Periodic Classification of Elements

Name a group of elements which was not known when Mendeleev formulated

his periodic table.

Answer explanation

Noble gases were not known when Mendeleev formulated his periodic table.

This is because the noble gases such as He, are etc. where not discovered at that time.

Element M is in the first group of the periodic table. Write the formula of its

oxide.

Answer explanation

First group of the periodic table is mainly known as alkali metals and they

have valency = 1 and oxide with the valency = 2 so the formula of oxide would be M2O.

Why do you think the noble gases are placed in a separate group?

Answer explanation

The noble gases are placed in a separate group because these are unreactive

and present in a very low concentration in the atmosphere and their properties do not

resemble with any other group so they are placed in a separate group.

An element belongs to the 3rd period and 2nd group of the periodic table. Find

out the valence electrons and valency of this element.

Answer explanation

The valency of an atom refers to the number of electrons present in the

outermost shell.

The element in the 3rd period and 2nd group is Magnesium (Mg). Its configuration is 2, 8,

2 which shows that its outermost shell has 2 valence electrons and its valency is 2.

Name two elements which will be chemically similar to aluminum. What is the

basis of your choice?

Answer explanation

In periodic table elements of the same group have the same chemical

properties so aluminium will have same chemical properties are Gallium (Ga) and

Indium (In).

These all elements belong to the same group so they have similar chemical properties.

Amongst elements with atomic number 11 and 14, which has a bigger atomic

size?

Answer explanation

Both the elements with atomic number 11 and 14 lie in the same group so

when we move from right to left in a period atomic size decreases. So, an element with

atomic number 11 will be in bigger in size.

Element X with atomic number 12 and Y atomic number 17 reacts with

hydrogen to form hydrides. Which of them is expected to have a high melting

point?

Answer explanation

The element with atomic number 12 is Magnesium (Mg) and element with

atomic number 17 is Chlorine (Cl).

When Mg reacts with hydrogen it forms MgH2 and chlorine forms HCl. MgH2 is solid

and HCl is liquid so MgH2 has a higher melting point.

An element X has E.C. = 2, 8, 8, 1 while Y has 2, 8, 7. Which of these is a

metal?

Answer explanation

‘X’ has 1 valence electron and it belongs to a 4th period so it is Potassium (K)

and the ‘Y’ is of the 3rd period and it has 7 electrons in an outermost cell with atomic

number 17 so it is Chlorine (Cl). We know that poopt5assium is a metal.

Name the most reactive element present in group 17.

Answer explanation

Fluorine is a most reactive element in group 17 because of its small size and

high electronegativity by which it accepts electron easily and forms a bond with another

element.

Give the formula of oxide formed by a metal if it belongs to the 3rd period

and 13th group

Answer explanation

13th group consists of the following elements:

Boron (B), Aluminium (Al), Gallium (Ga), Indium (In), Thallium (Ti).

‘Al’ lies in the 3rd period so; the formula of oxide of Aluminium is ‘Al2O3’.