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

Chemistry

Electricity

Optics

Mechanics
Mechanics lessons
Water
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
Decantation
Centrifugation
Filtration
Vaporization of water
Distillation
Chromatography
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

Air and atmosphere
Atmosphere
Earth's atmosphere protect us
Threats to the Earth's atmosphere
Composition of air
Air and life
Pressure
Atoms and molecules
Molecules
Molecules in mixtures and pure substances
Molecules and states of matter
Atoms
Composition of molecules
Constituents of the atoms
The electrical neutrality of the atom

Combustions
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
Metals

Most common metals

How to distinguish metals ?

Corrosion of metals

Electrical conductivity of solid materials

Electrons and free electrons

Electric current in metals
Ions
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



















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Atoms and molecules

Constituents of the atom

   
An atom always consists of two kinds of particles: electrons and a nucleus.

1) The nucleus

- The shape: the nucleus is spherical.
- Location: it is in the center of the atom.
- Size: it is about 100,000 times smaller than the atom to which it belongs (the size of an atom is of the order of 10-10 m while that of a nucleus is 10-15 m.
- Electric charge: each nucleus has several positive electrical charges (except for the hydrogen atom which has only one charge).
Different atoms have different nuclei which are characterized by their number of positive charges.


2) Electrons

- Size: the electrons are much smaller than the nucleus.
- Location: They are mobile and move around the nucleus.
- Electric charge: each electron has a negative electrical charge.
All electrons are identical even if they belong to different atoms.

 
3) Example: the carbon atom

Atom of carbon:
Carbon atom

Comment:
The dotted line is the trajectory of the electron farthest from the nucleus and is also the limit of the atom.



 


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