Chapter 5 Periodic Classification of Elements

Question 4

Use Mendeleev’s Periodic Table to predict the formulae for the oxides of the following elements: K, C, AI, Si, Ba.

Answer:

Question 5

Besides gallium, which two other elements have since been discovered that fill the gaps left by Mendeleev in his periodic table?

Answer:

Since discovery, germanium(ge) and scandium(Sc) are the two elements that are left by Mendeleev.

Question 6

What were the criteria used by Mendeleev in creating his periodic table?

Answer:

Mendeleev used atomic masses of elements while creating periodic table. In the table, the elements were arranged in increasing order of atomic masses.

Question 7

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

Answer:

Noble gases are the gases which are inert in nature and have low concentration in our atmosphere. They were not discovered by Mendeleev and were placed in a group called Zero Group.

Question 8

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

Answer:

  • Modern periodic table are arranged in increasing order of atomic number. Which removes the anomaly of the Mendeleev’s periodic table.
  • Placement of cobalt and nickel is wrong in Mendeleevs periodic table. They are not arranged according to atomic mass. Cobalt needs to be placed before nickel.
  • All isotopes of same elements have different atomic masses, but same atomic number; hence they are kept  at the same position in the modern periodic table.

Question 9

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

Answer:

Calcium and beryllium are similar to magnesium. Because all three elements belong to the same group. And have 2 valence electrons in there outer shell.

Question 10

Name:

  1. three elements that have a single electron in their outermost shells.
  2. three elements that have two electrons in their outermost shells.
  3. three elements with filled outermost shells.

Answer:

  1. These are known as Alkali metals. The elements are Lithium, Sodium and potassium
  2. These are alkaline earth metals. The elements are Beryllium, Magnesium and Calcium.
  3. These are known as noble gases. The elements are Helium, Neon and Argon.

Question 11

(a) Lithium, sodium and 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?

Answer:

(a) They are having one valence electron in their outermost shell, which makes them very unstable. Hence they readily react with water to liberate hydrogen. Like mentioned above, they are known as Alkali Metals.

(b) Because there outermost shells are full and they react only in extreme circumstances and hence are known as noble gases.

Question 12

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

Answer:

Lithium and Beryllium are the known metals among ten elements.

Question 13

By considering their position in the Periodic Table, which one of the following elements would you expect to have maximum metallic characteristics? Ga, Ge, As, Se, Be

Answer:

Be and Ge are expected to be most metallic. Out of both these, Ga is bigger in size and hence has higher chances of losing electrons. Hence Ga is more metallic than Be.

Question 14

Which of the following statements is not a correct statement about the trends when going from left to right across the periods of periodic table?
(a) The elements become less metallic in nature.
(b) The number of valence electrons increases.
(c) The atoms lose their electrons more easily.
(d) The oxides become more acidic.

Answer:

(c) The atoms lose their electrons more easily

Question 15

Element X forms a chloride with the formula XCI, which is a solid with high melting point. X is most likely to be in the same group of the periodic table as:
(a) Na
(b) Mg
(c) Al
(d) Si

Answer:

(b) Mg

Question 16

Which element has:
(a) two shells, both of, which are completely filled with electrons?
(b) the electronic configuration 2, 8, 2?
(c) a total of three shells with four electrons in the valence shell?
(d) a total of two shells with three electrons in the valence shell?
(e) twice as many electrons in the second shell – as in the first shell?

Answer:

(a) Neon has two shells, which are completely filled with electrons

(b) Silicon has the electronic configuration of 2,8,2.

(c) Carbon has 3 shells and 4 electrons in the valence shell.

(d) Boron

(e) Magnesium has twice as many electrons in the second shell as in the first shell.

Question 17

(a) Which property do all elements in the same column of the periodic table as boron have in common?
(b) Which property do all elements in the same column of the periodic table as fluorine have in common?

Answer:

(a) Boron belongs to Group 13 which is the only non metal. All other elements in the group are metals. They all have 3 valence electrons in their shells. Ex: Aluminium, gallium, indium and thallium

(b) All the elements in the column of the periodic table have 3 valence electrons in their valence shells.

Question 18

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? (Atomic numbers are given in parentheses.) N(7), F(9), P(15), Ar(18)

Answer:

(a) The atomic number of element (2+8+7) = 17. The element is chlorine.

(b) Any atom with electronic configuration 2,8,7 would be chemically similar to F.

Question 19

Answer:

Question 20

Nitrogen (atomic number 7) and phosphorus (atomic number 15) belong to group 15 of the periodic table. Write their electronic configuration. Which of these will be more electronegative and why?

Answer:

As the size of nitrogen is small in comparison to phosphorus, it has a greater tendency to take up electrons. It is therefore, more electro-negative than phosphorus.

Question 21

How does the electronic configuration of an atom relate to its position in the Modern Periodic Table?

Answer:

The number of valence electrons decides an atoms position in the periodic table.

Question 22

In the Modern Periodic Table, calcium (atomic number 20) is surrounded by elements with atomic numbers 12, 19, 21 and 38. Which of these have physical and chemical properties resembling calcium?

Answer:

Calcium has an atomic number of 20, and thus have an electronic configuration of 2,8,8,2. Thus calcium has 2 valence electrons. Thus the element having atomic number 12 is 2,8,2. Thus this element resembles calcium the most.

Question 23

Compare and contrast the arrangement of elements in Mendeleev’s Periodic Table and the Modern Periodic Table.

Answer:

 

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