(a) Atom — An atom is the smallest particle of an element that exhibits all the properties of that element. It may or may not exist independently but takes part in every chemical reactions.
Example : O — Oxygen atom, H — Hydrogen atom
(b) Molecule — A molecule is the smallest particle of an element or compound, made up of two or more atoms of an element or compound which has independent existence exhibiting all of their properties.
Example : O2 — Oxygen molecule, H2O — Water molecule
(c) Radical — A radical is an atom of an element or a group of atoms of different elements that behaves as a single unit with a positive or negative charge on it.
Example : NH4+ — Ammonium radical
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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