p-block elements:

i) The last electron of these elements goes into p-subshell.
ii) The outermost electronic configuration of these elements is ns2 np1-6
iii) They may be metals or non-metals.
iv) They form ionic as well as covalent compounds.
v) They have high ionization energies.
vi) They form generally acidic oxides.

# Properties or characteristics of d-block elements:

i) The last electron of these elements goes into d-subshell.
ii) The outermost electronic configuration of these elements is (n-1)d1-10ns1-2
iii) They are metals (and commonly known as transition metals).
iv) They show variable oxidation states.
v) They have generally variable valency.
vi) They form ionic as well as covalent compounds.
vii) They show catalytic activites (hence they are used as catalyst).

# Properties or characteristics of f-block elements:

i) The last electron of these elements goes into f-subshell.
ii) The outermost electronic configuration of these elements are (n-2)f1-14(n-1)d0-1ns2.
iii) They are metals and known as inner transition metals.
iv) They show variable oxidation states.
v) They generally form coloured compounds.
vi) They are radio-active.

Periodic Properties:

(a.) Atomic size or Atomic radius: The distance between centre of the nucleus to the outermost orbit of an atom containing electron, is called atomic size or atomic radius.

Factors affecting atomic size or atomic radius:
1. Number of orbits or shells: As the number of orbits increases, atomic size also increases i.e. Atomic size ∝ No. of orbits.
For eg: the atomic size of Lithium is smaller than that of sodium due to lesser number of orbits.
Li(3) ----> 2,1
No. of orbits= 2
(smaller size)

2. Nuclear charge or effective nuclear charge: As the nuclear charges increases, the force of attraction between the nucleus and the electrons of an atom increases.
If the number of orbits remains same than atomic size decreases with the increase in nuclear charge. This is due to the greater force of
attraction between the nucleus and valence electrons which causes contraction.

Variation of atomic size in group and periods:



On moving from top to bottom in a group, atomic size generally increases due to increase in number of orbits at each step. For example:
Lithium(3) -----> 2,1 No. of orbits= 2, smaller size


Sodium(11) -----> 2,8,1 No. of orbits= 3, larger size
On moving from left to right in a period, atomic size generally decreases because the number of orbits remains the same but nuclear charge increases at each step.

Ionic size or ionic radius: The distance between centre of the nucleus to the outermost orbit
containing electron of an ion is called ionic size or ionic radius.

Comparison of size of atom and its ion:
i) Cations (positively charged ions): The ionic size of a cation is smaller than the atomic size of its parent atom.
For eg: ionic size of Na+ ion is smaller than the atomic size of Na atom.
ii) Anions (negatively charged ions): The ionic size of an anion is larger than the atomic size of its parent atom.
For eg: ionic size of Cl- ion has larger ionic size than the atomic size of Cl atom.
iii) Isoelectronic ions: Ions having same number of electrons are called isoelectronic ions. In case of isoelectronic ions,
as the nuclear charge increases ionic size due to greater force of attraction between the nucleus and outermost electrons.
Ionic size decreases from left to right.

3. Ionization Energy: The minimum amount of energy required to remove the outermost electron
from a monopositive cation is called second ionization energy.

* Order of energy:
1st ionization energy < 2nd ionization energy < 3rd ionization energy

Factors affecting Ionization energy:
a. Atomic size: As the atomic size increases, lesser amount of energy is required to remove the outermost electron and hence ionization energy also decreases i.e.
I.E ∝ 1/atomic size
b. Nuclear charge or effective nuclear charge: As the nuclear charge increases, the force of attraction between
the nucleus and outermost electrons increases. Due to this greater amount of energy is required to remove the outermost electrons. i.e.
I.E ∝ Nuclear charge.
c. Electronic configuration: Atoms having half filled and full filled sub-shells are more stable.
Greater amount of energy is required to remove electrons from such atoms. Hence, the ionization energies of such atoms are high. For example:
Na(7) -----> 1s22s22p3 [Where 2p3 is half filled ane it is more stable.]
O(8) ------> 1s22s22p4 [Where 2p4 is neither half filled nor full filled and it is less stable.]
d. Variation of ionization energy in groups and periods:



4. Electron Affinity (Electron Gain Enthalpy): The amount of energy released (or changed) when an electron is added
to an isolated gaseous atom to form anion is called electron affinity. It is represented as E.A. i.e.
A(isolated present atom) + e- ----------> A-(anion) + Energy
The value of electron affinity depends on the tendency to gain electron. Greater the tendency to gain electron higher
is the value of electron affinity.
(The electron affinity values of inert gases are nearly zero because they have no any tendency to gain electron.)
Factors affecting Electron Affinity:


i) Atomic size: The value of Electron Affinity decreases with the increase in atomic size and vice-versa.
This is due to the decrease in tendency to gain electron with the increase in atomic size.
i.e. E.A. ∝ 1/atomic size
ii) Nuclear charge: As the nuclear charge increases, the tendency to gain electron also increases.
Due to this, the value of electron affinity increases with the increase in nuclear charge.
i.e. E.A. ∝ nuclear charge
iii) Electronic Configuration: Atoms having half filled and full filled sub-shells are more stable and they have either no any or very less tendency to gain electron. Hence, the electron affinity values of such atoms are either zero or nearly zero.
iv) Variation of Electron affinity in groups and periods:



v) Electronegativity: The tendency of an atom to attract the shared pair of electrons towards itself in a covalent molecule is called electronegativity.
As the tendency to attract electrons increases, electronegativity also increases.
Fluorine is the most electronegative element of the periodic table followed by oxygen and nitrogen.
 

Factors affecting electronegativity:

1. Atomic size: As the atomic size increases, the tendency to attract electrons decreases. Hence
electronegativity decreases with the increase in atomic size.
i.e. Electronegativity ∝ 1/atomic size.

2. Nuclear charge: As the nuclear charge increases, the tendency to attract electrons also increases.
Hence, electronegativity increases with the increase in nuclear charge and vice-versa.
Electronegativity ∝ nuclear charge.

3. Ionization energy (I.E) and electron affinity (E.A.): The value of electronegativity increases with the
increase in the values of ionization energy and electron affinity.
Electronegativity= (I.E. + E.A.)/2

Variations of electronegativity in groups and periods:


 

Advantages of Modern Periodic Table:

The main advantages of modern periodic table are as follows:
a.) It is based on atomic number which is more fundamental than atomic weight.
b.) Since the classification is based on the atomic numbers, the position of isotopes is fully justified.
c.) The position of elements in the periodic table is governed by electronic configuration which determines their properties.
d.) It is easy to memorize and easy to reproduce.
e.) Classification of elements into four block has made the study of element simple.
f.) It separates metals and non metals.