Q1: Who first used the term ‘atomos’ and what does it mean?
Ans: The Greek philosophers Leucippus and Democritus used the term atomos, which means indivisible in Greek.
Q2: What did Acharya Kanada call the smallest indivisible particles of matter?
Ans: Acharya Kanada called them parmanus. His ideas are recorded in the Sanskrit text Vaisesika Sutras.
Q3: In which year did John Dalton propose his atomic theory and what was its main idea?
Ans: In 1808, Dalton proposed that all matter is composed of indivisible particles called atoms, which are the fundamental building blocks of matter.
Q4: What are cathode rays?
Ans: Cathode rays are streams of negatively charged particles (electrons) that move from the cathode (negative electrode) to the anode (positive electrode) in a discharge tube.
Q5: What is the charge of an electron?
Ans: The charge of an electron is C, which is taken as as a matter of convention.
Q6: Why is Thomson’s model of the atom called the plum pudding model?
Ans: Thomson proposed the atom as a sphere of positive charge with electrons distributed throughout it, just like plums embedded in a pudding, so it is called the plum pudding model.
Q7: What is the gold foil experiment also known as?
Ans: The gold foil experiment is also called the α-ray scattering experiment. It was performed by Geiger and Marsden under Ernest Rutherford in 1911.
Q8: What is the nucleus of an atom?
Ans: The nucleus is the extremely small, dense, positively charged region at the centre of an atom where most of the atom’s mass is concentrated.
Q9: State the main limitation of Rutherford’s model of the atom.
Ans: Rutherford’s model could not explain the stability of the atom. According to classical physics, an accelerating electron should lose energy and spiral into the nucleus, causing atoms to collapse.
Q10: Who discovered the proton and what is its charge relative to the electron?
Ans: Rutherford discovered and named the proton. Its charge is equal and opposite to that of an electron, taken as .
Q11: What are the stationary orbits (shells) in Bohr’s model of the atom?
Ans: According to Bohr, electrons revolve around the nucleus in fixed circular paths called stationary states, orbits, or shells, in which an electron does not lose energy.
Q12: Who discovered the neutron and in which year?
Ans: James Chadwick discovered the neutron in 1932. It is a neutral subatomic particle with a mass nearly equal to that of a proton.
Q13: What is the atomic number of an element? Give its symbol.
Ans: The atomic number is the number of protons present in the nucleus of an atom of an element. It is denoted by the symbol .
Q14: Write the formula for mass number and define nucleons.
Ans: The protons and neutrons present in the nucleus are collectively called nucleons.
Q15: What is the maximum number of electrons that can be filled in the K, L, and M shells?
Ans: Using the formula : K-shell holds 2 electrons, L-shell holds 8 electrons, and M-shell holds 18 electrons.
Q16: What are valence electrons?
Ans: The electrons present in the outermost shell of an atom are called valence electrons. The outermost shell itself is known as the valence shell.
Q17: Define valency of an element.
Ans: Valency is the combining capacity of an atom. It is equal to the number of electrons gained, lost, or shared by an atom to complete its octet and become stable.
Q18: What are isotopes? Give one example.
Ans: Isotopes are atoms of the same element that have the same atomic number but different mass numbers. For example, hydrogen has three isotopes: protium , deuterium , and tritium .
Q19: What are isobars? Give an example.
​Ans: Isobars are atoms of different elements that have the same mass number but different atomic numbers. For example, calcium , potassium , and argon all have a mass number of 40.
Q20: Give one use each of the isotopes 614C and 53131I.
​Ans: is used in archaeology and geology to determine the age of ancient fossils and artefacts. is used to treat goitre and thyroid cancer.