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Atoms, Molecules and Radicals
ICSE Class 7 Chemistry • Chapter 3
Chapter Overview
To grasp Chemistry, we must understand the fundamental building blocks of all matter. In this chapter, we define elements and compounds, and explore how atoms combine to form molecules. We also introduce Valency and Radicals, the essential tools for writing chemical formulas.
3.1 Pure Substances: Elements and Compounds
Matter can be classified basically as either pure substances or impure mixtures. A pure substance consists exclusively of only one type of particle.
- Element: A pure substance that cannot be broken down into simpler substances by any physical or chemical means. It consists entirely of only one kind of atom. Examples: Oxygen ($O$), Gold ($Au$), Iron ($Fe$).
- Compound: A pure substance formed when two or more distinct elements chemically combine together in a rigidly fixed ratio by mass. Examples: Water ($H_2O$), Carbon dioxide ($CO_2$).
3.2 Atoms and Molecules
John Dalton proposed that all matter is made of extremely small particles called atoms.
- Atom: The smallest particle of an element that cleanly takes part in a chemical reaction. An atom generally does not exist independently in nature (except for Noble gases like Helium).
- Molecule: The smallest particle of a substance (element or compound) that actually physically exists independently and exhibits all the properties of that substance.
Atomicity
The total number of atoms physically present in a single molecule of an element is called its atomicity.
| Type | Definition | Examples |
|---|---|---|
| Monoatomic | Molecules containing only 1 atom. | Helium ($He$), Neon ($Ne$), Argon ($Ar$). |
| Diatomic | Molecules containing 2 atoms. | Hydrogen ($H_2$), Oxygen ($O_2$), Chlorine ($Cl_2$). |
| Polyatomic | Molecules containing more than 2 atoms. | Ozone ($O_3$), Phosphorus ($P_4$), Sulphur ($S_8$). |
3.3 Valency and Radicals
Valency
Valency is firmly defined as the chemical combining capacity of an element. It indicates exactly how many hydrogen atoms can chemically combine with one atom of that specific element.
- Valency of Hydrogen ($H$) = 1
- Valency of Oxygen ($O$) = 2
- Valency of Carbon ($C$) = 4
Radicals
A radical is a specific group of atoms (belonging to different elements) that chemically behaves entirely like a single unit and possesses a fixed overall charge. They have their own specific valency.
Common Radicals You MUST Memorize
- Hydroxide: $OH$ (Valency = 1)
- Nitrate: $NO_3$ (Valency = 1)
- Carbonate: $CO_3$ (Valency = 2)
- Sulphate: $SO_4$ (Valency = 2)
- Ammonium: $NH_4$ (Valency = 1, Basic Radical)
AI Image Prompt: A colorful 3D diagram showing the Carbon atom in the center with 4 empty slots physically waiting to connect (indicating valency 4). Four smiling Hydrogen atoms (indicating valency 1) are snapping perfectly into those slots to form a stable Methane ($CH_4$) molecule.
Q1. Nitrogen gas exists as $N_2$. What is its atomicity?
Ans: Because its molecule contains precisely 2 atoms, Nitrogen is uniquely diatomic. Its atomicity is 2.
Q2. State the exact valency of the Sulphate radical.
Ans: The overall valency of the Sulphate ($SO_4$) radical is 2.