Structure of Atom in One-Shot Class 9 Science Chemistry Full Chapter CBSE Class 9 Exams BYJUS

Structure of the Atom: Charged Particles, Models, and Electronic Configuration

The discovery that atoms are not indivisible — as Dalton had proposed — but are composed of even smaller subatomic particles, was one of the most revolutionary developments in science. This chapter in CBSE Class 9 Science traces the historical discovery of electrons, protons, and neutrons, the evolution of atomic models from Thomson to Bohr, and the modern system of electronic configuration that explains how electrons are arranged in atoms.

J.J. Thomson discovered the electron in 1897 through cathode ray tube experiments, determining that electrons are negatively charged particles with a very small mass compared to atoms. E. Goldstein discovered positively charged canal rays (protons). James Chadwick discovered the neutron in 1932 — a neutral particle found in the nucleus with a mass similar to that of a proton. Thomson proposed the "plum pudding" model: a sphere of positive charge with electrons embedded in it like seeds in a pudding. Ernest Rutherford's gold foil experiment (1911) revolutionised this understanding — when alpha particles were directed at a thin gold foil, most passed straight through, some were deflected at small angles, and a very few bounced back. Rutherford concluded that the atom is mostly empty space, with a tiny, dense, positively charged nucleus at the centre containing all the mass, and electrons orbiting around it. However, Rutherford's model had a fatal flaw: according to classical physics, orbiting electrons should continuously radiate energy and spiral into the nucleus, making atoms unstable. Neils Bohr resolved this in 1913 by proposing that electrons orbit in specific, fixed energy levels (shells) without radiating energy. An electron can jump from a lower to a higher energy level by absorbing energy, and drop from a higher to a lower level by emitting energy as radiation.

The modern picture of the atom consists of a nucleus containing protons (positively charged, each with one unit of charge and a mass of approximately 1 atomic mass unit) and neutrons (neutral, similar mass). Electrons (negatively charged, negligible mass) revolve around the nucleus in discrete energy levels called shells, designated K, L, M, N (or n = 1, 2, 3, 4). The maximum number of electrons in a shell is given by 2n², where n is the shell number: K holds 2, L holds 8, M holds 18, N holds 32. However, the outermost shell (valence shell) can hold a maximum of 8 electrons. The atomic number (Z) equals the number of protons in the nucleus, and in a neutral atom, Z also equals the number of electrons. The mass number (A) equals the total number of protons plus neutrons: A = Z + N. Isotopes are atoms of the same element (same Z) with different mass numbers (different number of neutrons). For example, hydrogen has three isotopes: protium (¹H, no neutrons), deuterium (²H, one neutron), and tritium (³H, two neutrons). Isobars are atoms of different elements with the same mass number but different atomic numbers, such as ⁴⁰Ca (Z = 20) and ⁴⁰Ar (Z = 18). The valency of an element is determined by the number of electrons in the outermost shell — elements tend to gain, lose, or share electrons to achieve a stable octet (8 electrons in the valence shell). The electronic configuration of sodium (Na, Z = 11) is 2, 8, 1, giving it a valency of 1 (it loses one electron to form Na⁺). Chlorine (Cl, Z = 17) has the configuration 2, 8, 7, giving it a valency of 1 (it gains one electron to form Cl⁻).

• Atoms consist of protons (+charge, in nucleus), neutrons (neutral, in nucleus), and electrons (−charge, orbiting in shells).
• Rutherford's gold foil experiment revealed the nuclear atom — tiny dense nucleus, mostly empty space, electrons outside.
• Bohr's model: electrons orbit in fixed energy levels (K=2, L=8, M=18); valence shell holds max 8 electrons.
• Atomic number Z = number of protons; mass number A = protons + neutrons; isotopes have same Z but different A.
• Valency is determined by valence shell electrons — atoms gain, lose, or share electrons to achieve a stable octet.