Page 140 - Chemistry ICSE Class IX

128 ICSE Chemistry – 9
Q.4. (a) What are the two properties that all elements in the same column of the periodic table as boron have in
common?
(b) What are the two properties that all elements in the same column of the periodic table as uorine have in
common?
Ans. (a) (i) All elements belonging to boron group will have the same, K L , outer electronic con guration.
2 3
(ii) All elements belonging to boron group will show a valency of +3, and are metallic in nature.
K L
(b) (i) All elements belonging to the uorine group will have the same, , outer electronic con guration.
2 5
(ii) All the elements belonging to the uorine group are nonmetals.
Q.5. Name:
(a) three elements that have only a single electron in their outermost shell.
(b) two elements that have two electrons in their outermost shell.
(c) three elements with lled outermost shell.
(d) one element that has a single electron in its outermost shell.
Ans. (a) Lithium, sodium and potassium have only a single electron in their outermost shell.
(b) Magnesium and calcium have two electrons in their outermost shell.
(c) Helium, neon and krypton have completely lled outermost shell.
(d) Hydrogen has a single electron in its outermost shell.
Q.6. In how many groups and periods, the modern periodic table of elements is divided?
Ans. Modern periodic table (Long Form, Böhr) has nine groups (or 16 subgroups) and 7 periods.
Q.7. An element of group IVA has the atomic number 14. Examine if this element will have metallic properties or not.
Ans. The electronic con guration of the element having atomic number 14 is 2, 8, 4. So, it lies in the middle of the 3rd
period. This element, therefore, has no tendency to lose its valence electrons. As a result, this element does not
have metallic properties.
Q.8. Indicate the atomic numbers of elements of period 3 of the modern periodic table having the following
characteristics:
(a) nonmetals (b) elements forming negative ions
(c) elements with high melting points (d) elements forming positive ions
(e) metals (f) elements with low boiling points
Mention the atomic numbers only.
Ans. The atomic numbers of the elements of period 3 of the modern periodic table (from left to right) are 11, 12, 13, 14,
15, 16, 17 and 18.
(a) Nonmetals. Elements having atomic numbers 15, 16 and 17 are nonmetals.
(b) Elements forming negative ions. Elements having atomic numbers 15, 16 and 17 form negative ions.
(c) Elements with high melting points. Elements having atomic numbers 12, 13 and 14 have high melting
points.
(d) Elements forming positive ion. Elements having atomic numbers 11, 12 and 13 form positive ions.
(e) Metals. Elements having atomic numbers 11, 12 and 13 are metals.
(f) Elements with low boiling points. Elements having atomic numbers 17 and 18 have low boiling points.
Q.9. Sodium and aluminium have atomic numbers of 11 and 13, respectively. These are separated by one element in
the periodic table, and have valencies of one and three, respectively.
Chlorine (at. no. 17) and potassium (at. no. 19) are also separated by one element in the periodic table, but both
show valency of one. Why is it so?
Ans. Writing the electronic con gurations of the various elements, we have
Sodium (11) 2, 8, 1 Chlorine (17) 2, 8, 7
X (12) 2, 8, 2 Y (18) 2, 8, 8
Aluminium (13) 2, 8, 3 Potassium (19) 2, 8, 8, 1
This difference is because sodium and aluminium belong to the same period. These elements would lose 1 and 3
electrons, respectively to attain the stable con guration of the nearest noble gas and show different valencies.
On the other hand, chlorine and potassium lie in different periods. These elements are separated by a noble gas
element. So, Cl attains the nearest noble gas con guration by gaining one electron, and potassium does so by
losing one electron. As a result, both chlorine and potassium show valencies equal to 1.