Periodic Classification of Elements
NCERT Solutions
Exercises
1. Which of the following statements is not a correct statement about the trends when going from left to right across the periods of the periodic Table?
a. The elements become less metallic in nature.
b. The number of valence electrons increases.
c. The atoms lose their electrons more easily.
d. The oxides become more acidic.
(c) The atoms lose their electrons more easily.
2. Element X forms a Chloride with the Formula XCl_2 , Which is a Solid with a High Melting Point. X would most likely be in the same group of the Periodic Table as
Mg . Valency of X in the given compound is 2. Mg is in the group 2 in the periodic table.
3. Which Element has
a. Two shells, both of which are completely filled with electrons?
Neon has two shells, both shells are completely filled with Electrons
b. The Electronic Configuration 2, 8, 2?
The electronic configuration of Magnesium is 2, 8, 2.
c. A Total of Three Shells, With Four Electrons in its Valence Shell?
Silicon has three shells, with four electrons in its valence shell .
d. A total of two shells, with three electrons in its valence shell?
Boron has two shells, with three electrons in its valence shell
e. Twice as many electrons in its second shell as in its first shell?
Carbon has twice as many electrons in its second shell as in its first shell.
4.
a. What property do all elements in the same column of the Periodic Table as boron have in common?
Valence electrons of all element in the column is 3, so its valency is 3.
b. What property do all elements in the same column of the Periodic Table as fluorine have in common?
All the elements have the same number of valence electrons, i.e., 7 and they all have valency equal to 1.
5. An Atom Has Electronic Configuration 2, 8, 7.
a. What is the atomic number of this element?
Atomic number of element is 17.
b. To which of the following elements would it be chemically similar? (Atomic numbers are given in parentheses.)
N(7) F(9) P(15) Ar(18)
Ans: It is similar to F(9) with electronic configuration as 2, 7.
6. The Position of Three Elements A, B and C in the Periodic Table are Shown Below
a. State whether A is a metal or nonmetal.
A is a non-metal.
b. State whether C is more reactive or less reactive than A.
C is less reactive than A.
c. Will C be larger or smaller in size than B?
Atomic size decrease as we move from left to right of the periodic table, so C is smaller than B.
d. Which type of ion, cation or anion, will be formed by element A?
A is in group 17 and the valence electrons will be 7. So it form an anion.
7. Nitrogen (atomic number 7) and Phosphorus (atomic number 15) Belong to Group 15 of the Periodic Table. Write the Electronic Configuration of These Two Elements. Which of These Will Be More Electronegative? Why?
The Electronic configuration of Nitrogen is 2,5.
The Electronic configuration of Phosphorus is 2, 8, 5.
The electronegativity of an atom in a molecule is its tendency to attract the shared pair of electrons present between bonded atoms toward itself. Electronegativity increases from left to right and decreases from top to bottom. The electronegativity decreases in group 15. So Nitrogen is more electronegative than phosphorus.
8. How Does the Electronic Configuration of an Atom Relate to its Position in the Modern Periodic Table?
Number of electrons in the outermost shell represents the group number. Atomic number is represented by the total number of electrons in all shells. Number of shells represent the period number.
9. In the Modern Periodic Table, Calcium (Atomic Number 20) is Surrounded by Elements With Atomic Numbers 12, 19, 21, and 38. Which of These Have Physical and Chemical Properties Resembling Calcium?
Let’s first write the electronc configuration of the elements with the help of atomic number.
From the above table we need to find the elements with same number of valence electrons. Because the elements with same number of valence electrons. shows similar properties. So Elements with atomic number 12 and 38 have the same chemical properties as that of calcium.
10. Compare and Contrast the Arrangement of Elements in Mendeleev’s Periodic Table and the Modern Periodic Table.
Intext Exercises
Pg. No. 81
1. Did Dobereiner’s Triads Also Exist in the Columns of Newlands’ Octaves? Compare and Find Out.
In the columns of Newland’s octaves, only one of Dobereiner’s triads is found. In the columns of Newlands’ octaves, the elements Li, Na, and K of Dobereiner’s triads also occur.
2. What Were the Limitations of Dobereiner’s Classification?
Dobereiner’s triads can only be formed by a few elements.
3. What Were the Limitations of Newlands’ Law of Octaves?
Limitations of Newland’s law of octaves:
(i) It was not applicable throughout the arrangements. It was applicable up to calcium only.
(ii)Newlands table does not include the elements discovered after Newlands octaves.
(iii) There was no explanation for the position of cobalt and nickel in the group of elements containing F and Cl.
(iv) It is also not clear why iron was placed far away from cobalt and nickel, which have similar properties as iron.
Pg.No: 85
1. Use Mendeleev’s Periodic Table To Predict the Formula for the Oxides of the Following Elements: K, C, Al, Si, Ba.
Potassium is in group 1. The oxide will be K_2O .
Carbon is in group 4. The oxide will be CO_2 .
Aluminium is in group 3. The oxide will be Al_2O_3 .
Silicon is in group 4. The oxide will be SiO_2 .
Barium is in group 2. The oxide will be BaO.
2. Besides Gallium, Which Other Elements Have Since Been Discovered That Were Left by Mendeleev in His Periodic Table? (Any Two)
Gallium(Ga) and Scandium(Sc)
3. What Were the Criteria Used by Mendeleev in Creating His Periodic Table?
He found that the properties of the elements are the periodic function of their atomic masses. The main criteria he followed is to arrange the elements in the increasing order of their atomic masses and the properties gets repeated after a regular interval.
4. Why do you think noble gases are placed in a separate group?
Its because their properties are different from all other elements. They are inert elements.
Pg. No. 90
1. How could the Modern Periodic Table remove various anomalies of Mendeleev’s Periodic Table?
(a) In the modern periodic table, elements are arranged in the increasing order of their atomic number, not in the atomic mass.
(b) In the modern periodic table, the isotopes are placed in the same place as they having the same atomic number but different atomic masses.
(c) Now, the same group elements have the same number of valence electrons.
(d) Hydrogen is now placed in the group containing the same valence electrons.
2. Name two elements you would expect to show chemical reactions similar to magnesium. What is the basis for your choice?
Calcium and Beryllium show chemical reactions similar to Magnesium. All the three elements 2 valence electrons in their outer shell.
3. Name
(a) Three elements that have a single electron in their outermost shells.
Lithium (Li), Sodium (Na) and potassium (k)
(b) Two elements that have two electrons in their outermost shells.
Magnesium (Mg) and Calcium (Ca)
(c) Three elements with filled outermost shells
Neon (Ne), Argon (Ar) and Xenon (Xe)
4.
a) Lithium, sodium, potassium are all metals that react with water to liberate hydrogen gas. Is there any similarity in the atoms of these elements?
Outermost shells of all these elements have one one electron. So they are unstable and they react with water to liberate hydrogen gas.
(b) Helium is an unreactive gas and neon is a gas of extremely low reactivity. What, if anything, do their atoms have in common?
Outermost shells of Helium and neon are filled.
5. In the Modern Periodic Table, which are the metals among the first ten elements?
Lithium and Beryllium
6. By considering their position in the Periodic Table, which one of the following elements would you expect to have maximum metallic characteristic?
Ga Ge As Se Be
Gallium (Ga)