Page 153 - Chemistry ICSE Class IX

The Periodic Table 141
Ans. (a) The electronic con gurations of potassium and lithium are shown alongside. The valence electron of
potassium is in the 4th shell while that of lithium is in the K L M N
2nd shell. Thus, the valence electron in potassium is less Potassium (atom no. 19) 2 8 8 1
rmly bound to the nucleus. So, it is easier to remove electron
from the valence shell of potassium than from lithium. As a Lithium (atomic no. 3) 2 1
result, potassium is more reactive than lithium.
(b) The electronic con gurations of uorine and chlorine are shown alongside. Both these elements attain stable
con gurations by gaining one electron each. Fluorine atom is K L M
smaller than chlorine. So, uorine has a greater tendency to Fluorine (atom no. 9) 2 7
gain an electron than chlorine. As a result, ƀWQTKPG KU OQTG
reactive than chlorine. Chlorine (atomic no. 17) 2 8 7
Q.5. For each of the following groups, name the element with the characteristics speci ed below:
Element Least atomic diameter Chemically least reactive
(i) F, Cl, Br — —
(ii) Li, Na, K — —

Ans. Element Least atomic diameter Chemically least reactive
(i) F, Cl, Br F Br
(ii) Li, Na, K Li Li

Q.6. Element X forms a chloride with the formula XCl , which is a solid with a high melting point. X would most
2
likely be in the same group of the periodic table as
(a) Na (b) Mg (c) Al (d) Si (e) P
Ans. From the information given, the compound XCl is an ionic compound.
2
The element X must have lost two electrons to form XCl . So, X belongs to group 2. Therefore, the element X lies
2
in the same group as magnesium (Mg). So, the answer (b) is correct.
Q.7. (a) Lithium, sodium, potassium are all metals that react with water to liberate hydrogen gas. What, if anything,
do atoms of each of these elements have in common?
(b) Helium is an unreactive gas and neon is a gas of extremely low reactivity. What, if anything, do their atoms
have in common?
Ans. (a) Atoms of these elements tend to lose electrons to reduce hydrogen in water to free hydrogen gas.
(b) Both helium and neon have their outermost shells completely lled with electrons.

Let us Answer Some More Questions

1. Write a typical Dobereiner’s triad.

2. State the law of octaves as given by Newlands.
3. What was the basis of Mendeleev’s classi cation of elements?
4. Why did Mendeleev leave a few gaps in his periodic table?
5. State the modern periodic law.

6. What is the basis of the classi cation of elements in the modern periodic table?
7. By which common name are the elements of Group IA and Group VIIA called?
8. Name an element which is (a) most metallic (b) most nonmetallic
9. How is the position of an element in the periodic table related to the number of valence electrons?
10. What is meant by horizontal relationship between the elements in the periodic table?

11. To which electronic shell would an incoming electron go in the case of elements belonging to