(a) Valency equal to 3.
(b) Valency equal to 1.

(a) The atomic number of this element is 17.
(b) It would be chemically similar to F(9) with configuration as 2, 7.

The position of three elements A, B and C in the Periodic Table are shown below -

Group 16	Group 17
-	-
-	A
-	-
B	C

(a) State whether A is a metal or non-metal.

(b) State whether C is more reactive or less reactive than A.

(c) Will C be larger or smaller in size than B?

(d) Which type of ion, cation or anion, will be formed by element A? 

Answer

(a) A is a non-metal.
(b) C is less reactive than A, as reactivity decreases down the group in halogens.
(c) C will be smaller in size than B as moving across a period, the nuclear charge increases and therefore, electrons come closer to the nucleus.
(d) A will form an anion as it accepts an electron to complete its octet.

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?

Element	K L M
Nitrogen Phosphorus	2 5 2 8 5

Nitrogen is more electronegative than phosphorus. On moving down a group, the number of shell increases. Therefore, the valence electrons move away from the nucleus and the effective nuclear charge decreases. This causes the decrease in the tendency to attract electron and hence electronegativity decreases.

In the modern periodic table, atoms with similar electronic configurations are placed in the same column. In a group, the number of valence electrons remains the same.
Elements across a period show an increase in the number of valence electrons.

The element with atomic number 12 has same chemical properties as that of calcium. This is because both of them have same number of valence electrons (2).

	Mendeleev's periodic table		Modern periodic table
1.	Elements are arranged in the increasing order of their atomic masses.	1.	Elements are arranged in the increasing order of their atomic numbers.
2.	There are a total of 7 groups (columns) and 6 periods (rows).	2.	There are a total of 18 groups (columns) and 7 periods (rows).
3.	Elements having similar properties were placed directly under one another.	3.	Elements having the same valence shell are present in the same period while elements having the same number of valence electrons are present in the same group.
4.	The position of hydrogen could not be explained.	4.	Hydrogen is placed above alkali metals.
5.	No distinguishing positions for metals and non-metals.	5.	Metals are present at the left hand side of the periodic table whereas non-metals are present at the right hand side.

1. Upto which element, the Law of Octaves was found to be applicable?
(a) Oxygen
(b) Calcium
(c) Cobalt
(d) Potassium
Ans. (b) Calcium

2. According to Mendeleev's ′ Periodic Law, the elements were arranged in the periodic table in the order of
(a) increasing atomic number
(b) decreasing atomic number
(c) increasing atomic masses
(d) decreasing atomic masses
Ans. (c) Increasing atomic masses

3. In Mendeleev′s Periodic Table, gaps were left for the elements to be discovered later. Which of the following elements found a place in the periodic table later?
(a) Germanium
(b) Chlorine
(c) Oxygen
(d) Silicon
Ans. (a) Germanium
Explanation: Mandeleev named the undiscovered elements as Eka-boron, Ekaaluminium and Eka-silicon. These were replaced with scandium, gallium and germanium respectively in due course of time.

4. Which of the following statement (s) about the Modern Periodic Table are incorrect?
(i) The elements in the Modern Periodic Table are arranged on the basis of their decreasing atomic number
(ii) The elements in the Modern Periodic Table are arranged on the basis of their increasing atomic masses
(iii) Isotopes are placed in adjoining group (s) in the Periodic Table
(iv) The elements in the Modern Periodic Table are arranged on the basis of their increasing atomic number
(a) (i) only
(b) (i), (ii) and (iii)
(c) (i), (ii) and (iv)
(d) (iv) only
Ans. (b) (i), (ii) and (iii).

Explanation: Atomic mass is not a criterion in Modern Periodic Table. Hence, option (ii) is incorrect. Isotopes were given the same position in Periodic Table, hence option (iii) is incorrect.

5. Which of the following statements about the Modern Periodic Table is correct:
(a) It has 18 horizontal rows known as Periods
(b) It has 7 vertical columns known as Periods
(c) It has 18 vertical columns known as Groups
(d) It has 7 horizontal rows known as Groups
Ans. (c) It has 18 vertical columns known as groups
Explanation: Modern Periodic Table has 18 Groups and 7 periods. Columns are called groups, while rows are called periods

6. Which of the given elements A, B, C, D and E with atomic number 2, 3, 7, 10 and 30 respectively belong to the same period?
(a) A, B, C
(b) B, C, D
(c) A, D, E
(d) B, D, E
Ans. (b) B, C, D
Explanation: Element with atomic number 2 belongs to the first period. Elements from 3 to 10 belong to the second period.

7. The elements A, B, C, D and E have atomic number 9, 11, 17, 12 and 13 respectively. Which pair of elements belong to the same group?
(a) A and B
(b) B and D
(c) A and C
(d) D and E
Ans. (c) A and C
Explanation: Fluorine (9) and Chlorine (17) belong to the same group.

8. Where would you locate the element with electronic configuration 2, 8 in the Modern Periodic Table?
(a) Group 8
(b) Group 2
(c) Group 18
(d) Group 10
Ans. (c) Group 18
Explanation: In this element, the outermost shell is completely filled. Hence, this belongs to Group 18.

9. An element which is an essential constituent of all organic compounds belongs to
(a) Group 1
(b) Group 14
(c) Group 15
(d) Group 16
Ans. (b) Group 14
Explanation: Carbon belongs to group 14

10. Which of the following is the outermost shell for elements of period 2?
(a) K shell
(b) L shell
(c) M shell
(d) N shell
Ans. (b) L shell
Explanation: In period 2, there are 2 shells, i.e. K and L
11. Which one of the following elements exhibit maximum number of valence
electrons?
(a) Na
(b) Al
(c) Si
(d) P
Ans. (d) P
Explanation: Electronic configuration of Na: 2, 8, 1
Al: 2,8,3 Si: 2,8,4 and P: 2,8,5
Phosphorus has 5 valence electrons, while others have less number of valence electrons.

12. Which of the following gives the correct increasing order of the atomic radii of O, F and N?
(a) O, F, N
(b) N, F, O
(c) O, N, F
(d) F, O, N
Ans. (d) F, O, N
Explanation: Atomic radius decreases as we move from left to right across a period. N(7), O(8) and F(9) are in ascending order of atomic numbers.

13. Which among the following elements has the largest atomic radii?
(a) Na
(b) Mg
(c) K
(d) Ca
Ans. (d) Ca
Explanation: Na K and Ca belong to the same group, i.e. Group I. Out of these, calcium is at the bottom and hence has the largest atomic radius.