Solutions of Periodic Classification Of Elements (Page No - 304) - Chemistry Lakhmir Singh, Class 10 | Extra Documents, Videos & Tests for Class 10 PDF Download

Question 25:
(a) Explain why :
 All the elements of a group have similar chemical properties.
 All the elements of a period have different chemical properties.
 The atomic radii of three elements X, Y and Z of a period of the periodic table are 186 pm; 104 pm and 143 pm respectively. Giving a reason, arrange these elements in the increasing order of atomic numbers in the period.
 Solution :
(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

Question 26:
(a) How does the electropositive character of elements change on going down in a group of the periodic table ?
 State how the valency of elements varies (i)in a group, and (ii)in a period, of the periodic table.
 Solution :
(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.

Question 27:
(a) What is the fundamental difference in the electronic configurations between the group 1 and group 2 elements ?
 (b) On the basis of electronic configuration, how will you identify :
 (i) chemically similar elements ?
 (ii) the first element of a period ?
 Solution :
(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.

Question 28:
(a) What is the usual number of valence electrons and valency of group 18 elements of the periodic table ?
 (b) What happens to the number of valence electrons in the atoms of elements as we go down in a group of the periodic table ?
 Solution :
(a) Usual number of valence elec trons is 8; Valency is 0 (zero).
(b) The number of valence electrons remains the same .

Question 29:
(a) What is the main characteristic of the last elements in the periods of the periodic table ? What is the general name of such elements ?
 (b) What is the number of elements in : (a) 1st period, and (b) 3rd period, of the modern periodic table ?
 Solution :
(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.

Question 30:
(a) How does the atomic size vary on going down from top to bottom in a group of the periodic table ? Why does it vary this way ?
 (b) Lithium, sodium and potassium are all metals that react with water to liberate hydrogen gas. Is there any similarity in the atoms of these elements ? Explain your answer.
 Solution :
(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.

Question 31:
 (a) How does the tendency to lose electrons change as we go down in group 1 of the periodic table ? Why does it change this way ?
 (b) How does the tendency to gain electrons change as we go down in group 17 of the periodic table ? Why does it change this way ?
 Solution :
(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.

Question 32:
(a) Why does the size of the atoms progressively become smaller when we move from sodium (Na) to chlorine (Cl) in the third period of the periodic table ?
 (b) Helium and neon are unreactive gases. What, if anything, do their atoms have in common ?
 Solution :
(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) .

Question 33:
(a) In the modern Periodic Table, why does cobalt with higher atomic mass of 58.93 appear before nickel having lower atomic mass of 58.71 ?
 (b) Why could no fixed position be given to hydrogen in Mendeleev’s periodic table ?
 Solution :
(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.

Question 34:
 (a) What are the periods and groups in a periodic table ? Give two characteristics of each.
 (b) In terms of electronic configurations, explain the variation in the size of the atoms of the elements belonging to the same period and same group.
 (c) Given alongside is a part of the periodic table. As we move vertically downward from Li to Fr :
 (i) What happens to the size of atoms ?
 What happens to their metallic character ?
 (d) Name two properties of elements whose magnitudes change when going from top to bottom in a group of the periodic table. In what manner do they change ?
 (e) Rewrite the following statement after correction, if necessary :
 Groups have elements with consecutive atomic numbers.

Solution :
(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.
(e) Periods have elements with consecutive atomic numbers .