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
(a) Identification of the element: The element placed in Group 2 and Period 3 is magnesium (Mg).
(b) Electronic configuration: \( 1s^2 \ 2s^2 \ 2p^6 \ 3s^2 \). Using shell representation: K = 2, L = 8, M = 2.
(c) Balanced equation for burning in air:
\[ 2Mg(s) + O_2(g) \rightarrow 2MgO(s) \]
(d) Reaction of the oxide with water:
\[ MgO(s) + H_2O(l) \rightarrow Mg(OH)_2(aq) \]
(e) Electron dot structure:
Magnesium loses two electrons to form \( Mg^{2+} \), and oxygen gains two electrons to form \( O^{2-} \). The final dot structure represents ionic bonding: \( Mg^{2+} : O^{2-} \).
An element X (atomic number 17) reacts with an 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
(a) Position of elements:
Element X (Z = 17): Group 17, Period 3.
Element Y (Z = 20): Group 2, Period 4.
(b) Classification:
X is a non-metal (halogen). Y is a metal (alkaline earth metal).
(c) Nature of oxide and bonding:
Element Y forms a basic oxide because it is a Group 2 metal. The compound formed between X and Y is ionic because Y donates two electrons and X accepts one electron each.
(d) Electron dot structure:
\( Y^{2+} \Rightarrow [ X^{-} : X^{-} ] \). Each halogen atom gains one electron from Y, forming a divalent ionic halide \( YX_2 \).
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
(a) Identification:
10 → Neon (Ne)
20 → Calcium (Ca)
7 → Nitrogen (N)
14 → Silicon (Si)
(b) Group numbers:
Ne → Group 18
Ca → Group 2
N → Group 15
Si → Group 14
(c) Period numbers:
Ne → Period 2
Ca → Period 4
N → Period 2
Si → Period 3
(d) Electronic configurations:
Ne: \( 1s^2 2s^2 2p^6 \)
Ca: \( 1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 \)
N: \( 1s^2 2s^2 2p^3 \)
Si: \( 1s^2 2s^2 2p^6 3s^2 3p^2 \)
(e) Valency:
Ne → 0 (inert gas)
Ca → 2
N → 3
Si → 4
Complete the crossword puzzle (Figure 5.1) using the given clues.
Across:
(1) An element with atomic number 12
(3) Metal used in making cans and a member of Group 14
(4) A lustrous non-metal which has 7 electrons in its outermost shell
Down:
(2) Highly reactive and soft metal which imparts yellow colour when subjected to flame and is kept in kerosene
(5) The first element of the second Period
(6) An element used in making fluorescent bulbs and second member of Group 18
(7) A radioactive element which is the last member of halogen family
(8) Metal important in steel and forms rust in moist air
(9) The first metalloid in the Modern Periodic Table used in making bullet-proof vests
Completed crossword answers:
(1) Magnesium
(3) Tin
(4) Iodine
Down:
(2) Sodium
(5) Lithium
(6) Neon
(7) Astatine
(8) Iron
(9) Boron
In the ladder (Figure 5.2) symbols of elements are jumbled. Rearrange these symbols of elements:
(a) In the increasing order of their atomic number
(b) In the order of their groups
(a) Increasing order of atomic number:
H, He, Li, Be, B, C, N, O, F, Ne, Na, Mg, Al, Si, P, S, Cl, Ar, K, Ca
(b) Order based on groups:
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
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 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?
(a) Modern equivalents:
Eka-silicon → Germanium (Ge)
Eka-aluminium → Gallium (Ga)
(b) Group and period:
Ge → Group 14, Period 4
Ga → Group 13, Period 4
(c) Classification:
Ge → Metalloid
Ga → Metal
(d) Valence electrons:
Ge → 4
Ga → 3
Based on the trends of the Periodic Table, answer the following about the elements with atomic numbers 3 to 9:
(a) Name the most electropositive element
(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
Answers based on periodic trends:
(a) Lithium (most electropositive)
(b) Fluorine (most electronegative)
(c) Fluorine (smallest atomic size)
(d) Boron (metalloid)
(e) Carbon (maximum valency)
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 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
(d) What would be the nature (acidic/basic) of oxides formed?
(e) Locate the position of the element in the Modern Periodic Table
(a) Identification: The element X is sulphur (S).
(b) Electronic configuration: \( 1s^2 2s^2 2p^6 3s^2 3p^4 \). Shell notation: K = 2, L = 8, M = 6.
(c) Thermal decomposition of ferrous sulphate:
\[ 2FeSO_4(s) \xrightarrow{\text{Heat}} Fe_2O_3(s) + SO_2(g) + SO_3(g) \]
(d) Nature of the oxides: Both \( SO_2 \) and \( SO_3 \) are acidic oxides.
(e) Position in the Modern Periodic Table: Period 3, Group 16.
An element X of group 15 exists as a diatomic molecule and combines with hydrogen at 773 K in presence of a catalyst to form ammonia. Ammonia 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 state the type of bond formed in it
(a) Identification: X is nitrogen (N). It has 5 valence electrons.
(b) Electron dot structure of \( N_2 \): The two nitrogen atoms share three electron pairs forming a triple covalent bond.
(c) Electron dot structure of ammonia (NH3): Nitrogen forms three single covalent bonds with three hydrogen atoms. The structure shows three shared pairs and one lone pair on nitrogen.
Which group of elements could be placed in Mendeleev’s Periodic Table without disturbing the original order? Give reason.
The noble gases (Group 18) could be added to Mendeleev’s Periodic Table without disturbing the existing arrangement. This is because they are inert and chemically unreactive. Their placement as a separate group does not affect the order of other elements, which were arranged based on their valency and atomic mass.
Give an account of the process adopted by Mendeleev for the classification of elements. How did he arrive at the “Periodic Law”?
Mendeleev examined all known 63 elements and studied their properties, including the formation of oxides and hydrides. He grouped together the elements showing similar properties and arranged them in order of increasing atomic masses.
He observed a periodic recurrence of similar physical and chemical properties at regular intervals when arranged by atomic mass. This led him to propose the Periodic Law: “The properties of elements are a periodic function of their atomic masses.”