Lesson: Periodic Classification of Elements

Question:1

Did Döbereiner’s triads also exist in the columns of Newlands’ Octaves?

Compare and find out?

Solution:

Yes, Döbereiner's triads also existed in the columns of Newlands' Octaves.

The second column of the classification has Li, K, and Na which constitutes Döbereiner’s triads.

Question:2

What were the limitations of Döbereiner’s classification?

Solution:

Dobereiner’s triads had a major limitation that all known elements could not be classified into groups of triads having similar properties.

Question:3

What were the limitations of Newlands’ Law of Octaves?

Solution:

Limitations of Newlands' law of octaves are:

· It was applicable up to calcium only. The properties of the elements listed

after calcium did not obey Newlands’ law of octaves.

· To fit elements in his table, Newland had put two elements together in

one slot that too having different properties.

· Newland assumed the existence of only 56 natural elements. Several new elements that were discovered later could not be fit into the law of octaves.

Question:4

Use Mendeleev’s Periodic Table to predict the formulae for the oxides of the following elements:

K, C, AI, Si, Ba.

Solution:

· K is in group 1. Therefore, the oxide will be $K_{2} O$ .

· C is in group 4. Therefore, the oxide will be $C O_{2}$ .

· Al is in group 3. Therefore, the oxide will be $A l_{2} O_{3}$ .

· Si is in group 4. Therefore, the oxide will be $S i O_{2}$ .

· Ba is in group 2. Therefore, the oxide will be $B a O$ .

Question:5

Besides gallium, which other elements have since been discovered that were left by Mendeleev in his Periodic Table? (Any two)

Solution:

Scandium and Germanium.

Question: 6

What were the criteria used by Mendeleev in creating his Periodic Table?

Solution:

Mendeleev used atomic mass of the elements as the unique criteria for creating his Periodic Table.

Mendeleev’s Periodic Table contains vertical columns called ‘groups’ and horizontal rows called ‘periods’. He proposed that the chemical properties of elements are the periodic function of their atomic masses.

Question: 7

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

Solution:

The noble gases are placed in a separate group in the Periodic Table as the valence shell of each of these gases is completely filled with electrons. All noble gases show similar chemical properties.

Question: 8

How could the Modern Periodic Table remove various anomalies of Mendeleev’s Periodic Table?

Solution:

The modern periodic table is an improvement over Mendeleev’s periodic table. In case of the modern periodic table, the elements are arranged in the increasing order of their atomic numbers.

The isotopes of an element have the same atomic number with different atomic masses. They have similar chemical properties and cannot be placed separately. Using the atomic number as the basis of classification helps explaining the position of isotopes in the periodic table.

The modern periodic table also clarifies the position of hydrogen as it is kept in the group with the elements having the same number of valence electrons.

Question: 9

Name two elements you would expect to show chemical reactions similar to magnesium. What is the basis for your choice?

Solution:

Calcium (Ca) and strontium (Sr) are expected to show chemical reactions similar to magnesium (Mg). This is because the number of valence electrons in all these three elements is 2.

Question: 10

Name:

(a) Three elements that have a single electron in their outermost shells.

(b) Two elements that have two electrons in their outermost shells.

Solution:

(a) Lithium (Li), sodium (Na), and potassium (K)

(b) Magnesium (Mg) and calcium (Ca)

Question: 11

a) Lithium, sodium, potassium are all metals that react with water to liberate hydrogen gas. Is there any similarity in the atoms of these elements?

b) Helium is an unreactive gas and neon is a gas of extremely low reactivity. What, if anything, do their atoms have in common?

Solution:

a) Yes, the atoms of all the three elements lithium, sodium, and potassium have one electron in their outermost shells.

b) Both helium (He) and neon (Ne) have filled outermost shells. Helium has

2 electrons in its K shell, while neon has 8 in its L shell.

Question: 12

In the Modern Periodic Table, which are the metals among the first ten elements?

Solution:

Lithium (Li) and beryllium (Be)

Question: 13

By considering their position in the Periodic Table, which one of the following elements would you expect to have maximum metallic characteristic?

$Ga \to Ge \to As \to Se \to Be$

Solution:

The metallic character of an element decreases as one moves from left to right in a period. Since Be lies to the extreme left-hand side of the periodic table, it will have maximum metallic characteristic among the given elements.