Chapter 9: Atomic Foundations of MatterClass 9 Science — summary, notes, extra questions & NCERT solutions

Summary

This chapter explains how atoms combine into molecules and compounds and how to describe them quantitatively. Two basic laws are established by experiment: the law of conservation of mass (matter is neither created nor destroyed in a chemical reaction, so total mass of reactants equals total mass of products, proposed by Lavoisier) and the law of constant (definite) proportions (a compound always contains its elements in a fixed ratio by mass whatever its source, e.g. water is always hydrogen to oxygen 1:8, proposed by Proust). These laws underlie Dalton’s atomic theory, whose postulates state that matter is made of indivisible atoms that combine in simple whole-number ratios. A molecule is an electrically neutral group of two or more atoms that can exist independently. Atoms bond to become stable by completing their octet: covalent bonds form by sharing electrons (single, double or triple bonds, as in \(\text{H}_2\), \(\text{O}_2\), \(\text{HCl}\), \(\text{H}_2\text{O}\)), while ionic bonds form by transfer of electrons creating cations and anions held by electrostatic attraction (as in NaCl, which forms a 3-D crystal lattice). The chapter teaches naming of covalent and ionic compounds and writing chemical formulae by criss-crossing valencies. Ionic compounds are usually water-soluble, high-melting and conduct electricity when dissolved or molten, while covalent compounds are usually low-melting and non-conducting. Finally it shows how to calculate molecular mass (for covalent compounds) and formula unit mass (for ionic compounds) by adding atomic masses.

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