# WBBSE Class 10 Physical Science Notes Chapter 8.1 Periodic Table and Periodicity of the Properties of Elements

Comprehensive WBBSE Class 10 Physical Science Notes Chapter 8.1 Periodic Table and Periodicity of the Properties of Elements can help students make connections between concepts.

## Periodic Table and Periodicity of the Properties of Elements Class 10 WBBSE Notes

Classification: It is the method of arranging similar elements together and separating them from dissimilar elements.

(i) Law of triads (Dobereiner, 1817) : In a triad of elements having similar properties, the atomic weight of the central element was merely the arithmetic mean of the atomic weights of the other two elements. e.g. Li (At. wt. = 6-94), Na (At. wt. = 23), (At. wt.= 39-1)
Atomic weight of$$\mathrm{Na}=\frac{6.94+39.1}{2}=23.02$$ (which is nearest value of 23.)

(ii) Law of Octave : (J.A.R. Newlands, 1864) : The elements when arranged in the increasing order of their atomic weights, the eight succeeding element was the repetion of the first one like the 8th note of the musical scale.

(iii) Lother Meyer curve : Lother Meyer plotted a graph between atomic volumes of elements and their atomic weights. He found that similar elements occupied similar positions of the curve.

(iv) Mendeleef’s Periodle Laws (Mendeleef, a Russian Chemist in 1896): It states that the physical and chemical properties of the elements are the periodic function of their atomic weights.

Mendeleef’s periodic table :

• Periodic table : When the elements are arranged in their increasing order of atomic weight in seven periods and nine groups, the total distribution or chart, that is obtained is called periodic table.
• Period : The horizontal rows in a periodic table are known as periods. At present the periodic tabic contains seven periods.
• Group : The vertical columns of the periodic table are known as groups. The present Mendeleef’s periodc table has nine groups. The groups in periodic table-form the first group to the eight group are designated by roman number 1, II, III, IV, V, VI, VII and VIII. The ninth group is designaged by ‘O’ number.

• Sub-group : Except group ‘VIII’ and ‘O’ each group from I to VII has two sub-groups-A and B. As a result, H find Li, Na, K, Rb, Css, Fr place in IA sub-group and Cu, Ag, Au find place in sub-group IB. In a similar way, the sub-group VIIA contains Mn, Te, Re while F- Cl, Br, I, At are included in sub group VIIB.

• The elements when arranged in this manner were found that elements of the same sub-group show similarity in properties.

• Modern Periodic Law : (Moseley, 1913-14) : It states that the physical and chemical properties of the elements are the periodic function of their atomic numbers.

• Periodicity : It is the recurrence of elements with similar properties after certain regular intervals when these elements are arranged in the increasing order of their atomic numbers.

• Cause of periodicity; It is the recurrence of similar electronic configuration.

Long form periodic table Rohr’s table) : It is based on the modern periodic law. It has eighteen vertical columns or groups and seven horizontal rows of periods. The periodic table can be divided into four main regions according to whether the s, p, d or f levels are being filled

 Period Fill orbitjil in period No. of elements present in period 1st Period Is 2 2nd Period 2s- 2p (2 + 6) = 8 3rd Period 3s 3p (2 + 6) = 8 4th Period 3s 3d 4p (2 + 10 + 6) = 18 5th Period 5s 4d 4p (2 + 10 + 6) = 18 6th Period 6s 4f 5d 6p (2 + 14 + 10 + 6) = 32 7th Period 7s 5f 6d (2 + 14 + 10) = 26 (incomplete) (Incomplete)

N.B, The elements with atomic numbers (58-7) and (90-103) are called lanthanoids and actinoids respectively. Elements from 93 onwards are purely synthestic and are called trans-uranium elements. These are radioactive in nature.

• The groups are denoted by IA to VIIA, IB to VIIB, VIII (There columns) and The elements in which the last added electron goes to s or p sub-levels are placed in A sub-groups, while those in which it goes to d or f sub-levels are placed in B-sub-groups. In accordance with the 1984, IUPAC recommendations the groups are numbered (1-18) and this notation replaces the old numbering scheme.

• The relationship between odd and new notations is as follows :

 Old IIA IIIB IVB VB VIB VIIB VIII IB IIB IIIA-VIIA Zero(O) New 2 3 4 5 6 7 8-10 11 12 13-17 18

Division of elements into s, p, d, f blocks :
s-block elements : These are the elements of IA or 1 group (alkali metal; outermost electronic configuration ; ns1) and IIA or 2 group (alkaline earth metals, outermost electronic configuration : ns2 ) These are so named because the last electron in them enters s-orbitals.

Characteristics of s-block elements :

• These are soft metals having low melting point and boiling point.
• These have low ionisation energies.
• These are highly reactive and readily from univalent or bivalent positive
• The metals and their salts give characteristic colours of the flame.
• In the molten state or solution form these are good conductors of heat and
• These are good reducing agents due to their low ionisation energies.
• The hydroxides of s-block elements are strong bases.

(b) p-block elements : The last electron enters p-orbital of the valence shell of p-block elements. The elements with configurations ns2np1 to ns2np6 constitute this block. Thus, p-block consists of elements of group IIIA (13), IVA(14),VA(15), VIA(16), VIIA (17) and zero group 18. Their (n-1) s, p and orbitals are fully filled. Thus, only the outermost orbitals in these elements are incomplete.

Characteristics of p-block elements :

• They include both metals and non-metals.
• They have quite high ionisation energies.
• They form mostly covalent compounds.
• In a period, these are gradation from reducing to exidising properties.
• Generally bad conductors to heat and electricity.
• Atomic radii decrease across a period and increase down a group.
• Heavier elements exhibit variable valency.

(c) d-block elements : The last electron enters a (n-1)d orbital. These have configurations
(n-1)d110ns02. This block is present in between s and p-block elements d-block elements are also known as transition elements.

Characteristics of d-block elements :

• All these elements are metals and have high melting and biding points.
• These are good counductors of heat and electricity.
• They show variable oxidation states.
• They generally form coloured compounds.
• d-block elements have a strong tendency to form complexes.
• They are used as catalyst.

(d) f-block elements : The last electron enters a (n-2) f -orbital. Lanthanoids and actinoids constitute the f-block and are also shown separately at the bottom of the periodic table.

They have (n-2)f 1-14(n-1)d1-10 ns2 configuration.
They are also known as inner trasition elements. Elements in which the last electron enters 4f-orbitals are known as lanthanides or lanthanoids (Atomic

Number 58 to 71) and elesments in which the last electron enters a 5f-orbitals are known as actinides or actinoids (Atomic number 90 to 103). Characteristics of f-block elements :

• They are heavy metals having high melting and boiling points.
• f-block elements show variable valency.
• They form coloured ions.
• They are paramagnetic due to the presence of unpaired electrons.
• Actinides are radioactive in nature.

Periodic properties and their variations in periodic and group :
(i) Atomic size : The distance of the outermost orbit from the nucleus of spherically shaped atom is called the atomic size.

Atomic size decreases across a period due to increase in nuclear change and increases down a group due to the successive addition of new shells,

(ii) Metallic and non-metallic character : Metallic character decreases and non-metallic character increases across a period and metallic character increases and non-metallic character decreases down a group

(iii) Electronegativity : It is the relative tendency of an element in a molecule to attract the shared pair of electrons towards itself. It increases across a period due to increase in nuclear charge and decreases down a group owing to increase in atomic size and shielding of inner electron shells, e.g. Period 2 :

(iv) Ionisation energy (I.E.) It is the amount of energy required to remove a valence electron from a neutral gaseous atom.
I.E increases across a period and decreases down a group.

(v) Electron affinity : It is the energy released when an electron is added to a neutral gaseous atom.
Electron affinity increases across a period and decreases down a group.

(vi) Diagonal relationship : Elements of second and third row which are present diagonally have similar properties because on moving along the diagonal, the decrease and increase of size, electropositive character and polarizing power partly cancel each other.

(vii) Oxidising and Reducing Character : Oxidising character of the elements increases across a period and decreases down a group. Conversely, reducing character decreases across a period and increases down a group.

(viii) Density: It increases in a group from top to bottom and in a period, it increases in a regular manner from the left.

(ix) Electronic configuration and poistion in the Periodic Table :

Position of hydrogen in the periodic table :
In the periodic table, hydrogen may find a place either in Group I or Group VII when valency and property are taken in account. The reasons are :

Similarities with Group I elements :

• Hydrogen is monovalent and an electropositive element like the alkali metals.
• On electrolysis of hydrogen associated compounds, hydrogen is liberated at the cathode like an alkali metal.
• Hydrogen is a reducing agent like an alkali metal.
• Hydrogen forms stable oxide (H2O) like an alkali metal.
• Hydrogen forms compounds with non-metals like an alkali metal.

Similarities with Group VII elements :

• Hydrogen and the halogens have valency 1.
• Hydrogen and the halogens are diatomic gaseous non-metals.
• Hydrogen forms hydrides with metals like the halogens e.g. NaH.

Differences with the alkali meals (Group I) :

• Hydrogen is a non-metal but the Group 1 elements are metals.
• Hydrogen is gas at the ordinary temperature the alkali metals are solid at the ordinary
temperature (exception : Cs is liquid).
• Hydrogen molecule is diatomic, but the alkali metals are monoatomic.

Differences with the halogen elements (Group VII) :

• Hydrogen is a reducing agent but the halogens are strong oxidising agent.
• In electronic configuration also, hydrogen shows dissimilarity with the halogens. From the above discussion, it is understood that hydrogen may be included in group I and Group VII. So, the actual poistion of hydrogen has not been identified till date. So, hydrogen is called ‘rouge element’.