Chemistry (NTSE/Olympiad)  

3. Atoms & Molecules


A combination of atoms is called a molecule. The forces which hold the atoms together in a molecule are called covalent bonds.
A molecule is the smallest particle of a substance which has the properties of that substance and can exist in the free state.
There are two types of molecules.
Molecules of elements
The molecule of an element contains two (or more) similar atoms chemically combined together.
Example. A molecule of hydrogen element contains 2 hydrogen atoms combined together, and it is written as H2.
Molecules of compounds
The molecule of a compound contains two (or more) different types of atoms chemically combined together.
Example. Hydrogen chloride is a compound. The molecule of hydrogen chloride (HCl) contains two different types of atom. Hydrogen(H) and chlorine atom (Cl)
Molecules of some compounds
Compound Combining elements Formula Ratio by mass
Water Hydrogen and oxygen H2O 1:8
Ammonia Nitrogen and Hydrogen NH3 14:3
Carbon doxide Carbon and Oxygen CO2 3:8

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