Frank Solutions for Chapter 7 Atomic Structure Class 9 Chemistry ICSE
1. Compare the characteristics of an electron, proton and neutron.
Answer
2. State the number of electrons, protons and neutrons in
Answer
Number of protons = 12
Number of neutrons = 24 – 12 = 12
Number of protons = 19
Number of neutrons = 39 – 19 = 20
Number of electrons = 20
Number of protons = 20
Number of neutrons = 40 – 20 = 20
Number of electrons = 15
Number of protons = 15
Number of neutrons = 31 – 15 = 16
3. Name the constituent particles in the atom that determine the following:
(a) Mass of an atom
(b) Size of an atom
Answer
(a) Mass of an atom - Protons and neutrons
(b) Size of an atom - Electrons
4. Name three fundamental particles of an atom.
Answer
Three fundamental particles of an atom are-
- Proton
- Electron
- Neutron
5. Define (a) an atom, (b) an element.
Answer
(a) An atom - An atom is the smallest particle of an element which may or may not be capable of independent existence.
(b) An element - An element is usually defined as a pure substance that contains only one kind of particles. These particles may be atoms or molecules.
6. What do you mean by the term atomic number and which letter is used to represent it?
Answer
Atomic number - The number of protons present in the nucleus of an atom is called the atomic number of the atom.
It is denoted by ‘Z’.
7. What are nucleons?
Answer
The protons and neutrons are collectively called as nucleons.
8. What are isotopes? Name three isotopes of hydrogen.
Answer:
Isotopes are atoms of the same element, having the same atomic number, same chemical properties but different mass number, i.e., the atoms differ in the number of neutrons. The three isotopes of hydrogen atoms are:
- Protium
- Deuterium
- Tritium
9. Why do isotopes of an element have the same chemical properties?
Answer
Electrons take part in a chemical reaction. Therefore, the chemical properties of an element depend upon the electronic configuration. Since, isotopes of an element have the same atomic number and hence same electronic configuration. So, they exhibit the same chemical properties.
10. Compare Dalton’s atomic theory with Modern Atomic Theory?
Answer
11. What is meant by valence shell and valence electron ?
Answer
The outermost orbit of an element is called valence shell. The electrons present in the outermost orbit (valence shell) of an element are called valence electrons.
12. Why do atoms combine by transferring electrons?
Answer
Atoms may have incomplete octet. During the formation of a molecule, an atom of a particular element gains, losses or shares electrons until it acquires a stable configuration of eight electrons in its valence shell.
13. An atom contains 20 protons, 20 electrons and 20 neutrons. Mention the following details of that atom.
(a) Atomic number
(b) Mass number
(c) Electronic configuration
(d) Valency
Answer
(a) Atomic number = Number of protons = 20
(b) Mass number = Number of protons + Number of electrons = 20 + 20 = 40
(c) Electronic configuration = 2, 8, 8, 2
(d) Valency = 2
14. The atomic number and mass number of atoms of five elements are given below:
(b) Which of these contains the greatest number of neutrons?
(c) Which of the these contains the least number of electrons ?
(d) Which of these contains equal number of electrons and neutrons?
(e) Which of them is a metal ?
(f) Which is the most reactive of the non-metals ?
(g) What is common between C and D ?
(h) Which of them belongs to a particular family ?
(i) If atoms of A combines with atoms of B, what will be the formula of the compound formed ?
(j) By what type of valency will B and C combine ?
Answer
(a) A = 1
B = 3
C = 2
D = 1
E = 1
(b) E contains the greatest number of neutrons.
(c) A contains the least number of electrons.
(d) B contains equal number of electrons and neutrons.
(e) A is a metal.
(f) Fluorine is the most reactive of the non-metals.
(g) Both are electronegative and non-metals.
(h) D and E belongs to a particular family.
(i) A3B will be the formula if atoms of A combines with atoms of B.
(i) ‘B’ will combine in a trivalent element while ‘C’ will combine as a brivalent element.
15. What is the octet rule ?
Answer
“During the formation of the molecule, an atom of a particular element gains, losses electrons or shares electrons until it acquires a stable configuration of eight electrons in its valence shell”. i.e., until it acquires octet.
16. What are the maximum number of electrons that the K, L and M shells can accommodate?
Answer
K can accommodate maximum of 2 electrons.
L can accommodate maximum of 8 electrons.
M can accommodate maximum of 18 electrons.
17. Name the rays which are formed at the negative electrode of the discharge tube experiment ?
Answer
Cathode rays are formed at the negative electrode of the discharge tube experiment.
18. Calculate the maximum number of electrons which can be accommodated in (i) K, (ii) L (iii) M and (iv) N-shells.
Answer
(i) In K maximum number of 2 electrons can be accommodated.
(ii) In L maximum number of 8 electrons can be accommodated.
(iii) In M maximum number of 18 electrons can be accommodated.
(iv) In N maximum number of 32 electrons can accommodated.
19. The relative atomic mass of Boron is 10.8. Calculate the percentage of its isotope 
an occurring in nature.
Answer
20. Magnesium (atomic number 12) is a metallic element. Find its valency.
Answer
Electronic configuration of magnesium is -2, 8, 2. Since, it has 2 electrons in its valence shell, so its valency is 2.
21. The atomic number and mass number of sodium are 11 and 23 respectively. What information is conveyed by this statement ?
Answer
Number of electrons in Sodium = 11
Number of protons in sodium = 11
Number of neutrons in sodium = 12
Number of nucleons in sodium = 23
22. What are inert elements? Why do they exist as monoatoms in molecule ?
Answer
Inert elements are the elements which have completely filled valence shell. Since, they are already stable and do not need more electrons, they do not combine with other atoms. So, they exist as monoatoms in molecule.
23. Name or state the following :
(a) The element which does not contain any neutron in its nucleus.
(b) The shell closest to the nucleus of an atom.
(c) Metal with valency 2.
(d) Two elements having same number of protons and electrons but different number of neutrons.
(e) An element having valency ‘zero’.
Answer
(a) Hydrogen
(b) K shell.
(c) Magnesium
(d) Isotopes
(e) Helium has zero valency.
24. An atom of fluorine may be written as
, write down:
(a) The number of protons.
(b) The number of neutrons.
(c) The number of electrons.
Answer
(a) The number of protons = 9
(b) The number of neutrons = 19 – 9 = 10
(c) The number of electrons = 9
25. Atomic number is unique to an atom, but the mass number is not. Explain this briefly.
Answer
Atomic number is the number of protons of an atom which is unique to an atom but mass number is the total of number of protons and number of neutrons which may or may not be same to other atoms since there is probability of combination of number of protons and number of neutrons be same for two atoms.