Page 94 - ICSE Chemistry 8

4. Which of the following elements does not exhibit variable valency?
(a) Copper (b) Iron (c) Phosphorus (d) Lead

SHORT AND PRECISE

A. Show the given text in the form of fl owcharts.

1. Classifi ca on of elements on the basis of valency
2. Types of valency
3. Types of chemical bonds

B. Give reasons for the following statements.
1. The atoms of one element are diff erent from the atoms of all other elements.
2. Cathode rays consist of nega vely charged par cles.
3. Anode rays consist of posi vely charged par cles.

4. An atom is electrically neutral.
5. The valence shell cannot have more than eight electrons except in case of the fi rst shell which
cannot have more than two electrons.

C. Answer in short.
1. State the condi ons when gases become good conductors of electricity.
2. Describe Thomson’s model of an atom.

3. Defi ne the following terms: atomic number and mass number. Explain with the help of examples.
4. Defi ne rela ve atomic mass.
5. What are isotopes? Give examples.

6. What is electronic confi gura on? What is required to write down the electronic confi gura on of an
element?
7. Discuss Bohr-Bury scheme of electronic confi gura on.
8. Why do atoms of elements combine or react?
9. What do you understand by valence shell, valence electrons and valency?

10. What is variable valency? Give examples.
11. What are radicals? Give examples.
12. What do you understand by electrovalent compound and covalent compound?

AT LENGTH

A. Diff eren ate between the following.
1. Characteris cs of electrons and protons

2. Characteris cs of cathode rays and anode rays
3. Posi ve valency and nega ve valency
4. Electrovalent bond and chemical bond

80