Class-10th

CHEMISTRY STUDY NOTES & WORKSHEET

PERIODIC CLASSIFICATION OF ELEMENTS

PERIODIC CLASSIFICATION OF ELEMENTS

1. INTRODUCTION

There are 118 different elements currently on the periodic table. As of now there are 115 known elements, although these 115 are not in sequence. Elements 1 to 112, Hydrogen to Ununbium are known, with Ununbium only been discovered in 1996. Elements 114, 116 and 118 have also been discovered more recently, bringing the total to 115 known elements. Some of the elements such as 118 (Ununoctium) are very unstable, although Ununoctium technically should be a noble gas and thus stable, it has an incredibly short life and will decay to element 116 in a fraction of a second. It has been theorized that elements with atomic numbers approaching 125 will be stable at normal conditions and could be used for various industrial purposes. Note: Elements 113, 115 and 117 have not yet been discovered. The number of naturally occurring elements is 98.

As different elements were being discovered, scientists gathered more and more information about the properties of these elements. They found it difficult to organise all that was known about the elements. They started looking for some pattern in their properties, on the basis of which they could study such a large number of elements with ease. So, all the elements have been divided into a few groups in such a way that elements in the same group have similar properties.

2. DOBEREINER’S TRIADS

In the year 1829, a German chemist Dobereiner observed that certain elements had similar properties and that he could put them together in groups of three elements each. These groups of three elements were called triads. All the three elements of a particular triad had similar chemical properties.

2.1 Dobereiner’s law of triads

When elements are arranged in the order of increasing atomic masses, groups of three elements (known as triads), having similar chemical properties are obtained. The atomic mass of the middle element of the triad being equal to the arithmetic mean of the atomic masses of the other two elements.

(i) The Alkali Metal Group. The elements lithium, sodium and potassium have similar chemical properties and form a triad.

(ii) The Alkaline Earth Metal Group. The elements calcium, strontium and barium have similar chemical properties and form a triad. Döbereiner’s triads

WIN POINT

Page 1

Given Table represents some groups of three elements. These elements are arranged downwards in order of increasing atomic masses.

2.2 The limitations of Dobereiner’s classification

It failed to arrange all the then known elements in the form of triads of elements having similar chemical properties.

Dobereiner could identify only three triads from the elements known at that time.

3. NEWLANDS’ LAW OF OCTAVES

Newlands arranged the then known elements in the order of increasing atomic masses and found that the properties of every eighth element are similar to the properties of the first element.

3.1 Newlands’ law of Octaves

When elements are arranged in the order of increasing atomic masses, the properties of the eighth element (starting from a given element) are a repetition of the properties of the first element. This repetition in the properties of elements is just like the repetition of eighth note in an octave of music, so it is known as the law of octaves.

Newlands’ Octaves

sa (do)

re (re)

ga (mi)

ma (fa)

pa (so)

da (la)

ni (ti)

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

Co and Ni Cu Zn Y In As Se Br Rb Sr Ce and La Zr

3.2 Limitations

(a) Newlands’ law of octaves worked well with lighter elements only. (b) Newlands assumed that only 56 elements existed in nature and no more

elements would be discovered in the future. (c) In order to fit elements into his table, Newlands put even two elements

together in one slot and that too in the column of unlike elements having very different properties. For example, cobalt (Co) and nickel (Ni).

Group A elements

Atomic mass

Group B elements

Atomic mass

Group C elements

Atomic mass

N 14.0 Ca 40.1 Cl 35.5 P 31.0 Sr 87.1 Br 79.9

As 74.9 Ba 137.3 I 126.9

WIN POINT

Page 2

(d) Iron element (Fe) which resembles cobalt and nickel elements in properties, was placed far away from these elements.

3.3 Similarity of Dobereiner’s law and Newlands’ classification

Dobereiner’s triads also exist in the columns of Newlands’ classification of elements based on the law of octaves. For example, the second column of Newlands’ classification has the elements lithium (Li), sodium (Na), and potassium (K) which constitute a Dobereiner’s triad.

4. MENDELEEV’S PERIODIC TABLE

Note: The periodic table is a chart of elements prepared in such a way that the elements having similar properties occur in the same vertical column or group. It is called periodic because the elements having similar properties are repeated after certain intervals or periods; and it is called a table because the elements are arranged in the tabular form. A periodic table consists of horizontal rows of elements called periods and vertical columns called groups. When elements are arranged in the order of increasing atomic masses, the elements with similar properties occur at regular intervals.

4.1 Mendeleev’s periodic law

The properties of elements are a periodic function of their atomic masses.

Mendeléev’s Periodic Table

WIN POINT

Page 3

4.2 Features of Mendeléev’s Periodic Table

(a) Mendeleev arranged all the then known 63 elements in the order of increasing atomic masses in horizontal rows.

(b) The similar properties used by Mendeleev to classify elements into groups were the similar formulae of their oxides and hydrides.

(c) There were seven periods (horizontal rows) and eight groups (vertical columns) in the original periodic table of Mendeleev.

(d) Noble gases were not known at that time. So, there was no group of noble gases in Mendeleev’s original periodic table.

(e) In order to make sure that the elements having similar properties fell in the same vertical column or group, Mendeleev left some gaps in his periodic table.

(f) The undiscovered elements (or unknown elements) at that time for which gaps were left in the periodic table were named by Mendeleev as eka-boron, eka-aluminium and eka-silicon by prefixing the term ‘eka’ to the name of the preceding element in the same group. When these elements were discovered later on, then eka-boron was named as scandium (symbol Sc), eka-aluminium was named as gallium (symbol Ga), and eka-silicon was named as germanium (symbol Ge).

(g) Again, in order to make sure that the elements having similar properties fell in the same vertical column (or group), Mendeleev placed a few elements in the wrong order of their atomic masses by keeping the element with higher atomic mass first and the element with lower atomic mass later.

4.3 Merits of Mendeleev’s Classification of Elements

(i) Mendeleev’s periodic law predicted the existence of some elements that had not been discovered at that time.

Properties of eka–aluminium and gallium

Property Eka-aluminium Gallium Atomic Mass 68 69.7

Formula of Oxide E2O3 Ga2O3 Formula of Chloride ECl3 GaCl3

(ii) Mendeleev’s periodic table could predict the properties of several elements on the basis of their positions in the periodic table.

(iii) Mendeleev’s periodic table could accommodate noble gases when they were discovered.

WIN POINT

Page 4

4.4 Anomalies (or Limitations) of Mendeleev’s Classification of Elements

(i) The position of isotopes could not be explained. The isotopes were not given separate places in Mendeleev’s periodic table. The isotopes are placed at the same place in the Mendeleev’s periodic table. For example, the element chlorine has two isotopes, Cl-35 and Cl-37, having atomic masses of 35 and 37 respectively.

(ii) Wrong order of atomic masses of some elements could not be explained. When certain elements were put in their correct group on the basis of their chemical properties, it was found that the element with higher atomic mass comes first and the element with lower atomic mass comes later.

(iii) A correct position could not be assigned to hydrogen in the periodic table. In Mendeleev’s periodic table, hydrogen (H) has been placed in group I with alkali metals. Hydrogen could also be placed in group VII of halogen elements. Mendeleev’s periodic law could not assign a correct position to hydrogen in the periodic table.

Compounds of H Compounds of Na HCl NaCl H2O Na2O H2S Na2S

5. PRESENT BASIS FOR THE

CLASSIFICATION OF ELEMENTS

The present basis for the classification of elements is the atomic number of elements. In 1913, Moseley showed that the atomic number of an element is a more fundamental property than atomic mass and hence atomic number is a better basis for the classification of elements. The atomic number of every element is fixed. No two elements can have the same atomic number.

5.1 Explanation of the Anomalies of Mendeleev’s Classification of Elements

When the elements are arranged according to their atomic numbers on the basis of modern periodic law, then all the anomalies (or defects) of Mendeleev’s classification disappear.

(i) Explanation for the position of isotopes. (ii) Explanation for the position of Cobalt and Nickel.

WIN POINT

Page 5

5.2 Modern Periodic Law

The modern periodic law is based on the atomic numbers of elements. According to modern periodic law: The properties of elements are a periodic function of their atomic numbers. When elements are arranged according to increasing atomic numbers, there is a periodicity in the electronic configurations of elements. The periodicity in electronic configurations of elements leads to the periodicity in their chemical properties.

5.3 Explanation of Modern Periodic Law

When the elements are arranged according to increasing atomic numbers, then the elements having same number of valence electrons occur at regular intervals (or periods). Since the number of valence electrons in the elements show periodicity (regular repetition), the chemical properties also show periodicity.

6. MODERN PERIODIC TABLE 6.1 Features

(a) The modern periodic table was prepared by Bohr. (b) It is also known as long form of periodic table. (c) The elements are arranged in the order of increasing atomic numbers in

horizontal rows called periods. (d) We place under each element all other elements having the same number of

valence electrons so that all the elements having same number of valence electrons come in the same vertical column or group.

(e) Since the elements having same number of valence electrons show similar properties, therefore, all the elements in a particular group of the periodic table have similar properties.

6.2 Periods

The horizontal rows of elements in a periodic table are called periods. There are seven periods in the long form of periodic table. The elements in a period have consecutive (continuous) atomic numbers.

1st period contains 2 elements. It is called very short period. 2nd period contains 8 elements. It is called a short period. 3rd period contains 8 elements. It is also a short period. 4th period contains 18 elements. It is a long period. 5th period contains 18 elements. It is also a long period. 6th period contains 32 elements. It is called very long period. 7th period contains rest of the elements. It is incomplete.

The number of elements in a period is fixed by the maximum number of electrons which can be accommodated in the various shells of an atom.

WIN POINT

Page 6

Tab

le 5

.6 M

oder

n P

erio

dic

Tab

le

6.3 Groups

The vertical columns in a periodic table are called groups. There are 18 groups in the long form of periodic table. These groups are numbered as 1 to 18. Group 1 is on the left side of the periodic table whereas group 18 is on the extreme right side of the periodic table. The elements in a group do not have consecutive atomic numbers. All the elements in a group have similar electronic configurations and show similar properties.

7. POSITION OF HYDROGEN

Hydrogen element has been placed at the top of group 1, above the alkali metals because the electronic configuration of hydrogen is similar to those of alkali metals. Both, hydrogen as well as alkali metals have 1 valence electron each. Since hydrogen atom is very small in size, many properties of hydrogen are different from those of alkali metals. Therefore while discussing the alkali metals of group 1, hydrogen is never included. In some of the periodic tables, however, hydrogen is not placed in any group. Hydrogen is treated as a very special element and placed alone at the head of the periodic table.

8. CHARACTERISTICS OF PERIODS

The physical and chemical properties of elements change on moving across a period. These are called characteristics of periods in the modern periodic table:

(a) Valence Electrons (or Outermost Electrons) On moving from left to right in a period, the number of valence electrons in elements increases from 1 to 8.

(b) Valency On moving from left to right in each short period, the valency of elements increases from 1 to 4 and then decreases to 0 (zero).

(i) The valency of an element is determined by the number of valence electrons (or outermost electrons) present in the atom of the element.

(ii) The number of electrons lost or gained (or shared) by one atom of an element to achieve the nearest inert gas electron configuration, gives us the valency of the element.

WIN POINT

Page 7

(c) Size of Atoms (or Atomic size) On moving from left to right in a period of the periodic table, the size of atoms decreases (or atomic size decreases).As we move from left to right in a period, the atomic number of elements increases which means that the number of protons and electrons in the atoms increases. Due to large positive charge on the nucleus, the electrons are pulled in more close to the nucleus and the size of atom decreases.

In any period, the alkali metal atom like Li, Na, etc., is the biggest in size. Halogen atom like F, Cl, Br, etc., is the smallest in size. The size of the atom of an inert gas is bigger than that of the preceding

halogen atom. The greater size of the inert gas atom (or noble gas atom) in a period is due to the structural stability of its outermost shell consisting of an octet of electrons.

(d) Metallic Character On moving from left to right in a period, the metallic character of elements decreases (but the non metallic character increases). On the left side in a period we have metals. On the right side in a period we have non-metals. Some elements in-between the metals and non-metals are known as

metalloids.

The greatest metallic character is found in the elements on the extreme left side of a period and the greatest non- metallic character is found in the elements on the right side of a period. On moving from left to right in a period, the electropositive character of elements decreases, but the electronegative character increases. On moving from left to right in a period, the tendency of atoms to lose electrons decreases. On moving from left to right in a period, the tendency of atoms to gain electrons increases.

(e) Chemical Reactivity On moving from left to right in a period, the chemical reactivity of elements first decreases and then increases. In the first element of the third period, sodium, there is 1 valence electron which it can lose easily to react with other substances, so it is very reactive. The second element magnesium has 2 valence electrons. It is not so easy for an atom to lose 2 electrons, so magnesuim is less reactive than sodium. Aluminium and silicon are still more unreactive (because they have 3 and 4 valence electrons respectively).

WIN POINT

Page 8

(f) Nature of Oxides On moving from left to right in a period, the basic nature of oxides decreases and the acidic nature of oxides increases.

9. CHARACTERISTICS OF GROUPS

The characteristics of elements in different groups are discussed below: (a) Valence Electrons (or Outermost Electrons)

All the elements of a group of the periodic table have the same number of valence electrons. Group 1 elements are monovalent (having valency of 1). On moving down in a particular group of the periodic table, the number of valence electrons in the elements remains the same. All the elements of group 2 have 2 valence electrons each in their atoms. Group 2 elements are divalent (having valency of 2). Group 13 elements have 3 valence electrons each, group 14 elements have 4 valence electrons each, group 15 elements have 5 valence electrons, and group 16 elements have 6 valence electrons each in their atoms. All the elements of group 17 have 7 valence electrons each in their atoms. Group 17 elements are monovalent and form negatively charged ions. All the elements of group 18 have 8 valence electrons each in their atoms, except helium which has only 2 valence electrons in its atom. All the elements of group 18 are zerovalent (having zero valency) and unreactive. If some elements have the same number of electrons (n electrons) in the outermost shell of their atoms, then they belong to the same group of the periodic table.

(b) Valency All the elements in a group have the same valency.

1. Valency of group 1 elements is 1 5. Valency of group 15 elements is 3 2. Valency of group 2 elements is 2 6. Valency of group 16 elements is 2 3. Valency of group 13 elements is 3 7. Valency of group 17 elements is 1 4. Valency of group 14 elements is 4 8. Valency of group 18 elements is 0

(c) Size of Atoms (or Atomic size)

On going down in a group of the periodic table, the size of atoms increases (or atomic size increases). Decrease in size of the atoms due to increased attraction between nucleus and electrons (due to increase in atomic number) is much less as compared to the increase in size due to the addition of an extra shell of electrons. In group 17 of halogens, the atomic size increases on going down from fluorine to iodine.

WIN POINT

Page 9

(d) Metallic Character

On going down in a group of the periodic table, the metallic character of elements increases. The greatest metallic character is found in the elements in the lowest part of a group. On going down in a group of the periodic table, the electropositive character of elements increases. The tendency of an atom to lose electrons increases on moving down in a group of the periodic table. The tendency of an atom to gain electrons decreases on going down in a group of the periodic table.

(e) Chemical Reactivity

The chemical reactivity of metals increases on going down in a group of the periodic table. The chemical reactivity of non- metals decreases on going down in a group of the periodic table.

(f) Nature of Oxides

On going down in a group of the periodic table, there is no change in the nature of oxides of elements. All the elements of group 1 (like Li, Na, K) form basic oxides, whereas all the elements of group 17 (like F, Cl, Br) form acidic oxides.

10. MERITS OF THE MODERN PERIODIC TABLE

The modern periodic table has following merits:

(a) The modern periodic table is based on the atomic numbers of elements which is the most fundamental property of elements.

(b) The modern periodic table helps us understand why elements in a group show similar properties.

(c) The modern periodic table explains the reasons for the periodicity in properties of elements.

(d) The modern periodic table tells us why the properties of elements are repeated after 2, 8, 18 and 32 elements. Since the electronic configurations of the elements are repeated after 2, 8, 18 and 32 elements, therefore, the properties of elements are also repeated after 2, 8, 18 and 32 elements.

(e) There are no anomalies in the arrangement of elements in the modern periodic table.

WIN POINT

Page 10

11. ADVANTAGES OF THE PERIODIC TABLE

There are following advantages of the periodic table: (a) The periodic table has made the study of chemistry systematic and easy. (b) It is easier to remember the properties of an element if its position in the

periodic table is known. (c) The type of compounds formed by an element can be predicted by knowing

its position in the periodic table. If an element is on the left side of the periodic table, it will be a

metal and hence form only ionic compounds. If an element is on the right side of the periodic table, then it will be

a non-metal and can form ionic as well as covalent compounds. (d) A periodic table chart is used as a teaching-aid in chemistry in schools and

colleges.

12. PERIODIC TABLE AND CHEMICAL BONDING

(a) When a metal combines with a non-metal, transfer of electrons takes place from metal atoms to non-metal atoms and an ionic bond is formed. Since metals are placed on the left side in the periodic table and non-metals are on its right side, we can say that when an element from the left side of the periodic table combines with an element from the right side of the periodic table, ionic bond is formed. Whenever an element from groups 1, 2 or 13 combines with an element from groups 14, 15, 16 or 17, an ionic bond is formed.

(b) When a non-metal combines with another non-metal, sharing of electrons takes place between their atoms and a covalent bond is formed. When an element from the right side of the periodic table combines with another element from the same side, a covalent bond is formed.

13. ELECTRONIC CONFIGURATION

Elements in the same vertical column or group have similar valence shell electronic configurations, the same number of electrons in the outer orbitals, and similar properties. For example, the Group 1 elements (alkali metals) all have ns1 valence shell electronic configuration as shown on next page.

WIN POINT

Page 11

14. IONIZATION ENTHALPY

Some atoms and ions which contain the same number of electrons are called isoelectronic species. For example, O2–, F–, Na+ and Mg2+ have the same number of electrons (10). Their radii would be different because of their different nuclear charges. The cation with the greater positive charge will have a smaller radius because of the greater attraction of the electrons to the nucleus. Anion with the greater negative charge will have the larger radius.

A quantitative measure of the tendency of an element to lose electron is given by its Ionization Enthalpy. It represents the energy required to remove an electron from an isolated gaseous atom (X) in its ground state. In other words, the first ionization enthalpy for an element X is the enthalpy change (iH) for the reaction depicted in following equation:

X (g) → X+ (g) + e– The ionization enthalpy is expressed in units of kJ mol–1. The second ionization enthalpy is defined as the energy required to remove the second most loosely bound electron; it is the energy required to carry out the reaction shown in given equation:

X+(g) → X2+ (g) + e– Energy is always required to remove electrons from an atom and hence ionization enthalpies are always positive. The second ionization enthalpy will be higher than the first ionization enthalpy because it is more difficult to remove an electron from a positively charged ion than from a neutral atom.

15. ELECTRON GAIN ENTHALPY

When an electron is added to a neutral gaseous atom (X) to convert it into a negative ion, the enthalpy change accompanying the process is defined as the Electron Gain Enthalpy (eg).Electron gain enthalpy provides a measure of the ease with which an atom adds an electron to form anion as represented by following equation:

X (g) + e– → X– (g)

WIN POINT

Page 12

Depending on the element, the process of adding an electron to the atom can be either endothermic or exothermic. For many elements energy is released when an electron is added to the atom and the electron gain enthalpy is negative. For example, group 17 elements (the halogens) have very high negative electron gain enthalpies because they can attain stable noble gas electronic configurations by picking up an electron. On the other hand, noble gases have large positive electron gain enthalpies because the electron has to enter the next higher principal quantum level leading to a very unstable electronic configuration.

16. ELETRONEGATIVITY

A qualitative measure of the ability of an atom in a chemical compound to attract shared electrons to itself is called electronegativity. Unlike ionization enthalpy and electron gain enthalpy, it is not a measureable quantity. Electronegativity generally increases across a period from left to right and decreases down a group in the periodic tale.

****

WIN POINT

Page 13

Class-10th (Chemistry) Topic: Periodic Classification of Elements

Worksheet

1. The three elements A, B and C with similar properties have atomic masses X, Y and Z respectively. The mass of Y is approximately equal to the average mass of X and Z. What is such an arrangement of elements called as ? Give one example of such a set of elements. 2. Elements have been arranged in the following sequence on the basis of their increasing atomic masses.

F, Na, Mg, Al, Si, P, S, Cl, Ar, K (a) Pick two sets of elements which have similar properties. (b) The given sequence represents which law of classification of elements? 3. Can the following groups of elements be classified as Dobereiner's triad ?

(a) Na, Si, Cl (b) Be, Mg, Ca Atomic mass of Be 9; Na 23; Mg 24; Si 28; Cl 35; Ca 40 Explain by giving reason. 4. In Mendeleev′’s Periodic Table the elements were arranged in the increasing order of their atomic masses. However, cobalt with atomic mass of 58.93 amu was placed before nickel having anatomic mass of 58.71 amu. Give reason for the same. 5. “Hydrogen occupies a unique position in Modern Periodic Table”. Justify the statement. 6. Write the formulae of chlorides of Eka-silicon and Eka-aluminium, the elements predicted by ′Mendeleev. 7. Three elements A, B and C have 3, 4 and 2 electrons respectively in their outermost shell. Give the group number to which they belong in the Modern Periodic Table. Also, give their valencies. 8. If an element X is placed in group 14, what will be the formula and the nature of bonding of its chloride ? 9. Compare the radii of two species X and Y. Give reasons for your answer. (a) X has 12 protons and 12 electrons (b) Y has 12 protons and 10 electrons 10. Arrange the following elements in increasing order of their atomic radii.

(a) Li, Be, F, N (b) Cl, At, Br I 11. Identify and name the metals out of the following elements whose electronic configurations are: (a) 2, 8, 2 (b) 2, 8, 1 (c) 2, 8, 7 (d) 2, 1 12. Write the formula of the product formed when the element A combines with the element B . Draw its electronic dot structure. What is the nature of the bond formed ? Atomic Numbers of A and B are 19 and 17 respectively

WIN POINT

Page 14

13. Arrange the following elements in the increasing order of their metallic character

Mg, Ca, K, Ge, Ga 14. Identify the elements with the following property and arrange them in increasing order of their reactivity

(a) An element which is a soft and reactive metal (b) The metal which is an important constituent of limestone (c) The metal which exists in liquid state at room temperature

15. Properties of the elements are given below. Where would you locate the following elements in the periodic table ?

(a) A soft metal stored under kerosene (b) An element with variable (more than one) valency stored underwater. (c) An element which is tetravalent and forms the basis of organic chemistry (d) An element which is an inert gas with atomic number 2 (e) An element whose thin oxide layer is used to make other elements corrosion resistant by the process of “ anodising”

16. An element is placed in 2nd Group and 3rd Period of the Periodic Table, burns in presence of oxygen to form a basic oxide.

(a) Identify the element (b) Write the electronic configuration (c) Write the balanced equation when it burns in the presence of air (d) Write a balanced equation when this oxide is dissolved in water (e) Draw the electron dot structure for the formation of this oxide

17. An element X (atomic number 17) reacts with element Y (atomic number 20) to form a divalent halide.

(a) Where in the periodic table are elements X and Y placed ? (b) Classify X and Y as metal (s), non-metal (s) or metalloid (s) (c) What will be the nature of oxide of element Y ? Identify the nature of bonding in the compound formed (d) Draw the electron dot structure of the divalent halide

18. Atomic number of a few elements are given below 10, 20, 7, 14 (a) Identify the elements (b) Identify the Group number of these elements in the Periodic Table (c) Identify the Periods of these elements in the Periodic Table (d) What would be the electronic configuration for each of these elements ? (e) Determine the valency of these elements

19. Which group of elements could be placed in Mendeleev's′ Periodic Table without disturbing the original order ? Give reason. 20. Give an account of the process adopted by ′Mendeleev for the classification of elements. How did he arrive at “Periodic Law” ?

WIN POINT

Page 15

21. In this ladder (as shown in figure) symbols of elements are jumbled up. Rearrange these symbols of elements in the increasing order of their atomic number in the Periodic Table. Also, arrange them in the order of their group.

22. Mendeleev′ predicted the existence of certain elements not known at that time and named two of them as Eka-silicon and Eka-aluminium.

(a) Name the elements which have taken the place of these elements (b) Mention the group and the period of these elements in the Modern Periodic Table. (c) Classify these elements as metals, non-metals or metalloids (d) How many valence electrons are present in each one of them ?

23. Read the following: Electropositive nature of the element(s) increases down the group and

decreases across the period Electronegativity of the element decreases down the group and increases

across the period Atomic size increases down the group and decreases across a period (left to

right) Metallic character increases down the group and decreases across a period.

On the basis of the above trends of the Periodic Table, answer the following about the elements with atomic numbers 3 to 9.

(a) Name the most electropositive element among them (b) Name the most electronegative element (c) Name the element with smallest atomic size (d) Name the element which is a metalloid (e) Name the element which shows maximum valency.

24. An element X which is a yellow solid at room temperature shows catenation and allotropy. X forms two oxides which are also formed during the thermal decomposition of ferrous sulphate crystals and are the major air pollutants.

(a) Identify the element X (b) Write the electronic configuration of X (c) Write the balanced chemical equation for the thermal decomposition of ferrous sulphate crystals ?

WIN POINT

Page 16

(d) What would be the nature (acidic/ basic) of oxides formed ? (e) Locate the position of the element in the Modern Periodic Table.

25. An element X of group 15 exists as diatomic molecule and combines with hydrogen at 773 K in presence of the catalyst to form a compound, ammonia which has a characteristic pungent smell.

(a) Identify the element X. How many valence electrons does it have ? (b) Draw the electron dot structure of the diatomic molecule of X. What type of bond is formed in it ? (c) Draw the electron dot structure for ammonia and what type of bond is formed in it ?

*****

WIN POINT

Page 17