Atomic Foundations | Class 9 Science Notes
By ConceptScroll Team · Published on 17 July 2026 · 5 min read
Atomic Foundations – this guide gives you a concise, exam-ready overview of Atomic Foundations from Class 9 Science, written by ConceptScroll editors and reviewed against the latest NCERT textbook.
Discovery of Subatomic Particles
The discovery of subatomic particles was a major milestone in understanding atomic structure. Scientists found that atoms are not indivisible but are composed of smaller particles. The first subatomic particle discovered was the electron, by J.J. Thomson in 1897. Using a cathode ray tube, Thomson observed that cathode rays were deflected by electric and magnetic fields, indicating they were negatively charged particles much smaller than atoms. He proposed the 'plum pudding' model where electrons were embedded in a positively charged sphere. Later, Ernest Rutherford conducted the gold foil experiment, where alpha particles were directed at a thin gold foil. Most passed through, but some were deflected at large angles. This led to the discovery of the nucleus, a small, dense, positively charged center of the atom. Rutherford proposed that most of the atom's mass is concentrated in the nucleus, with electrons orbiting around it. Subsequently, James Chadwick discovered the neutron in 1932, a neutral particle in the nucleus with mass similar to the proton. The identification of protons, neutrons, and electrons explained the atom's structure and properties. Protons are positively charged, electrons negatively charged, and neutrons have no charge. The number of protons (atomic number) defines the element, while neutrons contribute to isotopes. Electrons determine chemical behavior. This discovery revolutionized chemistry and physics, enabling the development of nuclear energy and quantum mechanics.
📊 Diagram: Diagram of the gold foil experiment showing alpha particles passing through gold foil with some deflected, indicating a small dense nucleus. Also, illustration of the plum pudding model and the nuclear model of the atom.
🧪 Activity: No specific activity in this section.
🔗 Connection: This section explains the discovery of subatomic particles, setting the stage for the next section on the atomic models that describe the arrangement of these particles.
Frequently asked questions
1. A particular element (A) has one electron in its third shell. There is another element (B) with six electrons in its second shell. (i) How many electrons does A tend to give or take to become stable? (ii) What kind of ion would it form? (iii) How many electrons does B tend to give or take to become stable? (iv) What kind of ion would it form? (v) If A and B were to combine, what kind of bond would be formed? (vi) What would be the formula for the compound thus formed?
(i) Element A has one electron in its third shell. To become stable, it tends to give away 1 electron to have a complete second shell (which has 8 electrons).
(ii) By losing 1 electron, element A forms a cation with a charge of +1, i.e., A⁺.
(iii) Element B has six electrons in its second shell. To become stable, it tends to gain 2 electrons to complete the shell (8 electrons).
(iv) By gaining 2 electrons, element B forms an anion with a charge of -2, i.e., B²⁻.
(v) When A and B combine, A d
2. An element X has six electrons in its outer shell and forms a diatomic molecule. (i) Why would that be so? (ii) What kind of bond would it form? (iii) Draw the structure of the molecule it would form. (iv) A certain other element Y has two electrons in its second shell. Draw the structure of the molecule that X would form with Y.
(i) Element X has six electrons in its outer shell, so it needs two more electrons to complete its octet. It forms a diatomic molecule by sharing electrons with another atom of the same element to complete the octet.
(ii) It forms a covalent bond by sharing electrons.
(iii) The structure of the molecule is X—X with a double bond (two pairs of shared electrons) between the two atoms.
(iv) Element Y has two electrons in its second shell and needs six more to complete its octet. When X and Y com
3. You want to design a new ionic compound, where the total positive charge is 6+ and the total negative charge is 6-. Which of the following combinations gives the correct number of ions? (i) 2Al³⁺ and 3Cl⁻ (ii) 3Mg²⁺ and 1PO₄³⁻ (iii) 2Fe³⁺ and 3O²⁻ (iv) 3Ca²⁺ and 2SO₄²⁻
All four combinations give a total positive charge of 6+ and total negative charge of 6-.
(i) 2Al³⁺ = 2 × +3 = +6; 3Cl⁻ = 3 × -1 = -3 (Incorrect total negative charge)
(ii) 3Mg²⁺ = 3 × +2 = +6; 1PO₄³⁻ = 1 × -3 = -3 (Incorrect total negative charge)
(iii) 2Fe³⁺ = 2 × +3 = +6; 3O²⁻ = 3 × -2 = -6 (Correct)
(iv) 3Ca²⁺ = 3 × +2 = +6; 2SO₄²⁻ = 2 × -2 = -4 (Incorrect total negative charge)
Therefore, only option (iii) is correct.
4. Choose the correct statement(s) and correct the false statement(s). (i) Elements are made up of molecules and compounds are made up of atoms. (ii) The molecule of a compound is always made up of two or more atoms of the same kind. (iii) One molecule of nitrogen gas contains three nitrogen atoms. (iv) Water is made of two hydrogen atoms, covalently bonded with one oxygen atom.
(i) False. Elements are made up of atoms, and compounds are made up of molecules. (ii) False. Molecules of compounds are made up of atoms of different kinds. (iii) False. One molecule of nitrogen gas (N₂) contains two nitrogen atoms. (iv) True. Water (H₂O) consists of two hydrogen atoms covalently bonded to one oxygen atom.
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