Question 11
Explain the term – 'valency'. State giving reasons the valency of –
(a) chlorine in hydrogen chloride
(b) sulphate in sulphuric acid
(c) nitrogen in ammonia
(d) magnesium in magnesium oxide
Valency of an element or radical is the number of hydrogen atoms which can combine with or displace one atom of the element or radical forming a compound.
(a) One atom of hydrogen combines with one atom of chlorine to form HCl, hence the valency of chlorine in hydrogen chloride is 1-.
(b) Two atoms of hydrogen combine with one radical of sulphate to form sulphuric acid (H2SO4), hence the valency of sulphate in sulphuric acid is 2-.
(c) Three atoms of hydrogen combine with one atom of nitrogen to form ammonia (NH3) hence the valency of nitrogen in ammonia is 3-.
(d) We know that valency of oxygen is 2-. Hence, in magnesium oxide [MgO], valency of magnesium is 2+.
ICSE Class VII Chemistry Chapter 3 15 Marks
Atomic Structure — Quick Study Guide
Atoms are the building blocks of all matter. Understanding atomic structure is essential for all of chemistry.
Key Definitions
- Atom: Smallest particle of an element with its chemical properties
- Molecule: Group of two or more atoms bonded together
- Element: Pure substance with only one type of atom
- Compound: Two or more different elements chemically combined
- Atomic number (Z): Number of protons = Number of electrons
- Mass number (A): Protons + Neutrons
- Valency: Combining capacity of an element
- Isotopes: Same element, different mass numbers
Electronic Configuration Rules
K shell = max 2 electrons, L shell = max 8, M shell = max 8 (for first 20 elements). Fill inner shells first.
Exam Tips
- Memorise symbols of first 20 elements
- Practice calculating protons, neutrons, electrons from Z and A
- Practice drawing Bohr diagrams
- Valency: if outermost ≤ 4, valency = outermost; if > 4, valency = 8 - outermost
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