This chapter explains that atoms are not indivisible but are made up of smaller particles called electrons, protons, and neutrons. It describes how scientists discovered these particles through experiments and proposed different models of the atom to explain its structure. The chapter discusses Thomson’s model, Rutherford’s nuclear model, and Bohr’s model, showing how each model improved our understanding of atomic structure. It also introduces important ideas such as atomic number, mass number, valency, isotopes, and isobars, helping students understand how atoms are arranged and how elements differ from one another.
Key Points
Atoms are made up of smaller particles called electrons, protons, and neutrons.
Electrons carry negative charge, while protons carry positive charge.
Neutrons have no charge and are present in the nucleus.
J.J. Thomson discovered the electron and proposed the plum pudding model.
Rutherford’s gold foil experiment showed that atoms have a small, dense nucleus.
Most of the atom is empty space.
The nucleus contains almost all the mass of the atom.
Rutherford’s model could not explain the stability of atoms.
Bohr proposed that electrons move in fixed energy levels or shells.
Electrons do not lose energy while revolving in allowed shells.
The maximum number of electrons in a shell is given by 2n².
The outermost shell of an atom can have a maximum of 8 electrons.
Valency depends on the number of electrons in the outermost shell.
Atomic number is equal to the number of protons in an atom.
Mass number is the sum of protons and neutrons.
Isotopes are atoms of the same element with different mass numbers.
Isobars are atoms of different elements with the same mass number.
Isotopes have similar chemical properties but different physical properties.
👉 👉The structure of the atom shows that matter is made of well-organised particles. Understanding atomic structure helps us explain the behaviour of elements, their reactions, and their applications in science, medicine, and technology.