Important Diagrams: Structure of the Atom | Science Class 9 PDF Download

Q1: Answer the following questions based on the diagram given below:

(i) What does Thomson's model of an atom resemble?
Ans: Thomson's model of an atom is likened to a plum‑pudding or a positively charged sphere in which electrons are embedded like plums in a pudding. The model was proposed to explain the existence of electrons inside atoms.

(ii) How does Thomson's model describe the distribution of charge within an atom?
Ans: Thomson's model suggests that the positive charge is spread uniformly throughout the atom, with electrons embedded within it, dispersing the negative charge.

(iii) What is the nature of the particles in Thomson's atomic model?
Ans: The atom contains positively charged matter distributed throughout the sphere and negatively charged electrons embedded in that positive sphere. The electrons are discrete particles within the continuous positive background.

(iv) What is the role of electrons in Thomson's atomic model?
Ans: Electrons in Thomson's model are depicted as negatively charged particles embedded within the positively charged sphere, providing a balanced charge for the atom.

(v) How does Thomson's model differ from the current understanding of atomic structure?
Ans: Thomson's model places electrons embedded uniformly in a positive sphere. The modern view is different: atoms have a small, dense, positively charged nucleus (containing protons and neutrons) with electrons arranged in energy levels (shells) around the nucleus; electrons are not embedded in a uniform positive medium.

Q2: Answer the following questions based on the diagram given below:

(i) What were the unexpected observations from the α‑particle scattering experiment?
Ans:

• Most of the fast moving α‑particles passed straight through the thin gold foil without any deflection.
• Some α‑particles were deflected by small angles.
• Approximately one out of many thousands of α‑particles (about 1 in 12,000 in the actual experiment) was deflected by a very large angle and a few even rebounded.

(ii) What did Rutherford conclude from the α‑particle scattering experiment regarding the space inside an atom?
Ans:

• Most of the atom is empty space, since most α‑particles passed through without being deflected.
• The positive charge and most of the mass of the atom are concentrated in a very small central region (the nucleus), because a few α‑particles were strongly deflected.
• The nucleus must be very small compared to the atom, since only a tiny fraction of α‑particles experienced large deviations.

(iii) How did Rutherford calculate the radius of the nucleus based on the α‑particle scattering experiment?
Ans: From the data obtained in the α-particle scattering experiment, Rutherford calculated that the radius of the nucleus is about 10⁵ times smaller than the radius of the atom. This showed that the nucleus occupies only a very small space and that most of the atom is empty.

(iv) What were the features of Rutherford's nuclear model of an atom?
Ans:

• The atom has a positively charged centre called the nucleus, containing nearly all the atom's mass.
• Electrons revolve around the nucleus in circular orbits (classically assumed) occupying most of the atom's volume.
• The size of the nucleus is very small compared to the overall size of the atom.

(v) What was a major drawback of Rutherford's model of the atom, and why was it considered unstable?
Ans: According to Rutherford’s model, electrons revolve around the nucleus in circular orbits. Such revolving electrons should continuously lose energy and finally fall into the nucleus. If this happened, the atom would be unstable. However, atoms are known to be stable, so Rutherford’s model could not explain the stability of the atom.

Q3: Answer the following questions based on the diagram given below:

(i) What do the concentric circles around the nucleus represent in the given diagram?
Ans: The concentric circles represent energy shells (orbits) in which electrons revolve around the nucleus. These shells are commonly labelled K, L, M, N corresponding to principal quantum numbers n = 1, 2, 3, 4 respectively.

(ii) Why are electrons placed in different shells rather than all being in one shell?
Ans: Electrons are placed in different shells because each shell has a maximum capacity determined by the 2n² rule. Electrons fill the shells in order of increasing energy, starting with the shell closest to the nucleus. This arrangement provides stability to the atom.

(iii) What is the maximum number of electrons that can be present in the third orbit or M‑shell?
Ans: Using the formula 2n² for maximum electrons in the nth shell, for n = 3:
Maximum electrons = 2 × (3)² = 2 × 9 = 18 electrons.

(iv) How many electrons can the outermost orbit of an atom hold?
Ans: The outermost orbit of an atom can hold a maximum of 8 electrons.

(v) In what manner are electrons accommodated in different shells of an atom?
Ans: Electrons are not accommodated in a given shell unless the inner shells are filled. In other words, the shells are filled in a step-wise manner.

Q4: Find out the valency of the atoms represented by the Fig. (a) and (b).

Ans: Atom (a) has zero valency because it has a stable configuration with 8 electrons in the valence shell (complete octet). Atom (b) has valency 1 because its valence shell has 7 electrons and it needs one more electron to attain a stable octet configuration.

Q5: What information do you get from the Fig. about the atomic number, mass number and valency of atoms X, Y and Z? Give your answer in a tabular form.

Ans:

Q6: The given figure depicts the atomic structure of an atom of an element ‘X’. Write the following information about the element ‘X’.

(a) Atomic number of ‘X’
(b) Atomic mass of ‘X’
(c) Valence electrons
(d) Valency of ‘X’
(e) ‘X’ should be metal or non‑metal.

Ans: (a) 12
(b) 24
(c) 2
(d) 2
(e) Metal