Class 10 NCERT Solutions Chemistry Chapter 5 - Periodic Classification of Elements

Only one triad of Dobereiner's triads exists in the columns of Newland's octaves. The triad formed by the elements Li, Na, and K of Dobereiner's triads also occurred in the columns of Newlands' octaves.

Concept Insight: Recall the Dobereiner's triads:
Li, Na and K
Ca, Sr and Ba
Cl, Br and I

Limitation of Dobereiner's classification:
All known elements could not be classified into groups of triads on the basis of their properties.

Concept Insight: Recall that Dobereiner could only classify 3 triads successfully

Limitations of Newland's law of octaves:
(i) It was not applicable throughout the arrangements. It was applicable up to calcium only.
(ii) Those elements that were discovered after Newland's octaves did not find a place in Newlands table.
(iii) The position of cobalt and nickel in the group of the elements containing F and Cl of different properties could not be explained.
(iv) Placing of iron far away from cobalt and nickel, which have similar properties as iron, could also not be explained.

Concept Insight: The students should remember the demerits of Dobereiner's, Newland's as well as Mendeleev's classification.

K is in group 1. Therefore, the oxide will be K₂O.
C is in group 4. Therefore, the oxide will be CO₂.
Al is in group 3. Therefore, the oxide will be Al₂O₃.
Si is in group 4. Therefore, the oxide will be SiO₂.
Ba is in group 2. Therefore, the oxide will be BaO.

Concept insight: Remember that while writing the formula we cross multiply the valency of the two elements in order to balance the charges. The students should remember that the valency of oxygen is 2 and the valency of other elements can be calculated from their group number.

Mendeleev's periodic table was based on the observation that the properties of elements are a periodic function of their atomic masses. This means that if elements are arranged in the increasing order of their atomic masses, then their properties get repeated after regular intervals.

Concept insight: The students should remember that Mendeleev had studied the properties of oxides and hydrides of the known elements and observed that properties of the elements orderly recur in a cyclic fashion. He classified the elements on the basis of their physical and chemical properties in the increasing order of the atomic masses, in the form of a table.

Noble gases are inert elements. Their properties are different from all the other elements. Therefore, the noble gases are placed in a separate group.

Concept insight: Recall that the outermost/valence shell of noble gases is completely filled and therefore they show chemical inertness.

Modern periodic table is based on atomic numbers whereas Mendeleev's periodic table was based on atomic masses. So, properties of elements could be explained with more precision when elements were arranged on the basis of increasing atomic number
For example:
The position of isotopes is taken care of when the elements are arranged in the ascending order of their atomic numbers as they have the same atomic number they do not need to be given separate slots.
Modern Periodic table can justify the position of hydrogen in group 1 as it has one valence electron like other elements of the group.
The anomaly regarding few elements such as tellurium (Te), iodine (I) was solved in Modern table. If arranged in increasing order of their atomic numbers, (Te is 52 and I is 53.) Te will naturally come before I.

Concept insight: Recall that atomic number being a more fundamental property than atomic mass could explain periodic properties in a better manner.

Calcium (Ca) and Strontium (Sr) are expected to show chemical reactions similar to magnesium (Mg). This is because the number of valence electrons (2) is same in all these three elements. And since chemical properties are due to valence electrons, they show same chemical reactions. 
 

Concept Insight: The students should remember that elements belonging to same group have same number of valence electrons and hence similar chemical properties.

(a) Lithium (Li), sodium (Na), and potassium (K) have a single electron in their outermost shells.
(b) Magnesium (Mg) and calcium (Ca) have two electrons in their outermost shells.
(c) Neon (Ne), argon (Ar), and xenon (Xe) have filled outermost shells.

Concept Insight: Recall that elements of group I have 1 valence electron, group II have 2 valence electrons while group XVIII have completely filled valence shell.

(a) Yes. The atoms of all the three elements lithium, sodium, and potassium have one electron in their outermost shell.
(b) Both helium (He) and neon (Ne) have filled outermost shells. Helium has a duplet in its K shell, while neon has an octet in its L shell.

Concept Insight: The students must learn the names of first 20 elements of the modern periodic table along with their atomic number.

Among the first ten elements, lithium (Li) and beryllium (Be) are metals.

Concept Insight: The key to answering this question lies in fact that metals are present ususally on the left hand side of the Modern periodic table whereas non-metals are present on the right hand side of the Modern Periodic Table.

Galium (Ga)

Concept Insight: The key to answering this question lies in fact that metals are present ususally on the left hand side of the Modern periodic table whereas non-metals are present on the right hand side of the Modern Periodic Table. Also, remember that as we move down a group, metallic character increases.

(c) The atoms lose their electrons more easily.

Concept Insight: The students should remember that on moving from left to right across the periods of the periodic table, the non-metallic character increases. Hence, the tendency to lose electrons decreases.

(b) Mg
X would most likely be in the same group of the Periodic Table as magnesium (Mg).

Concept Insight: Recall that valency of Chlorine is 1, therefore we can deduce from the formula XCl₂ that the valency of X is 2. Since valency of magnesium is also 2, thus X and Mg belong to same group.

(a) Neon has two shells, both of which are completely filled with electrons (2 electrons in
K shell and 8 electrons in L shell).
(b) Magnesium has the electronic configuration 2, 8, 2.
(c) Silicon has a total of three shells, with four electrons in its valence shell (2 electrons in K shell, 8 electrons in L shell and 4 electrons in M shell).
(d) Boron has a total of two shells, with three electrons in its valence shell (2 electrons in K shell and 3 electrons in L shell).
(e) Carbon has twice as many electrons in its second shell as in its first shell (2 electrons in K shell and 4 electrons in L shell).

Concept Insight: The students must learn the names of first 20 elements of the Modern Periodic Table along with their atomic number. The students should also be able to recall the Bohr Bury's rules for distribution of electrons.

(a) All the elements in the same column as boron have the same number of valence electrons (3). Hence, they all have valency equal to 3.
(b) All the elements in the same column as fluorine have the same number of valence electrons (7). Hence, they all have valency equal to 1.

Concept Insight: Recall that elements of same group have same number of valence electrons and hence the same valency. (For group I, II and XIII the valency is equal to the number of valence electrons while for group XIV to XVII the valency is equal to 8 - number of valence electrons.)

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

Concept Insight: The key to answering this question lies in fact that elements with same number of valence electrons belong to the same group and have similar chemical properties.

(a) A is a non-metal.
(b) C is more reactive than A
(c) C will be smaller in size than B
(d) A will form an anion

Concept insight: The students should understand and remember the periodic tends as they are often asked in exams.

Element            K  L  M

Nitrogen            2  5

Phosphorus       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.

Concept insight: Recall that electronegativity of an element depends on its ability to gain electrons to complete their octet configuration.

In the Modern Periodic Table, atoms with similar electronic configurations are placed in the same group. In a group, the number of valence electrons remains the same. Elements across a period show an increase in the number of valence electrons in the same shell.

Concept insight: The students should remember that from the group of an element we can find out the number of valence electrons while from the period we can deduce the number of shells present in atom

The elements with atomic number 12 and 38 have same chemical properties as that of calcium. This is because both of them have same number of valence electrons (2) as calcium.

Concept insight: While answering this kind of a question the students should write the electronic configuration of all elements given in the question.

Atomic number

                                     K,  L,  M,  N,  O

12                                 2,8,2

19                                 2,8,8,1

21                                 2,8,8,3

38                                 2,8,18,8,2

Since elements with same number of valence electron shave similar properties, the elements which have 2 valence electrons will have physical and chemical properties resembling that of calcium

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. The position of hydrogen could not be explained.	3. Hydrogen is placed above alkali metals as it has one valence electron.
4.No distinguishing positions for metals and non-metals was assigned.	4.Metals are present at the left hand side of the periodic table whereas non-metals are present at the right hand side.

Concept insight: The students should be able to clearly list the differences between Mendeleev's and modern classification as it is a favourite with examiners.