Class 10 LAKHMIR SINGH AND MANJIT KAUR Solutions Chemistry Chapter 5 - Periodic Classification Of Elements

 (a) Mendeleev arrange the element in his periodic table on the basis of atomic masses.

(b) Now they are arranged on the basis of atomic numbers.

(a) False

(b) False

(c) False

Mendeleev said that the properties of elements are a periodic function of their atomic masses.

(a) Periods have elements with consecutive atomic numbers.

(b) Correct.

(c) Correct.

(a) Newlands.

(b) Dobereiner.

Six elements.

Mendeleev's basis for the classification of elements was the atomic mass of elements.

Mendeleev was guided by two factors:

(i) Increasing atomic masses.

(ii) Grouping together of elements having similar properties.

Gallium and Scandium.

Scandium, Gallium, Germanium.

Eka- Aluminium (gallium) and Eka-Silicon (germanium).

The elements lithium, sodium and potassium form a Dobereiner's triad. Lithium is the first element of this triad, sodium is the middle element whereas potassium is the third element of the triad. Sodium (middle element) has atomic mass 23.

According to Dobereiner,

Noble gases. Since they are chemically unreactive, so they got a place in the periodic table in the form of a separate group and hence did not disturb the original order of Mendeleev's periodic table .

(a) Atomic number.

(b) Periods.

(c) Alkali metals.

(d) Halogens.

(e) Noble Gases.

(f) Eighth.

(a) (i) The vertical columns in a periodic table are called groups.

      (ii) The horizontal rows of elements in a periodic table are called periods.

(b) There are seven periods and eighteen groups in the long form of periodic table.

(c) (i) Group 1: Lithium and Sodium.

     (ii) Group 17: Fluorine and Chlorine.

     (iii) Group 18: Neon and Argon.

(a) Lithium, Beryllium are metals.

(b) The real significance of atomic number in the modern periodic classification is that it relates the periodicity in the properties of elements to the periodicity in their electronic configurations.

Example: The atomic number increases from 3 in lithium to 11 in sodium, there is a repetition of electronic configuration from 2,1 to 2,8,1 (both having 1 valence electron).

(a) Position of isotopes: All the isotopes of an element have the same number of protons, so their atomic number is also the same. Since, all the isotopes of an element have the same atomic number; they can be put at one place in the same group of the periodic table. 

(b) Position of Cobalt and Nickel: The atomic number of cobalt is 27 and that of nickel is 28. According to modern periodic law, the elements are arranged in order of increasing atomic numbers. So, cobalt with lower atomic number (27) should come first and nickel with higher atomic number (28) should come later, even if their atomic masses are in the wrong order.

(c) Position of hydrogen: Hydrogen has been placed at the top of group 1, above the alkali metals in the modern periodic table because the electronic configuration of hydrogen is similar to those of alkali metals. Both, have 1 valence electron each.

(a) Left side.

(b) Right side.

(c) Metalloids.

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

For example: Alkali metal group (Dobereiner's triad): Lithium is the 1st element, sodium is the middle element whereas potassium is the 3rd element of the triad.

According to the 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.

For example: If we start with lithium as the first element, we find that the eighth element from it is sodium having the similar properties to lithium.

(a) Yes, Dobereiners triads also exist in the columns of Newlands' Octaves.

 Consider the elements lithium (Li), sodium (Na) and potassium (K) which are present in the second column of Newlands' classification of elements. Now, if we start with lithium as the 1st  element, then the 8th  element from it is sodium, and according to Newlands' law of octaves, the properties of 8th element, sodium should be similar to those of the 1st element, lithium. Again, if we take sodium as the 1st element, then the 8th element from it is potassium, and according to Newlands' law of octaves, the properties of 8th element, potassium should be similar to those of the 1st element, sodium. This means that according to Newlands' law of octaves, the elements lithium, sodium and potassium should have similar chemical properties. We also know that lithium, sodium and potassium form a Dobereiner's triad having similar chemical properties. From this, we conclude that Dobereiners triads also exist in the columns of Newlands Octaves.

(b) The main limitation of Dobereiner's classification of elements was that 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. So, his classification of elements was not much successful. Another limitation was that Dobereiner failed to explain the relation between atomic masses of elements and their chemical properties.

(c) Newlands' law of octaves for the classification of elements had the following limitations:

(i) Newlands' law of octaves was applicable to the classification of elements up to calcium only. After calcium, every eighth element did not possess the properties similar to that of the first element. Thus, this law worked well with lighter elements only.

(ii) Newlands assumed that only 56 elements existed in nature and no more elements would be discovered in the future. But later on, several new elements were discovered whose properties did not fit into Newlands' law of octaves.

(iii) 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, the two elements cobalt (Co) and nickel (Ni) were put together in just one slot and that too in the column of elements like fluorine, chlorine and bromine which have very different properties from these elements.

(a) According to Mendeleev's periodic law: The properties of elements are a periodic function of their atomic masses. It was the discovery of atomic number which led to a change in Mendeleev's periodic law which was based on atomic mass.

(b) The noble gases are placed in a separate group because they are chemically very inert or unreactive (having completely filled outermost electron shells).

(a) 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.

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

(iii) It could accommodate noble gases when they were discovered.

(b) Anomalies of Mendeleev's classification of elements:

(i) The position of isotopes could not be explained: If the elements are arranged according to atomic masses, the isotopes should be placed in different groups of the periodic table. But, the isotopes were not given separate places in Mendeleev's periodic table. They were placed at the same place in the table. This placing of the isotopes at same place could not be explained by Mendeleev's periodic law.

(ii) Wrong order of atomic masses of some elements could not be explained: In Mendeleev's periodic table, 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. Mendeleev's periodic law could not explain this abnormal situation of wrong order of atomic masses.

(a) Eka-aluminium and gallium are the two names of the same element as Eka-Aluminum has almost exactly the same properties as the actual properties of the gallium element. The properties: atomic mass, density, melting point, formula of chloride and formula of oxide are almost the same.

(b) (i) Eka boron.

      (ii) Eka aluminum.

      (iii) Eka-silicon.

(a) The elements are classified into groups so that the elements with similar properties fall in the same group and hence the study of a large number of elements is reduced to the study of a few group of elements.

(b) (i) Increasing atomic masses

      (ii) Grouping together of elements having similar properties.

(c) 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.

(d) Out of eight groups in the original periodic table of Mendeleev, first seven groups are of normal elements and eighth group is of transition elements. Noble gases were not known at that time. So, there was no group of noble gases in Mendeleev's table.

(e) The isotopes of chlorine, Cl-35 and Cl-37 are placed in the same slot because they have similar chemical properties and same atomic number.

(a) Mendeleev's periodic law: The properties of elements are a periodic function of their atomic masses. It was the discovery of atomic number which led to a change in Mendeleev's periodic law which was based on atomic mass.

(b) The elements having similar chemical properties form oxides and hydrides having similar formulae. Mendeleev used these properties for creating his periodic table.

(c) Limitations of Mendeleevs classification of elements:

(i) The position of isotopes could not be explained.

(ii) Wrong order of atomic masses of some elements could not be explained.

(iii) A correct position could not be assigned to Hydrogen in the periodic table.

(d) Silicon and Germanium.

(e) Noble gases were missing from Mendeleev's original periodic table.

(a) The modern periodic law states that the properties of elements are a periodic function of their atomic numbers.

(b) When elements are arranged according to increasing atomic numbers, there is a periodicity in the electronic configurations of the elements. The elements in a period have consecutive atomic numbers. The elements having same number of valence electrons in their atoms are placed in a group. All the elements in a group have similar electronic configurations and show similar properties.

(c)  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. This is discussed below:

(i) Explanation for the Position of Isotopes: All the isotopes of an element have the same number of protons, so their atomic number is also the same. Since all the isotopes of an element have the same atomic number, they can be put at one place in the same group of the periodic table. For example, both the isotopes of chlorine, Cl-35 and Cl-37, have the same atomic number of 17, so both of them can be put at one place in the same group of the periodic table.

(ii) Explanation for the Position of Cobalt and Nickel: The atomic number of cobalt is 27 and that of nickel is 28. Now, according to modern periodic law, the elements are arranged in the order of increasing atomic numbers. So, cobalt with lower atomic number (27) should come first and nickel with higher atomic number (28) should come later, even if their atomic masses are in the wrong order.

(iii) Explanation for the 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.

(d) Atomic number is always a simple whole number. It can either be 1 or 2. There can be no element with atomic number 1.5.

(e) The modern periodic table was prepared by Bohr.

(a) Lithium, Sodium, Potassium.

(b) Magnesium, Calcium.

(c) Helium, Neon, Argon.

(a)

(b) The average atomic mass of elements X and Z is equal to the atomic mass of element Y.

(c) Dobereiner's law of triads.

(d) X is lithium, Y is sodium and Z is potassium.

(e) Chlorine, Bromine, Iodine.

Sodium does not belong to the set. This is because all other elements belong to group 2 but sodium belongs to group 1.

Chlorine does not belong to the set. This is because all other elements belong to 2nd period whereas chlorine belongs to 3rd period.

(a) No. This is because the elements Na, Si and Cl do not have similar properties even though the atomic mass of middle element Si is almost equal to the average atomic mass of first element Na and third element Cl.

(b) Yes. This is because the elements Be, Mg and Ca have similar properties and the atomic mass of middle element Mg is almost equal to the average atomic mass of first element Be and third element Ca.

(a) Same period (Third period): Na, Mg, Al.

(b) Same group (First group): Li, Na, K.

(a) Neon (2, 8).

(b) Magnesium.

(c) Silicon (2, 8, 4).

(d) Boron (2, 3).

(e) Carbon (2, 4).

Li, Na, K : All these elements are metals having a valency of 1.

Ca, Sr, Ba : All these elements are metals having a valency of 2.

Cl, Br, I : All these elements are halogens.

(a) (i) Gallium (ii) Germanium.

(b) 4th period.

(c) Gallium: 13th group; Germanium: 14th group.

(d) Gallium: Metal; Germanium: Metalloid.

(e) Gallium: 3 ; Germanium: 4.

(a) Newlands' law of octaves.

(b) Newlands.

(c) This classification of elements is compared with a characteristic of musical scale because in this classification, the repetition in the properties of elements is just like the repetition of eighth note in an octave of music.

(d) This classification of elements could be applied only up to the element calcium and not beyond that.

(i) Metallic character decreases.

(ii) Atomic size decreases.

On moving from left to right in a period, the tendency of atoms to gain electrons increases.

On moving from left to right in a period, the tendency of atoms to lose electrons decreases.

(a) In group 1 of alkali metals, the chemical reactivity increases from lithium to francium.

(b) In group 17 of halogen elements, the chemical reactivity decreases from fluorine to iodine.

The element fluorine is in group 17 of the periodic table and has a valency of 1. So, all the elements in the same group of periodic table as fluorine will have a valency of 1.

(a) 1

(b) 8

False

The first elements in the periods of the periodic table have 1 valence electron. Such elements are called alkali metals.

On moving from left to right in a period, the atomic size decreases.

On going down in a group of the periodic table, the metallic character of elements increases.

(i) The number of valence electrons increases from 1 to 2 in the 1st period of the periodic table.

(ii) The valence electrons increase from 1 to 8 in the 2nd period of the periodic table.

The valency of elements increases from 1 to 4 and then decreases to zero in the 3rd period.

All the elements in a group have the same valency while going down the group.

(a) Sodium.

(b) Fluorine.

2, 1 ; 2, 2 ; 2,3 ; 2,4 ; 2, 5 ; 2, 6 ; 2, 7 ; 2, 8.

F < N < Be < Li.

Ga < Mg < Ca < K.

(i) Elements in the same group have equal valency.

(ii)The metallic character of elements in a period decreases gradually on moving from left to right.

N (2,5) ; P (2, 8, 5) ; Nitrogen will be more electronegative because its atom has small size due to which the attraction of its nucleus for the incoming electron is more.

(a) 2.

For groups 1 and 2, the number of valence electrons is equal to the group number.

(b) 2.

Valency is determined by the number of valence electrons present in the atom of the element.

(c) 5.

For groups 13 to 18, the number of valence electrons is equal to (group no. - 10).

(d) 3.

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

(a) The horizontal rows of elements in a periodic table are called periods. As we move from left to right in a period, the atomic number of elements increases which means that the no. of protons and electrons in the atom increases. Due to large positive charge on the nucleus, the electrons are pulled in more close to the nucleus and the size of the atom decreases.

(b) (i) On moving from left to right in a period, the chemical reactivity of elements first decreases and then increases.

Example: In the 3rd period of elements, sodium is a very reactive element, magnesium is less reactive whereas aluminium is still less reactive. Silicon is the least reactive in the third period. Now, phosphorus is quite reactive, sulphur is still more reactive whereas chlorine is very reactive.

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

Example: In the 3rd period of the periodic table, sodium oxide is highly basic in nature and magnesium oxide is comparatively less basic. The aluminium and silicon oxides are amphoteric in nature. Phosphorus oxides are acidic, sulphur oxides are more acidic whereas chlorine oxides are highly acidic in nature.

(a) On moving from left to right in a period of the periodic table, the atomic size decreases. As we move from left to right in a period, the atomic number of elements increases which means that the no. 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.

(b) On moving from left to right in a period, the metallic character of elements decreases.

(a) Periods

(b) Increases

(c) Decreases

(d) Increases

(e) Decreases

(a) (i) All the elements of a group have similar chemical properties because they have same no. of valence electrons in their outermost shell.

(ii) All the elements of a period have different chemical properties because they have different no. of valence electrons in their atoms.

(b) Order of atomic numbers of elements: X < Z < Y. Because as the atomic number increases in a period from left to right, the size of atoms goes on decreasing

(a) On going down in a group of the periodic table, the electropositive character of elements increases.

(b) (i) In a group, all the elements have the same valency.

(ii) In a period, on moving from left to right, the valency of elements first increases from 1 to 4 and then decreases to zero.

(a) The fundamental difference between the electronic configuration of group 1 and group 2 elements is that group 1 elements have 1 valence electron in their atoms whereas group 2 elements have 2 valence electrons in their atoms.

(b) (i) All the chemically similar elements will have same valence electrons.

(ii) The 1st element in a period is determined by the no. of valence electrons in its atoms. The 1st element of every period has 1 valence electron.

(a) Usual number of valence electrons is 8; Valency is 0 (zero).

(b) The number of valence electrons remains the same.

(a) The main characteristic of last elements in a period is that they all have 8 valence electrons in their atoms except helium. Such elements are called noble elements.

(b) (i) 2 (ii) 8.

(a) On going down in a group of the periodic table, the atomic size increases. When we move from top to bottom in a group, a new shell of electrons is added to the atoms at every step due to which the size of atom increases.

(b) The similarity in the atoms of lithium, sodium and potassium is that all of them have 1 valence electron each.

(a) The tendency of an atom to lose electrons increases on moving down in a group of the periodic table. As we go down in group 1, one more electron shell is added at every stage and the size of the atom increases. The valence electrons become more and more away from the nucleus and hold of the nucleus on valence electrons decreases. Due to this, the atoms can lose valence electrons more easily to form positive ions and hence electropositive character increases.

(b) The tendency of an atom to gain electrons decreases on going down in a group of the periodic table. When we move from top to bottom in group 17, a new shell of electrons is added to the atoms at every step, due to which the size of atom increases. The nucleus goes more deep inside the atom due to which the attraction of nucleus for the incoming electron decreases due to which the atom cannot form negative ions easily and hence the electronegative character decreases.

(a) As we move from Na to Cl in the 3rd period, the size of the atoms of the elements decreases. Na atom is the biggest whereas Cl atom is the smallest in size. As we move from left to right in a period, the atomic no. of elements increases i.e. the number of protons and electrons in the atoms increases. Due to large positive charge on nucleus, the electrons are pulled in more close to the nucleus and thus the size of the atom decreases from Na to Cl.

(b) Helium and neon atoms have completely filled outermost electron shells (containing the maximum number of electrons which can be accommodated in them).

(a) Modern periodic table arranges the elements according to increasing atomic numbers. So, the atomic number of cobalt (27) comes first whereas the atomic number of nickel (28) comes later.

(b) In Mendeleev's periodic table, hydrogen has been placed in group I since like alkali metals, hydrogen also combines with halogens, oxygen and sulphur to form compounds having similar formulae. This means that hydrogen resembles alkali metals in some of the properties.

Hydrogen also resembles halogens in some of the properties. So, hydrogen could also be placed in group VII of halogen elements.

Thus, Mendeleev's periodic law could not assign a correct position to hydrogen in the periodic table.

(a) The horizontal rows of elements in a periodic table are called periods.

Characteristics:

(i) The elements in a period have consecutive atomic numbers.

(ii) The no. of elements in period is fixed by the maximum no. of electrons which can be accommodated in various shells.

The vertical columns in a periodic table are called groups.

Characteristics:

(i) The elements in a group do not have consecutive atomic numbers.

(ii) All the elements in a group have similar electronic configurations and show similar properties.

(b) The size of atom decreases on moving from left to right in a period. As we move from left to right in a period, the atomic number of elements increases which means that the no. of protons and electrons in the atom increases. The electronic configuration of the atoms increases in the same shell. Due to large positive charge on the nucleus, the electrons are pulled in more close to the nucleus and the size of the atom decreases.

On going down in a group of the periodic table, the atomic size increases. The no. of electron shells in the atoms gradually increases and the electronic configuration also increases due to which the atomic size increases.

(c)

(i) The atomic size increases gradually from lithium to francium.

(ii) The metallic character increases from lithium to francium.

(d) On going down in a group of the periodic table, the atomic size and metallic character increases. When we move down from top to bottom in group 1 of alkali metals, the size of atoms increases gradually from lithium to francium.

In group 1 of alkali metals, lithium is the least metallic element whereas francium is the most metallic element.

(a) The 1st period has two elements because the 1st electron shell of an atom can take a maximum of two electrons only. The 2nd period of the periodic table has 8 electrons because the maximum no. of electrons which can be put in the 2nd shell of an atom is 8.

(b) The elements in the same group show similar properties because they have similar electronic configuration (having the same number of valence electrons) whereas the elements of different groups have different electronic configurations (different number of valence electrons) due to which they show different properties.

     (ii) Li; Li

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

(ii) It helps us to understand why elements in a group show similar properties but elements in different groups show different properties.

(iii) It explains the reasons for the periodicity in properties of elements.

(iv) It tells us why the properties of elements are repeated after 2, 8, 18 and 32 elements.

(a) The vertical columns in a periodic table are called groups.

(i) The greatest metallic character is found in the elements in the lowest part of the group.

(ii) The largest atomic size is found in the lowest part of the group.

(b)  Group 1 elements have 1 valence electron and are ionic in chemical reactions. Their chemical reactivity increases down the group. They are electropositive in nature and it increases down the group.

Whereas, the elements of group 17 have 7 valence electrons. They all are non-metals. Their chemical reactivity decreases down the group. They are electronegative in nature and it decreases down the group.

(c) The no. of valence electrons in the atoms of elements decides which element will be the 1st element in a period and which will be the last in a period.

(d) The properties of elements are repeated after 2, 8, 18 and 32 elements in the periodic table because the electronic configurations of the elements are repeated in this manner.

(e) Advantages of the periodic table:

(i) It has made the study of chemistry systematic and easy.

(ii) It is easier to remember the properties of an element if its position in the periodic table is known.

(iii) The type of compounds formed by an element can be predicted by knowing its position in the periodic table.

(iv) It is used as a teaching aid in chemistry in schools and colleges.

(i) X and Z.

X and Z have zero valency hence they belong to same group: noble gases.

(ii) Y and Z.

Y: 2,4 and Z: 2,8 so, both of them belong to second period with two shells filled.

(a) 9.

(b) 17Cl.

(c) Both have the same number of valence electrons (7 electrons each) in their atoms.

(i) 18Ar and 2He (Noble gases).

(ii) 20Ca and 4Be (no. of valence electrons in each = 2).

(iii) 8O and 16S (no. of valence electrons in each = 6).

(a) Mg since atomic size decreases from left to right in a period.

(b) K since atomic size increases on going down a group.

(i) C (2, 8).

(ii) B (2, 5).

(iii) A (2, 3).

(iv) 2nd period (2 shells are filled).

(a) 2.

(b) 2.

(c) Metal.

(d) Magnesium.

(i) a (size decreases from left to right in a period).

(ii) k (valency of k = 3; valency of o = 1).

(iii) i (metallic character decreases from left to right in a period).

(iv) g (non-metallic character increases from left to right in a period).

(v) b (or j).

(vi) f (or n).

Valency of X = 2

(a) XCl2

(b) XO

(i) Non-metal.

(ii) 6.

(iii) 2.

(iv) Oxygen.

(v) Na2Y.

(a) Group 1 (2, 8, 8, 1).

(b) Oxygen (X is monovalent so Y has to be divalent to form the compound X₂Y).

(a) 2.

(b) Group 2.

(c) XCl2.

(a) A in group 14; B in group 17.

(b) Covalent bond.

(c) AB4.

C and D.

(a) XY.

(b) Ionic bond.

Valency of group 1 metals is 1 so it will react with oxygen (valency = 2) to form X₂O.

(i) Covalent bond is formed between two non-metals (A and B).

(ii) AB2.

(a) Ionic compound.

(b) Yes.

(c) XY2.

(d) 2.

(e) 7.

(i) d.

(ii) c.

(iii) e.

(iv) Covalent bond.

(v) Ionic bond.

(a) B and C.

(b) A and C.

Argon atom, 18 electrons.

(a) 3rd period.

(b) Ionic compound.

(c) A and B.

(d) H.

(e) CG3.

Sodium (Na) and Potassium (K);

Sodium (Na) is a metal. So, sodium readily reacts with a halogen like chlorine (Cl) to form an ionic chloride called sodium chloride. This is illustrated below:

Ionic bond; Ionic compounds.

Physical properties of ionic compounds:

(i) Ionic compounds are usually hard, brittle.

(ii) They conduct electricity when molten or dissolved.

(iii) They have high melting and boiling points.

(iv) Most are soluble in polar solvents such as water.

(a) A is carbon (C); B is carbon monoxide (CO) ; C is carbon dioxide (CO2).

(b) 14th group.

(c) Silicon (Si).

(a) X is nitrogen gas, N2; Y is ammonia gas, NH3 and Z is ammonium sulphate, (NH4)2SO4.

(b) 15th group.

(c) 2nd period.

(d) Carbon, C.

(e) Oxygen, O.