Periodic Table, Chapter Notes, Class 10, Science PDF Download

Periodic Table

Introduction :

Before the beginning of the eighteenth centuray when there are only 30 elements were known, it was earier to study and rember their properties. In later years when number of elements discovered were increased then it became difficult to study them. So scientist fell the need of simple method to facilitale the study of the properties of various elements and their compounds. After numerous attempts they got sucess & elements were arranged in such manner that similar elements were grouped together and different elements were separated. This arrangement of elements is known as classification of elements which led to the formation of periodic table.

Periodic table may be defined as the arrangement of all the known elements according to their properties in such way that the elements of similar properties are grouped together in a tabular form.

Earlier attempts of classification of elements (development of periodic table) :

Earlier attempts to classify the elements resulted in grouping as metals and non-metals. Later on they were classified on the basis of their atomic masses.

Dobereiner Triads rule :

In 1817, Johann walfgang dobereiner. A German chemist, arranged the elements is group of three elements and in a manner that the atomic mass of middle element was roughly the average of the atomic masses of the other two elements of the triad.

Example : Element : Lithium, Sodium and Potassium.

Atomic mass 7 23 39

Average of the atomic masses of Lithium and Potassium is only three triads could be arranged in this manner at that time. They were :

Triads Elements Atomic masses Average of the atomic masses of first and third element

Short coming of Dobereiner's triads rule :

This classification was not found satisfactory as it could be applied to the limited number of elements. Now a days some more triads have been made they are

(i) Potassium Rubidium Cesium

K Rb Cs

(ii) Phosphours Arsenic Antimony

P As Sb

(iii) Sulphur Selenium Tellurium

S Se Te

(iv) Hydrogen Fluorine Chlorine

H F Cl

(v) Scandium Itrium Lanthanum

Sc Y La

For a Debereiner's triad all the three elements should belong to the same group and the difference in atomic number should be 8 or 18.

New lands law of octave : In 1866, J.A.R. Newlands correlated the chemical properties of the elements with the increasing order of atomic masses. i.e. to arrange the element having lowest atomic mass (H) firstely and ended to at secondly the element having highest atomic mass. (Thorium which was 56^th known element at that time).

Definition : When the elements are arranged in order of their increasing atomic masses, every eighth element has the properties similar to those of the first elements like the eighth note of an octav in music. Thus according to this law, the physical & chemical properties are repeated after an interval of eight elements and this is similar to eight notes of an octave on a musical scale shown below :

The properties of Lithium are similar to that of 8^th element i.e. Na, Be is similar to Mg and so on.

Limitations :

1. Law of octaves was applicable only up to calcium. It worked well with lighter elements only.

2. At that time only 56 elements were existed in nature, but later several elements were discovered which can not be kept in periodic table as per this law. Their properties were not in accordance with the law of octaves.

Law of octaves :

3. (i) In order to fit element in to his table New lands adjusted two elements in the same column. For example cobalt and nickel were placed in the same position and in the same column as fluorine, chlorine and bromine.

(ii) Iron which resembles cobalt and nickel in properties were placed far away from these elements.

4. After the discovery of inert gases & included in the periodic table it becomes the eighth element from alkali so this law has to be dropped out.

Mendeleev's Periodic table :

In the year 1861, D Mitri Ivanovich Mendeleev arranged all the known elements (63 elements) in the form of a table in which elements were arranged in the increasing order of their atomic mass and also on the similarities of chemical properties.

r The arrangement of element was based on the physical and chemical properties of the elements and also the formulae of the compounds they formed with oxygen and hydrogen. He selected hydrogen and oxygen as they are very reactive and formed compounds with most elements.

The table which classifies the elements in such a way that elements having similar properties are placed in same vertical column or group in known as periodic table. The term periodic means repetition of elements having similar properties after a certain regular interval. The periodic table consists of vertical columns which are called as groups and horizontal rows called as periods. Mendeleev's periodic table had six periods and eight groups as shown in the table, he arranged all the elements horizontally in the order of their increasing atomic masses and vertically according to their similarities in properties. Each group was further sub divided into two sub groups A & B.

Achievements of the Mendeleev's periodic table :

1. Systematic study of the elements : All the elements in general were arranged systematically in increasing order of their atomic masses. This arrangement helped to study the properties of various elements. If the nature of the element present in a group is known, it become easier to predict or guess the expected properties of other elements.

2. Prediction of new elements : Mendeleev predicted the properties of some unknown elements and left gaps for these elements to be filled as and when discovered. For eg. Scandium, Gallium and Germanium were not known at that time but Mendeleev already named these elements as eka-boron, eka-aluminium and

eka-silicon. When these elements were later on discovered, they were found to have more or less similar properties as predicted by Mendeleev.

3. Position of Noble gases : When noble gases were discovered they were placed in a new group without disturbing the existing order.

4. Correction of atomic masses : Atomic masses of several elements were corrected on the basis of periodic table. eg. Atomic mass of Beryllium was corrected from 135 to 9. Mendeleev predicted that atomic mass of gold is incorrect. Later on it was found to be so. similarly atomic masses of Indium, Uranium and Platinum were also corrected.

Drawbacks of Mandeleev's periodic table :

Position of Hydrogen is uncertain becomes it resemble with IA group alkali metals elements and VII A (halogens) group elements.

(i) Isotopes : Isotopes of an element have similar chemical properties but different atomic masses.

(ii) Position of isotopes : Since basis of periodic table was increasing atomic mass. So isotopes should be placed separately but no separate place was given to isotopes.

(iii) Anamolus pairs of certain elements : Certain elements were not arranged according to their increasing atomic mass eg.

(a) Argon (Atomic mass 39.9) was placed before potassium (atomic mass 39.0)

(b) Cobalt (58.95) before Nickel (58.70) (c) Tellurium (127.6) before Nickel (126.9)

(d) Thorium (232) before Protactimum (231) (iv) Similar elements were placed in different groups. eg.

(a) Silver and thallium (b) Barium and lead (c) Copper and mercury (d) Platinum and gold.

(v) Dissimilar elements were placed in same group eg. silver and gold were placed in a same group while there is little similarity in physical and chemical properties.

(vi) Cause of periodicity : Mendeleev did not explain the cause of periodicity in the physical and chemical properties of the elements.

(vii) Metals have not been separated from non-metals.

(viii) Position for elements of group (VIII) : There is no proper position for the elements of group (VIII) consisting of elements in three triads. These elements are placed out side the main structure of the periodic table.

The modern periodic table :

In 1913 Henry Moseley showed that properties of the elements are determined by atomic numbers instead of the atomic mass. It formed the basis of modern periodic law. The law is -

"The physical and chemical properties of the elements are periodic function of their atomic numbers". Since atomic mass is a nuclear property where as atomic number implies for the no. of electrons in neutral atom or no. of protons in nucleus. Nucleus is deep seated in the atoms and does not take part in chemical reactions. Therefore the physical and chemical properties depends upon the no. of electrons and their electronic configuration which in turn depends upon atomic number (Z). So when elements are arranged in the increasing order of atomic numbers, after an regular interval elements have similar no. of valence electrons therefore chemical properties are repeated i.e. periodicity in the chemical properties of the elements occurs.

Modern periodic table or long form of the periodic table :

It is also called as Bohr, Bury & Rang, Werner periodic table

(1) It is based on the Bohr-Bury electronic configuration concept and atomic number.

(2) This model is proposed by Rang and Werner.

This table is based on modern periodic law, the elements are arranged in the increasing order of atomic numbers in such a way that elements having the same number of valence electrons are placed in the same vertical column.

It consists of 18 vertical colums and seven horizontal rows. Vertical columns of periodic lable are known as groups while horizontal rows are known as periods.

The co-relation between the groups in long form of periodic table and in modern form of periodic table are given below:-

IA IIA IIIB IVB VB VIB VIIB VIII IB IIB

1 2 3 4 5 6 7 8,9, 10 11 12

III A IV A VA VIA VIIA 0

13 14 15 16 17 18

Elements belonging to same group having same number of electrons in the outer most shell so their properties are similar.

Description of periods :-

Description of periods :

Period n Sub shell No. of elements Element Name of Period

1. 1 1s 2 ₁H, ₂He Shortest

2. 2 2s, 2p 8 ₃Li _ ₁₀Ne Short

3. 3 3s, 3p 8 ₁₁Na _ ₁₈Ar Short

4. 4 4s, 3d, 4p 18 ₁₉K _ ₃₆Kr Long

5. 5 5s, 4d, 5p 18 ₃₇Rb _ ₅₈Xe Long

6. 6 6s, 4f, 5d, 6p 32 ₅₅Cs _ ₈₆Rn Longest

7. 7 7s, 5f, 6d, 26 ₈₇Fr _ ₁₁₂Uub Incomplete

Description of Groups :

1^st/IA/Alkali metals

H = 1s¹

Li = 1s² , 2s¹

Na = 1s² , 2s² 2p⁶ , 3s¹

K = 1s² , 2s² 2p⁶ , 3s² 3p⁶ , 4s¹

General electronic configuration = ns¹(n = Number of shell)

Number of valence shell e^_ = 1

2^nd/IIA/Alkali earth metals :

Be = 1s², 2s²

Mg = 1s², 2s², 2p⁶, 3s²

Ca = 1s², 2s², 2p⁶, 3s², 3p⁶, 4s²

General electronic configuration = ns²

Number of valence shell e^_ = 2

Periodicity

13^th/IIIA/Boron Family :

B = 1s², 2s², 2p¹

Al = 1s², 2s², 2p⁶, 3s², 3p¹

Ga = 1s², 2s², 2p⁶, 3s², 3p⁶, 3d¹⁰,4s²,4p¹

General electronic configuration = ns² np¹

Number of valence shell e^_ s = 3

14^th/IVA/Carbon Family :

C = 1s², 2s², 2p²

Si = 1s², 2s², 2p⁶, 3s², 3p²

Ge = 1s², 2s², 2p⁶, 3s², 3p⁶, 4s², 3d¹⁰, 4p²

General electronic configuration = ns² np²

Number of valence e^_ s= 4

15^th/VA/Nitrogen family/Pnicogen : (Used in fertilizer as urea)

N = 1s², 2s², 2p³

P = 1s², 2s², 2p⁶, 3s², 3p³

As = 1s², 2s², 2p⁶, 3s², 3p⁶, 4s², 3d¹⁰, 4p³

General electronic configuration = ns² np³

Number of valence shell e^_ = 5

16^th/VIA/Oxygen family/Chalcogen : (Ore forming)

O = 1s², 2s², 2p⁴

S = 1s², 2s², 2p⁶, 3s², 3p⁴

Se = 1s², 2s², 2p⁶, 3s², 3p⁶,3d¹⁰, 4s², 4p⁴

General electronic configuration : ns² np⁴

Number of valence shell e^_ s= 6

17^th/VIIA/Fluorine family/Halogens : (Salt forming)

F = 1s², 2s², 2p⁵

Cl = 1s², 2s², 2p⁶, 3s², 3p⁵

Br = 1s², 2s², 2p⁶, 3s², 3p⁶, 4s², 3d¹⁰, 4p⁵

General electronic configuration = ns² np⁵

Number of valence shell e^_ s= 7

18^th/Zero group/Inert gases / Noble gases :

Ne = 1s², 2s², 2p⁶

Ar = 1s², 2s², 2p⁶, 3s², 3p⁶

Kr = 1s², 2s², 3p⁶, 3s², 3p⁶, 3d¹⁰,4s², 4p⁶

General electronic configuration = ns² np⁶ ( except. He)

Number of valence shell e^_ = 8

Elements of group 16 are known as chalocogens Elements of group 17 are known as halogens.

Classification on the basis of subshell in which last electron (e^_) enters

 

Periodicity in properties :

The electronic configurations of atoms display a periodic variation with increase in atomic number. Since the properties of elements depends upon the electronic configurations. So the elements exhibits periodic variation of physical & chemical properties. Some properties of elements are :-

(A) Valency :- It is defined as the combining capacity of the element. Valency is determined by the number of electrons present in outer most shell. These electrons are known as valence electrons.

Variation of valency across a period :- The number of valency electrons increases from 1 to 8 on moving across a period. The valency of an element with respect to hydrogen and halogen increases from 1 to 4 and then decreases from 4 to zero. With respect to oxygen valency increases from 1 to 7.

Variation of valency along a group :- On moving down a group. The no. of valence electrons remains same so the valence of all the elements of a group is same.

Group (1) elements have valency - 1

Group (2) elements have valence - 2

Atomic size :- Atomic size means atomic radius of an atom which is defined as the distance between the centre of the nucleus of an atom and the valence shell containing electrons in an isolated atom since it is very difficult to measure the atomic radius because -

(i) The isolation of single atom is very difficult.

(ii) There is no well defined boundary for the atom.

So the more accurate definition of atomic radius is -

(Half the internuclear distance between the two atoms in a homatomic molecule is known as atomic radius)

This internuclear distance is also known as bond length. It depends upon the type of bond by which two atoms combine. Based on chemical bonds, atomic radius is divided in to four categories.

(a) Covalent radius (Single bonded covalent radius) For homoatoms

It is half of the internuclear distance between two singly bonded homoatoms.

(b) Covalent radius for hetero atoms.

(i) In case of hetero atomic molecule (A _ B), if the electronegativity difference is less. Then covalent radius of oxygen, nitrogen and carbon is taken from the compound H₂O₂, N₂H₄ and C₂H₆ respectively.

This radii is subtratied from the bond length of A_B molecules.

eg. C_I (electronegativity is almost same 2.5)

Internuclear distance C_I is 2.13Å, covalent radius of carbon in compound C₂H₆ is 0.77Å covalent radius of I^_ will be.

d_{C_I} = r_C + r_I (covalent radius of iodine)

(covalent radius of carbon)

i.e. 2.13 = 0.77 + r_I

r_I = 2.13 _ 0.77 = 1.36Å

(ii) When electronegativity difference is more. Then bond length is determined by the schole maker and Stevenson law _

d_{A_B} = r_A + r_B _ 0.09(X_A _ X_B)

where d_{A_B} = Bond length of d_{A_B} molecule

X_A = Electronegativity of A

X_B = Electronegativity of B

Example _ Bond length of F₂ = 1.44 Å

i,e. d_{F_F} = 1.44Å r_F = = 0.72Å

d_{H_H} = 0.74Å r_H = = 37Å

Electronegativity of Fluorine is 4.0 and Electronegativity of Hydrogen is 2.1

d_{H_F} = r_F + r_H + 0.09(X_F _ X_H)

= 0.72 + 0.37 _ 0.09(4 _ 2.1)

= 1.09 _ (0.09 × 1.9)

= 1.09 _ 0.171 = 0.919Å

(B) Ionic Radius -

(i) Cationic radius (ii) Aniotic radius

(i) Cationic Radius -

Size of cation µ

eq. Fe > Fe⁺² > Fe⁺³

(ii) Anionic radius _

Anionic radius is always greater than atomic radius because in an anion electrons are more than the protons so effective nuclear charge reduces and inter electronic repulsion increases so size of anion also increases.

(C) Metallic Radius _

Half of the inter nuclear distance between two adjacent metallic atoms.

Metallic radius µ

(D) Vander Waal's radius _

Those atoms which are not bonded with each other experiences a weak attractive force to come nearer. Half of the distance between the nuclei of adjacently placed atoms in solid state of a noble gas is vander waal's radius.

Variation of Atomic size in a group :

On moving down a group of periodic table, the size of the atom increases.

When we move from top to bottom in a group a new shell of electron is added in each period. This addition increases the size.

Variation of atomic size in period :

In general atomic radii decreases across a period from left to right eg. In II^nd period.L atom is largest and Fluorine is the smallest atom because nuclear charge increases with increase in atomic number. Electrons are also increasing but these are added to the same shell.

Atomic Size :- Decreases along the period. Increases down the group.

Metallic and Non-metallic character :

l Metallic character is the tendency of atoms of the elements to lose electrons and form positive ions. It can be expressed as

M ¾® M⁺ + e^_

Therefore metals are also called as electropositive elements.

The metallic character increases from top to bottom in a group the metallic character of the element goes on increasing eg. Li is least metallic element while caesium is most metallic element.

If we use the term electropositive in place of metallic character, we can say that electropositive character goes on increasing as we move from top to bottom in the periodic table. If we consider the electronegative character, it goes on decreasing as we move down in a group of the periodic table.

Ionization Enthalpy :

The minimum amount of energy required to remove the most losely bounded electron from an isolated gaseous neutral atom to form gaseous electropositive ion called Ionization enthalpy. Its unit is kilo joules per mole (kJ/mol)

M_(g) + Energy → M⁺_(g) + e

It is a measure of tendency to lose electrons by atoms. The tendency to lose electron increases from top to bottom in a group and it decreases on moving left to right in a period.

Electron gain enthalpy :

It is defined as the amount of energy released when an isolated gaseous atom in the ground state accepts an electron to form gaseous negative ion i.e. and anion. It is a measure of tendency of an atom to accept an extra electron to form an anion. Its unit is kilo joule mole (kJ/mole). Electron gain enthalpy of elements goes on increasing as we move from left to right in a period. In group it decreases from top to bottom.

• PREVIOUS YEARS' BOARD QUESTIONS :
• 1. Which physical and chemical properties of the elements were used by Mendeleev in creating his periodic table ? List two observation which posed a challenge of Mendeleev's periodic law. (C.B.S.E. 2008)
• Ans. The creation of Mendeleev's periodic table was based upon certain physical and chemical properties.
• Physical properties : The atomic masses of the elements was taken into account and the elements were arranged in order of increasing atomic masses. The influences of their physical properties such as melting points, boiling points, density etc.
• Chemical properties : The distribution of the elements into different groups was linked with formation of hydrides by combining with hydrogen and formation of oxides by combining with oxygen. This is linked with the valency of the elements.
• The two main observations which posed challenge to Mendeleev's periodic table are as follows:
• (i) Position of isotopes : Since the isotopes of an element differ in their atomic masses, they must be assigned separate slots or positions in the periodic table.
• (ii) Anomalous positions of some elements : In the Mendeleev's periodic table, certain elements with
• higher atomic masses precede or placed before the elements with lower atomic masses. For example,
• the element Ar (Atomic mass = 39.9) is placed before the element K (Atomic mass = 39.1)
• 2. Using the part of the periodic table given below, answer the questions that follow.
• (i) Na has physical properties similar to which elements and why?
• (ii) Write the electronic configuration of N and P
• (iii) State one property common to fluorine and chlorine. (C.B.S.E. All India 2008)
• Ans. (i) Na has physical properties similar to Li and K. All the three elements have one electron each in the valence of their atoms. These are known as alkali metals.
• (ii) Electronic configuration of N (z = 7) = 2, 5
• Electronic configuration of P (z = 15) = 2, 8, 5
• (iii) Both the elements have seven electrons in the valence shells as their atoms
• Fluorine (z = 19) = 2, 7
• Chlorine (z = 17) = 2, 8, 7
• 3. Table given below shows a part of the periodic Table
• Using this table explain why
• (a) Li and Na are considered as active metals.
• (b) Atomic size of Mg is less than that of Na.
• (c) Fluorine is more reactive than chlorine. (C.B.S.E. Foreign 2008)