(a) Identify the element
 Ans. Magnesium
(b) Write the electronic configuration
 Ans. 2, 8, 2
(c) Write the balanced equation when it burns in the presence of air
 Ans. 2 Mg +  O₂ → 2 MgO
(d) Write a balanced equation when this oxide is dissolved in water
 Ans. MgO + H₂O → Mg(OH)₂
(e) Draw the electron dot structure for the formation of this oxide
 Ans. Electron dot structure for the formation of magnesium oxide. 

 

(a) Where in the periodic table are elements X and Y placed?
Ans. X is in Group 17, period 3 and Y is in group 2, period 4
(b) Classify X and Y as metal (s), non-metal (s) or metalloid (s)
 Ans. X is a non-metal, While Y is a metal.
(c) What will be the nature of oxide of element Y? Identify the nature of bonding in the
compound formed
 Ans. Oxide of element Y would be basic. This compound is formed by ionic bond.
(d) Draw the electron dot structure of the divalent halide
 Ans.

(a) Identify the elements
 Ans. Neon (10), calcium (20), Nitrogen (7) and Silicon (14)
(b) Identify the Group number of these elements in the Periodic Table
 Ans. Neon belongs to group 18, calcium belongs to group 2, nitrogen belongs to group 7 and silicon belongs to group 14.
(c) Identify the Periods of these elements in the Periodic Table
 Ans. Nitrogen and Neon belong to period 2. Calcium and Silicon belong to period 3.
(d) What would be the electronic configuration for each of these elements?
 Ans. Ne: 2, 8
 Ca: 2, 8, 8, 2
 N: 2, 5
 Si: 2, 8, 4
(e) Determine the valency of these elements
 Ans. Ne: O, Ca: 2, N: 3 and Si: 4 

Across:
(1) An element with atomic number 12.
 Ans. Magnesium
 (3) Metal used in making cans and member of Group 14.
 Ans. Tin
(4) A lustrous non-metal which has 7 electrons in its outermost shell.
 Ans. Iodine.
 
Down:
(2) Highly reactive and soft metal which imparts yellow colour when subjected to flame and is kept in kerosene
 Ans. Sodium
(5) The first element of second Period
 Ans. Lithium 
 (6) An element which is used in making fluorescent bulbs and is second member of Group 18 in the Modern Periodic Table
 Ans. Neon
(7) A radioactive element which is the last member of halogen family.
 Ans. Astatine
(8) Metal which is an important constituent of steel and forms rust when exposed to moist air.
 Ans. Iron
(9) The first metalloid in Modern Periodic Table whose fibres are used in making bulletproof vests
 Ans. Boron 

(a) In this ladder (Figure 5.2) symbols of elements are jumbled up. Rearrange these symbols of elements in the increasing order of their atomic number in the Periodic Table.

 Ans. H, He, Li, Be, B, C, N, O, F, Ne, Na, Mg, Al, Si, P, S, Cl, Ar, K, Ca
 
(b) Arrange them in the order of their group also. Fig. 5.2
 Ans. Group 1: H, Li, Na< k
 Group 2: Be, Mg, Ca
 Group 13: B, Al
 Group 14: C, Si
 Group 15: N, P
 Group 16: O, S
 Group 17: F, Cl
 Group 18: He, Ne, Ar 

(a) Name the elements which have taken the place of these elements
 Ans. Eka-silicon was replaced by Germanium and Eka-aluminium was replaced by Gallium.
(b) Mention the group and the period of these elements in the Modern Periodic Table.
 Ans. Gallium: Group 13 Period 5
 Germanium: Group 14 period 5 
(c) Classify these elements as metals, non-metals or metalloids
 Ans. Gallium is metal, while Germanium is metalloid
(d) How many valence electrons are present in each one of them?
 Ans. Gallium has 4 valence electrons, while Germanium has 5. 

(a) Electropositive nature of the element(s) increases down the group and decreases across the period
(b) Electronegativity of the element decreases down the group and increases across the period
(c) Atomic size increases down the group and decreases across a period (left to right)
(d) 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
Ans. Element with 3 as atomic number, i.e. Lithium

(b) Name the most electronegative element
 Ans. Element with 9 as atomic number, i.e. Fluorine
 
(c) Name the element with smallest atomic size
 Ans. Fluorine
 
(d) Name the element which is a metalloid
 Ans. Boron (5)
 
(e) Name the element which shows maximum valency.
 Ans. Carbon, valency 4

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
 Ans. Sulphur
 
(b) Write the electronic configuration of X
 Ans. 2, 8, 6
 
(c) Write the balanced chemical equation for the thermal decomposition of ferrous sulphate crystals?
 Ans.2FeSO₄ →  Fe₂O₃ +SO₂ + SO₃ 
 
(d) What would be the nature (acidic/ basic) of oxides formed?
 Ans. Oxides of Sulphur are acidic in nature.
 
(e) Locate the position of the element in the Modern Periodic Table.
 Ans. Group 16, Period 3