Q.1.What were the criteria used by Mendeleyev in creating his periodic table?

Ans.In creating his periodic table, Mendeleev was guided by two factors:-
(a). Increasing atomic masses and;
(b) Grouping together of elements having similar properties.

Q.2.Besides Gallium, which other elements have since been discovered that fill the gaps left by Mendeleev in this periodic table?

Ans.Scandium and Germanium.

Q.3.Name two elements you would expect to show same kind of chemical reactivity as Magnesium. What is your best choice?

Ans.Beryllium, Calcium and Strontium will show same kind of chemical reactivity as Magnesium because they are placed in the same group. Elements placed in the same group have similar outer electronic configuration.

Q.4.Looking at the resemblance of Hydrogen to alkali metals and the Halogen family, to which group and period should Hydrogen be assigned in Mendeleev’s periodic table?

Ans.Since properties of Hydrogen resemble both alkali metals and Halogens, it can be placed either in group IA or group VII A. Since Hydrogen has the lowest atomic mass, it is placed in period I.

Q.5.An atom has electronic configuration 2, 8, 7.
(a) What is the atomic number of this element?
(b) To which of the following elements would it be chemically similar:-
(i) 7N
(ii) 9F

Ans. (a) The atomic number of the element is 2+ 8+7=17.
(b) Electronic configurations of the given elements are:-
Thus F having similar electronic configuration with 7 valence electrons would be chemically similar to the given element.

Q.6.The positions of three elements A, B, C in the periodic table are show below:-
Group 16 Group 17
– –
– A
– –

(a) State whether C 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 than B in size?
(c) Which type of ion (cation or anion) will be formed by element A?

Ans.(a) Elements of group 17 have 7 electrons in their respective valence shell and have a strong tendency to gain an electron to complete their octet. So, A is a non-metal.
(b) On moving down a group, the atomic size increases and thus the force of attraction of the nucleus for the incoming electrons decreases. Hence the reactivity decreases. Therefore C is less reactive than A.
(c) B and C belong to the same period. On going from left to right in a period the nuclear charge increases but the number of shells remain same. Therefore atomic size decreases from left to right. Thus C is smaller in size than B.

Q.7.Use Mendeleev’s periodic table to predict the formulae for the oxides of K, C, Al, Si and Ba.

Ans.K2O, CO2, Al2O3, SiO2, BaO.

Q.8.A metal M forms an oxide having the formula M2O3. It belongs to third period of the modern periodic table. Give its atomic number and valency.

Ans.Atomic number of the element is 2, 8, 3=13 and its valency is 3.

Q.9.What are the uses of Modern Periodic table?

Ans.In modern periodic table, the elements with similar properties are placed together in the same group. If we know the properties of one element of the group, the properties of other elements of the group can be predicted. Thus it makes the study of elements systematic.

Q.10.Why is long form periodic table regarded better than Mendeleev’s periodic table?

Ans.It is due to the following reasons:-
(a) It is based on atomic number.
(b)There is no confusion regarding the position of isotopes.
(c) All groups are independent and there are no sub-groups.
(d)It is more systematic.