Journey Inside | Class 9 Science Notes

8.7 How Are Electrons Distributed in Different Energy Levels?

Electrons in atoms are arranged in shells or energy levels around the nucleus. Bohr and Bury proposed rules for electron distribution: the maximum number of electrons in a shell is 2n², where n is the shell number (K=1, L=2, M=3, etc.). Thus, K shell can hold 2 electrons, L shell 8 electrons, M shell 18 electrons. The outermost shell can hold a maximum of 8 electrons (except the first shell which holds 2). Electrons fill shells starting from the innermost (K) outward in order. For example, hydrogen (atomic number 1) has its single electron in the K shell. Helium (atomic number 2) fills the K shell with 2 electrons. The electronic configuration represents this distribution. Fig. 8.11 shows the atomic structures and electron filling for the first eighteen elements. Table 8.4 details atomic numbers, protons, neutrons, electrons, and electron distribution in shells for these elements. Understanding electron distribution is essential for explaining chemical properties and valency.

📊 Diagram: Fig. 8.11: Schematic atomic structure of the first eighteen elements showing electron filling in K, L, M shells; Table 8.4: Atomic data and electron distribution of first eighteen elements.

🧪 Activity: Pause and Ponder questions on valence electrons and electronic configurations.

🔗 Connection: Leads to valency and combining capacity of atoms.