Science class online
Chemistry and physics lessons
atom physics lesson


Free online chemistry lesson for elementary school, middle school and high school.
chemistry high school

Science class




Mechanics lessons
Water on Earth
Changes of state in the nature: the water cycle
Water in human body
Test for water
Properties of water in different states
States of matter
States of matter and its changes
Boiling water
Water: freezing and melting
Changes of state: mass and volume
Molecules in different states of matter

Mixtures and solutions
Heterogeneous mixtures
Homogeneous mixtures
Vaporization of water
Dissolving a solid in water
Conservation of mass on dissolving
Miscibility in water

Mass and volume
Volume and its units
Measuring volume with a graduated cylinder
Mass and its units
Measuring the mass of a liquid
Mass of a liter of water
Volumetric mass density
Air and atmosphere
Earth's atmosphere protect us
Threats to the Earth's atmosphere
Composition of air
Air and life
Atoms and molecules
Molecules in mixtures and pure substances
Molecules and states of matter
Composition of molecules
Constituents of the atoms
The electrical neutrality of the atom

Basics of combustion
The combustion of carbon
The combustion of butane
Atoms and chemical reactions
Chemical reactions
Chemical equations
Law of conservation of mass
Chemical synthesis

Most common metals

How to distinguish metals ?

Corrosion of metals

Electrical conductivity of solid materials

Electrons and free electrons

Electric current in metals
The conductivity of aqueous solutions
Aqueous solutions and ions
The direction of movement of ions
Formation of ions
Tests for ions
Acidic and basic solutions
pH of aqueous solutions
Ions in acidic and basic solutions
Dilution of acids and bases
Composition of hydrochloric acid
Chemical reaction between iron and hydrochloric acid

Electrochemical cells and chemical energy
Chemical reaction beteween a copper sulphate solution and zinc
Copper sulfate and zinc battery
Basics of electrochemical cell


Atoms and molecules

Compostion of molecules

1) What is a molecule ?

Atoms can gather to form molecules.
A molecule is a group of several identical or different atoms linked together.

2) Chemical formulas

A molecule is represented by a formula that indicates:
- The different kinds of atoms within the molecule (Each atom is represented by its symbol)
- The number of each atom (with a subscript after the symbol)

For instance the water molecular formula is H2O:
  - The symbol H indicates that the water molecule consists of atoms of hydrogen and its subscript ( 2 ) indicates that there are two atoms.
- The symbol O indicates a water molecule is composed of oxygen atoms but the absence of subscript indicates that there is only one atom.
The water molecule thus consists of two hydrogen atoms and one oxygen atom.

Some examples of chemical formulas to know:
The molecule of dioxygen: formula O
The dinitrogen molecule: formula N2
The molecule of carbon dioxide: formula CO2
The methane molecule: formula CH4
The molecule of carbon monoxide: formula CO
The butane molecule: formula C


3) Molecular models

Atoms are represented by models which are colored spheres.

Main models
atom model

These sphere can be used to build molecules
Some simple molecular models
molecular model of water, dioxygen, methane and carbon dioxide