STRUCTURE OF THE ATOM-QnA

Q1: Who discovered the electron?

J.J. Thomson in 1897.

Q2: What is the charge on a proton?

+1 elementary charge.

Q3: Name the negatively charged particle of an atom.

Electron.

Q4: What is the charge on a neutron?

Neutron has no charge; it is neutral.

Q5: Who discovered the nucleus of an atom?

Ernest Rutherford.

Q6: What are the three fundamental particles of an atom?

Electron, proton, and neutron.

Q7: Who proposed the planetary model of the atom?

Rutherford.

Q8: Who discovered the neutron?

James Chadwick in 1932.

Q9: What is the atomic number?

Number of protons in an atom.

Q10: What is the mass number?

Sum of protons and neutrons in an atom.

Q11: What does the symbol ‘Na’ represent?

Sodium atom.

Q12: What is the charge on an alpha particle?

+2 charge.

Q13: Where are electrons located in an atom?

In energy shells around the nucleus.

Q14: Which particle is responsible for the identity of an element?

Proton.

Q15: What is the maximum number of electrons in the K shell?

2 electrons.

Q1: Explain Thomson’s atomic model briefly.

Thomson proposed that the atom is a positively charged sphere with electrons embedded in it, like plums in pudding.

Q2: State two observations of Rutherford’s a-particle scattering experiment.

Most a-particles passed straight through; a few were deflected or bounced back.

Q3: What conclusions did Rutherford draw from his experiment?

Atom has a small, dense, positively charged nucleus; most of the atom is empty space.

Q4: What are isotopes? Give one example.

Atoms of the same element with the same atomic number but different mass numbers; e.g., \(^1H,\, ^2H,\, ^3H.\)

Q5: What are isobars? Give one example.

Atoms of different elements with the same mass number but different atomic numbers; e.g., \(^{40}Ar\) and \(^{40}Ca\).

Q6: Define valency.

The combining capacity of an atom to gain, lose, or share electrons.

Q7: State the electronic configuration of oxygen (Z=8).

2, 6.

Q8: What is the relation between atomic number, mass number, and neutrons?

Number of neutrons = Mass number – Atomic number.

Q9: Write two limitations of Rutherford’s model.

Could not explain electron stability and discrete line spectra.

Q10: What is Bohr’s postulate about electron energy levels?

Electrons revolve in fixed orbits without losing energy; energy is absorbed/emitted during level changes.

Q1: Describe Rutherford’s model of the atom.

Atom has a dense, positively charged nucleus containing protons; electrons revolve in orbits around it; most space is empty.

Q2: Explain Bohr’s model of the atom in detail.

Bohr proposed that electrons revolve in fixed energy levels (shells) without radiating energy; when they jump levels, energy is absorbed or emitted as light.

Q3: Discuss the discovery of electrons, protons, and neutrons.

Electrons were discovered by J.J. Thomson (cathode rays), protons by Goldstein (canal rays), and neutrons by Chadwick (bombardment of beryllium).

Q4: What are isotopes, isobars, and isotones? Give examples.

Isotopes – same element, different mass \((^1\mathrm{H},\, ^2\mathrm{H})\). Isobars – same mass, different element \((^{40}\mathrm{Ar},\, ^{40}\mathrm{Ca})\). Isotones – same neutrons \((^{14}\mathrm{C}, ^{15}\mathrm{N})\).

Q5: Explain the limitations of the Bohr model.

Could not explain fine structure of spectra, Zeeman effect, or behavior of multi-electron atoms.

Q1: Describe in detail Rutherford’s a-particle scattering experiment and its significance.

Rutherford bombarded gold foil with a-particles. Most passed through, few deflected or rebounded. He concluded that atom has a small, dense, positively charged nucleus and most space is empty — leading to nuclear model of the atom.

Q2: Explain Bohr’s atomic model with diagram and postulates.

Bohr proposed quantized orbits (K, L, M...), electrons revolve without radiating energy, and energy is emitted/absorbed only when electrons change orbits. This explained hydrogen line spectrum.

Q3: Discuss the discovery and properties of cathode rays and anode rays.

Cathode rays (electrons) travel from cathode to anode, negatively charged; Anode rays (protons) travel in opposite direction, positively charged; their study led to discovery of subatomic particles.

Q4: What are isotopes? Mention their applications.

Isotopes are atoms of the same element with different mass numbers. Used in medical imaging (Cobalt-60), carbon dating (C-14), and nuclear energy (U-235).

Q5: Describe how modern atomic theory explains the stability of atoms.

Modern theory combines quantum mechanics with Bohr’s model; electrons exist in orbitals defined by probability, not fixed paths, ensuring energy stability and explaining spectra.

Q1: What are canal rays?

Q2: If an atom contains one electron and one proton, will it carry any charge or not?

If an atom contains one electron and one proton, it will not carry any charge. This is because the electron has a negative charge, and the proton has a positive charge of equal magnitude. When you add these charges together, they cancel each other out perfectly. So, the atom as a whole remains electrically neutral. In fact, the most common example of such an atom is the hydrogen atom, which has one electron and one proton and carries no overall charge.

Q3: On the basis of Thomson’s model of an atom, explain how the atom is neutral as a whole.

According to Thomson’s model, an atom is like a positively charged sphere with negatively charged electrons scattered throughout it—similar to plums in a pudding or seeds in a watermelon. In this model, the positive charge is spread uniformly across the entire sphere, and the electrons are distributed within it. The key is that the total positive charge exactly balances the total negative charge from all electrons. Because these two are equal in magnitude but opposite in nature, the atom as a whole is electrically neutral. This simple and balanced arrangement of charges is how, according to Thomson, atoms remain neutral overall, despite containing both positive and negative parts.

Q4: On the basis of Rutherford’s model of an atom, which sub atomic particle is present in the nucleus of an atom?

According to Rutherford’s model of the atom, the nucleus is a tiny, dense core at the center of the atom. It was in this central nucleus that Rutherford found all the positive charge of the atom to be concentrated. The subatomic particle present here is the proton. Protons are positively charged particles, and in Rutherford's famous gold foil experiment, he showed that these particles are all tightly packed into the nucleus, while the electrons orbit around it. So, based on Rutherford’s atomic model, the nucleus of an atom contains protons.

Q5: Draw a sketch of Bohr’s model of an atom with three shells.

To draw Bohr’s model of an atom with three shells, imagine the nucleus at the center as a small solid circle. This nucleus holds the protons and neutrons closely packed together. Around this nucleus, draw three concentric circles, which will represent the electron shells. Label these shells as K (the innermost), L (the middle), and M (the outermost).

In this model:

• The electrons move in fixed orbits or shells outside the nucleus, keeping a defined distance from it.
• You can place up to 2 electrons on the first shell (K), up to 8 on the second shell (L), and more on the third shell (M), depending on the element.

Q6: What do you think would be the observation if the a-particle scattering experiment is carried out using a foil of a metal other than gold?

If the a-particle scattering experiment was done with a metal foil other than gold, the main observations would stay the same: most alpha particles would pass straight through, a few would get deflected, and very few would bounce back. This is because all atoms have a tiny, positively charged nucleus.
However, the exact amount and angles of deflection might differ slightly depending on the metal’s atomic number, since heavier metals have larger nuclei.
Gold was preferred simply because it can be made into very thin sheets for clearer results, but the basic outcome would hold true for any metal foil.

Q7: Name the three sub-atomic particles of an atom.

Q8: Helium atom has an atomic mass of 4 u and two protons in its nucleus. How many neutrons does it have?

Q9: Write the distribution of electrons in carbon and sodium atoms.

Q10: If K and L shells of an atom are full, then what would be the total number of electrons in the atom?

Q11: How will you find the valency of chlorine, sulphur and magnesium?

Q12: If number of electrons in an atom is 8 and number of protons is also 8, then (i) what is the atomic number of the atom? and (ii) what is the charge on the atom?

Q13: For the symbol H,D and T tabulate three sub-atomic particles found in each of them.

Q14: Write the electronic configuration of any one pair of isotopes and isobars.