About 2000 years ago the early scientists suggested that matter was made up of very small particles called atoms. About 200 years ago the English scientist John Dalton also proposed that all mater is made of tiny particles called atoms.

In his atomic theory Dalton proposed that:                            

·       Atoms are the smallest particles of an element

·        Atoms are indivisible

·        Atom of the same element are identical

·        Each element is made up of atoms of the same mass

However, it is now known, through the work of other later scientists, that atoms are themselves made up of still smaller particles. These particles are called subatomic particles and there are three main kinds. These three kinds of particles differ in their mass and electrical charge.

They are called:

·         Electrons: The electron is a very light particle. It has a mass of 9.11 x 10⁻³¹ kg and carries one unit of negative electric charge which is the same as 1.6 x 10⁻¹⁹ C (Coulombs).

·         Protons: Each proton carries one unit of positive electric charge and has a mass of 1.6726 x 10⁻²⁷ kg.

·          Neutrons: The neutron is electrically neutral i.e. it carries no charge at all. Like the proton, it is much heavier than the electron and its mass is 1.6749 x 10⁻²⁷ kg

Shortly, atoms are made up of a positively charged nucleus constituted by protons and neutrons, surrounded by one or more negatively charged electrons.

References

1.      Manahan, Stanley E. "FRONTMATTER"Environmental Chemistry, Boca Raton: CRC Press LLC, 2000

2.      FHSST Authors, The Free High School Science Texts: Textbooks for High School Students Studying the Sciences Chemistry Grades 10 – 12, Version 0, November 9, 2008