Class 9 Science Unit 4 Structure of the Atom Fill in the blanks , MCQ , True false and Extra question answers.
Fill in the Blanks
1. The sub-atomic particle discovered by J.J. Thomson is the electron.
2. The sub-atomic particle discovered by E. Goldstein is the proton.
3. The nucleus of an atom is positively charged.
4. Neutrons were discovered by James Chadwick in 1932.
5. The maximum number of electrons in the L-shell is 8.
6. The electron has a negative charge.
7. A proton has a mass approximately 2000 times that of an electron.
8. The atomic number is equal to the number of protons in an atom.
9. Electrons revolve around the nucleus in fixed orbits.
10. The outermost shell can hold a maximum of 8 electrons.
11. The mass number is the sum of protons and neutrons.
12. Isotopes have the same atomic number but different mass numbers.
13. An element’s valency is related to the number of valence electrons.
14. Argon has 8 electrons in its outermost shell and is chemically inert.
15. The distribution of electrons in shells follows the formula 2n².
Multiple Choice Questions (MCQs)
1. Who discovered the electron?
a) Rutherford
b) Goldstein
c) Bohr
d) J.J. Thomson ✅
2. Which particle is neutral in charge?
a) Proton
b) Neutron ✅
c) Electron
d) Positron
3. What is the maximum number of electrons in the K-shell?
a) 4
b) 2 ✅
c) 6
d) 8
4. Which of the following is an isotope of hydrogen?
a) Deuterium
b) Protium
c) Tritium
d) All of the above ✅
5. Which scientist proposed the nuclear model of an atom?
a) Bohr
b) Rutherford ✅
c) Thomson
d) Chadwick
6. Which shell is the third shell from the nucleus?
a) K
b) L
c) M ✅
d) N
7. An atom with 6 protons and 6 neutrons will have a mass number of:
a) 6
b) 12
c) 12 ✅
d) 18
8. Isobars have:
a) Same atomic number
b) Same mass number ✅
c) Same chemical properties
d) Same number of neutrons
9. What is the valency of oxygen?
a) 6
b) 0
c) 2 ✅
d) 8
10. What is the charge on a proton?
a) –1
b) +1 ✅
c) 0
d) +2
11. Which model said electrons are embedded like seeds in a watermelon?
a) Bohr’s model
b) Thomson’s model ✅
c) Rutherford’s model
d) Quantum model
12. What is the atomic number of carbon?
a) 4
b) 5
c) 6 ✅
d) 8
13. Which sub-atomic particle was discovered last?
a) Electron
b) Proton
c) Neutron ✅
d) Positron
14. The element with a fully filled K and L shell is:
a) Fluorine
b) Sodium
c) Neon ✅
d) Magnesium
15. The average atomic mass of chlorine is:
a) 35
b) 35.5 ✅
c) 36
d) 37
True or False
1. An electron has a positive charge. – False
2. The nucleus contains both protons and neutrons. – True
3. Thomson proposed the nuclear model of an atom. – False
4. Electrons revolve in fixed orbits as per Bohr’s model. – True
5. The atomic number is the total number of neutrons in an atom. – False
6. Isotopes of an element have the same chemical properties. – True
7. Neutrons are heavier than protons. – False
8. The valency of helium is 2. – False
9. Electrons in discrete orbits do not lose energy. – True
10. Rutherford discovered the electron. – False
11. The mass of the atom is mainly due to electrons. – False
12. The atomic number of sodium is 11. – True
13. The proton has a charge of +1. – True
14. Bohr’s model could explain the stability of atoms. – True
15. An atom is electrically neutral when it has equal protons and electrons. – True
Questions & Answers
Who discovered the electron?
Answer: J.J. Thomson discovered the electron. It is a tiny particle with a negative charge present in atoms.
What are sub-atomic particles?
Answer: Electrons, protons, and neutrons are sub-atomic particles. They are smaller parts of an atom.
What is the charge on a proton?
Answer: A proton has a positive charge. It is found in the nucleus of an atom.
What is the nucleus of an atom?
Answer: The nucleus is the center of an atom. It contains protons and neutrons and holds most of the atom’s mass.
What did Rutherford’s gold foil experiment show?
Answer: It showed that atoms have a small, heavy, and positively charged nucleus, and most of the atom is empty space.
What is Bohr’s model of the atom?
Answer: Bohr said electrons move in fixed circular paths (orbits) around the nucleus and do not lose energy in these orbits.
What are energy levels in an atom?
Answer: Energy levels are the orbits where electrons move around the nucleus. They are labeled as K, L, M, N, etc.
What is valency?
Answer: Valency is the number of electrons an atom gains, loses, or shares to complete its outer shell.
What is atomic number?
Answer: Atomic number is the number of protons in an atom. It is unique for each element.
What is mass number?
Answer: Mass number is the total number of protons and neutrons in the nucleus of an atom.
What are isotopes?
Answer: Isotopes are atoms of the same element with the same number of protons but different number of neutrons.
What are isobars?
Answer: Isobars are atoms of different elements that have the same mass number but different atomic numbers.
Why are atoms electrically neutral?
Answer: Atoms are neutral because they have an equal number of protons (positive) and electrons (negative), which balance each other.
Why are noble gases inactive?
Answer: Noble gases are inactive because their outermost shells are completely filled, so they do not need to react with other elements.
