Journey Inside the Atom | Class 9 Science Notes
By ConceptScroll Team · Published on 17 July 2026 · 4 min read
Journey Inside the Atom – this guide gives you a concise, exam-ready overview of Journey Inside the Atom from Class 9 Science, written by ConceptScroll editors and reviewed against the latest NCERT textbook.
8.7 How Are Electrons Distributed in Different Energy Levels?
Bohr and Bury proposed rules for electron distribution in atomic shells:
- The maximum number of electrons in a shell is given by 2n², where n is the shell number.
- The K-shell (n=1) can hold 2 electrons, L-shell (n=2) 8 electrons, M-shell (n=3) 18 electrons, and so on.
- The outermost shell can hold a maximum of 8 electrons (except the first shell).
- Electrons fill shells starting from the innermost (K) moving outward sequentially.
For example, hydrogen (atomic number 1) has one electron in the K-shell. Helium (atomic number 2) has two electrons filling the K-shell.
The arrangement of electrons in shells is called electronic configuration. Fig. 8.11 shows the electronic configurations of the first eighteen elements.
Table 8.4 lists elements with their atomic numbers, protons, neutrons, electrons, and electron distribution in shells.
Understanding electron distribution helps explain chemical properties and valency.
📊 Diagram: Fig. 8.11 shows schematic atomic structures of first eighteen elements with electrons filled in K, L, M shells. Table 8.4 details atomic numbers, protons, neutrons, electrons, and shell distribution.
🧪 Activity: Pause and Ponder questions on valence electrons and electronic configurations.
🔗 Connection: Leads to valency and combining capacity of atoms.
Frequently asked questions
According to the Bohr’s atomic model? Give a reason for each statement. (i) Electrons lose energy while moving in fixed orbits and slowly fall into the nucleus. (ii) Electrons can exist anywhere around the nucleus with no fixed energy. (iii) Electrons revolve around the nucleus in orbits of fixed energy without losing energy. (iv) Electrons can be found between energy levels as they move around the nucleus.
According to Bohr's atomic model:
(i) Electrons do NOT lose energy while moving in fixed orbits and hence do not fall into the nucleus. This is because electrons revolve in fixed energy levels or orbits where they do not emit energy.
(ii) Electrons cannot exist anywhere around the nucleus with no fixed energy. Bohr's model states that electrons can only exist in certain fixed orbits with definite energies.
(iii) Electrons revolve around the nucleus in orbits of fixed energy without losing ene
The composition of the nuclei of three atomic species X, Y, and Z are given as follows. | | X | Y | Z | | --- | --- | --- | --- | | Number of protons | 18 | 17 | 17 | | Number of neutrons | 19 | 18 | 20 | Explain the relation between the following: (i) Y and Z (ii) Z and X
(i) Y and Z have the same number of protons (17) but different number of neutrons (18 and 20 respectively). Therefore, Y and Z are isotopes of the same element.
(ii) Z and X have different numbers of protons (17 and 18 respectively) and different numbers of neutrons (20 and 19 respectively). Therefore, Z and X are different elements.
What conclusion did Rutherford draw about the position and characteristics of the atom’s positively charged part based on the few alpha particles that bounced back or were deflected at large angles in the gold foil experiment?
Rutherford concluded that the atom has a small, dense, positively charged nucleus at its center. The fact that a few alpha particles were deflected at large angles or bounced back indicated that the positive charge and most of the mass of the atom are concentrated in a tiny central nucleus, while the rest of the atom is mostly empty space.
Explain and arrange the following statements in the correct chronological order to show how atomic models have evolved over time. (i) Bohr's model proposed that electrons move in fixed orbits around the nucleus, each with a definite energy. (ii) Thomson's model depicted the atom as a 'plum pudding' with electrons embedded in a sphere of positive charge. (iii) Rutherford's model proposed that atoms have a dense central nucleus. (iv) Dalton's model described atoms as indivisible particles.
Correct chronological order:
(iv) Dalton's model described atoms as indivisible particles. (ii) Thomson's model depicted the atom as a 'plum pudding' with electrons embedded in a sphere of positive charge. (iii) Rutherford's model proposed that atoms have a dense central nucleus. (i) Bohr's model proposed that electrons move in fixed orbits around the nucleus, each with a definite energy.
Explanation: Dalton's model was the earliest, describing atoms as indivisible. Thomson discovered electron
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