CBSE Class 10 Science Periodic Classification Of Elements Assignment Set B

Question. The electrons in the atoms of four elements A, B, C and D are distributed in three shells havingl, 3, 5 and 7 electrons in the outermost shell respectively. State the period in which these elements can be placed in the modern periodic table. Write the electronic configuration of the atoms of A and D and the molecular formula of the compound formed when A and D combine.
Answer : A: 2, 8, 1
B: 2, 8, 3
C: 2, 8, 5
D: 2, 8, 7g
They belong to the 3rd period.

AD is the molecular formula.

Question. Examine elements of the third period: Na, Mg, Al, Si, P, S, Cl and Ar Answer the following:
a. Choose (i) Metals, (ii) Non-metals out of these elements.
b. On which side of periodic table we find (i) metals (ii) non-metals.
c. Name metalloids out of the elements given above.
Where are they located in the periodic table?
Answer : a. (i) Na, Mg, A1 are metals.
(ii) P, S, Cl, Ar are non-metals.
b. (i) Metals are placed on the left hand side and middle part of the periodic table.
(ii) Non-metals are placed on the right hand side of periodic table.
c. Silicon is a metalloid. They are located between metals and non-metals at the border line in a zigzag manner.

Question. Two elements P and Q belong to the same period of the modern periodic table and are in Group-1 and Group-2 respectively. Compare their following characteristics in tabular form:
a. The number of electrons in their atoms.
b. The size of their atoms.
c. Their metallic character.
d. Their tendency to lose electrons.
e. The formula of their oxides.
f. The formula of their chlorides.
Answer : a. P has 1 valence electron, Q has 2 valence electrons.
b. P is bigger than Q.
c. P is more metallic than Q.
d. P can lose electrons more easily than Q.
e. P₂O and QO
f. PCl and QCl₂

Question. Explain the variation in the following properties of the elements in periodic table:
a. Atomic radius in the periodic table,
b. Metallic character in a period,
c. Valency in a group.
Answer : a. Atomic radius and Metallic character increases down the group. Valency remains the same in a group.
b. Atomic size and metallic character decrease along
a period from left to right in a period.

Question. Write the main aim of classifying elements. Name the basic property of elements which is used in the development of Modern Periodic Table. State the Modern Periodic Law. On which side (part) of the Modern Periodic Table do you find metals, metalloids and non- metals?
Answer : Classification is done so as to study the properties of 118 elements easily. Modern periodic table is based on atomic number of the atom an element. ‘Properties of elements are a periodic function of their atomic numbers.’ Metals are placed on the left and middle, non-metals are placed on the right and Metalloids are placed on the border line between metals and nonmetals in a zig-zag manner.

Question. The position of three elements A, B and C in the Periodic Table is shown below:

Giving reasons, explain the following:
a. Element ‘A’ is a non-metal.
b. Element ‘B’ has a larger atomic size than element ‘C’.
c. Element ‘C’ has a valency of 1.
Answer : a. A is a non-metal because it has 7 valence electrons, it can gain one electron to form anion.
b. B has less electrons and protons, less forces of attraction between the nucleus and valence electrons, therefore it is bigger in size.
c. C can gain 1 electron to become stable, therefore its valency is equal to 1.

Question. Atoms of seven elements A, B, C, D, E, F and G have a different number of electronic shells but have the same number of electrons in their outermost shells. The elements A and C combines with chlorine to form an acid and common salt respectively. The oxide of element A is liquid at room temperature and it is a neutral substance, while the oxides of the remaining six elements are basic in nature. Based on the above information, answer the following question:
a. What could the element A be?
b. Will elements A to G belong to the same period or same group of the periodic table?
c. Write the formula of the compound formed by the reaction of the element A with oxygen.
b. What would be the ratio of number of combining atoms in a compound formed by the combination of element A with carbon?
e. Which one of the given elements is likely to have the smallest atomic radius?
Answer : a. A is hydrogen.
b. A and G will belong to the same group.
c. H₂O is the formula of the compound.
d. CA₄ i.e., 1:4
e. A has smallest atomic radius.

Question. An element M with electronic configuration (2,8,2) combines separately with (NO₃)^-, (SO₄)^2- and (SO₄)^3-  radicals. Write the formula of the three compounds so formed. To which group and period of the Modern Periodic Table does the element M belong to? Will M form covalent or ionic compounds? Give reason to justify your answer.
Answer : M(2, 8, 2). It has valency equal to 2.

M belongs to group 2.
It belongs to 3rd period.
M will form ionic compound because M can easily lose electrons.
Bond will be formed by transfer of electrons.

Question. Given below are some elements of the Modern Periodic Table. Atomic number of the element is given in the parentheses:
A(4), B(9), C(14), D(19), E(20)
a. Select the element that has only one electron in the outermost shell. Also write the electronic configuration of this element.
b. Which two elements amongst these belong to the same group? Give reason for your answer.
c. Which two elements amongst these belong to the same period? Which among the two has bigger atomic radius?
Answer : a. D(19): 2,8,8,1 has 1 valence electron.
b. A(4): 2, 2 and E(20): 2, 8, 8, 2 belong to same group because they have the same number of valence electrons.
c. A(2, 2), B(2, 7) belong to the same period. A has bigger atomic radius than B.

Question. a. Why did Mendeleev leave gaps in his periodic table?
b. State any three limitations of Mendeleev’s periodic table.
c. How do the electronic configuration of atoms change in a period with increase in atomic number.
Answer : a. He left gaps for the new elements which are to be discovered.
b. (i) Increasing order of atomic mass could not be maintained.
(ii) Position of hydrogen was not justified.
(iii) Isotopes could not be placed in different slots due to different atomic mass but same properties.
c. Number of valence electrons keep on increasing along a period from left to right in a period.

Question. a. Predict which of the following will form anions and which will form cations.
(i) Na, (ii) Al, (iii) Cl, (iv) O
b Name two elements that are inert.
Answer : a. Cl and O will form anions i.e., Cl^- and O^2- Na and A1 will form cations i.e., Na⁺ and Al₃⁺.
b. He and Ne are inert elements.

Question. Two elements X and Y have atomic 12 and 17 respectively.
a. Write the electronic configuration of both these elements.
b. Which type of bond will they form?
Answer : a. X: 2, 8, 2; Y: 2, 8,7
b. They will form ionic bond.

Question. Out of the elements H(1), Be(4), Na(11) and Mg(12).
a. Write the pair of elements having similar chemical properties.
b. State the group number of each pair,
c. Name one another element belonging to each of these groups.
Answer : a. Be(4) and Mg(12) have similar chemical properties.
H( 1) and Na(ll) have similar chemical properties.
b. Be and Mg belong to group 2, H and Na belong to group 1.
c. K belongs to group 1 and Ca belongs to group 2.

Question. Calcium is an element with atomic number 20. Stating the reason, answer each of the following questions:
a. Is calcium a metal or a non-metal?
b. Will its atomic radius be larger or smaller than that of potassium with atomic number 19?
c. Write the formula of its oxide.
Answer : a. Calcium is a metal because it can lose electrons to form cations.
b. Its atomic radius will be smaller due to more number of protons and electrons, more forces of attraction.
c. CaO

Question. a. How does electropositivity of elements gets affected as we move (i) down the group, (ii) across the period?
b. Which atomic property increases both ways: as we move across the period or down the group?
Answer : a. (i) Electropositivity increases down the group.
(ii) Electropositivity decreases across the period from left to right.
b. Atomic number increases across the period as well as down the group.

Question. Study the following table in which positions of six elements A, B, C, D, E and F are shown as they are in the modern periodic table:

On the basis of the above table, answer the following questions:
a. Name the element which forms only covalent compounds.
b. Which element is a metal with a valency of one?
c. Which element is a non-metal with a valency of two?
d. Out of D and E, which has a bigger atomic radius and why?
Answer : a. E forms only covalent compounds.
b. D is a metal with valency of 1.
c. B is a non-metal with valency of 2.
d. D has bigger atomic size because it has less effective nuclear charge.

Question. The electronic configuration of two elements A and B are 2, 8, 3 and 2, 8, 7 respectively. Find the atomic number of these elements. State the nature and formula of the compound formed by union of these elements.
Answer : A has 2 + 8 + 3 = 13 as its atomic number B has 2 + 8 + 7 = 17 as its atomic number 

AB₃ is the formula of compound. It is an ionic compound.

Question. As we move across a period in the periodic table, what is gradation in the following properties: (a) Atomic size, (b) Atomic number, (c) Electronegativity?
Answer : a. Atomic size goes on decreasing across the period from left to right.
b. Atomic number goes on increasing along the period from left to right.
c. Electronegativity goes on increasing along a period from left to right.

Question. An element X belongs to 3rd period and group 16 of the Modern Periodic table.
a. Determine the number of valence electrons and the valency of X.
b. Molecular formula of the compound of X when it reacts with hydrogen and write its electron dot diagram.
c. Name the element X and state whether it is metallic or non-metallic.
Answer : a. X has 6 valence electrons and its valency is equal to 2.
b. H₂X is its formula,
c. X is sulphur. It is a non-metallic element.

Question. Why do all elements of the
a. same group have similar properties?
b. same period have different properties?
Answer : a. It is due to same number of valence electrons which will decide the chemical properties.
b. They differ in number of valence electrons, therefore they differ in chemical properties. They have the same number of shells.