Lesson One

To understand the structure of the atom and to have a rounded knowledge of it. It’s good to know, who and what assisted us to know what we know now.

John Dalton

 John Dalton was a scientist, who is credited with developing the modern theory of the atom. Following a long string of experiments, Dalton suggested that:

                                 Matter is made up of atoms.

                                Different elements have atoms which differ in mass.

                                Each element is characterized by the mass of it atoms.

This formed the basis for his theory, “ATOMIC THEORY”. It states that, all matter - elements, compounds and mixtures- is composed of extremely small particles called atoms. The postulates (assumptions) were that:

                Atoms of any given element are of the same kind.

                Atoms can’t be created nor destroyed.

                Atoms are indivisible.

                Atoms retain their identity during chemical reactions.

During a chemical reaction atoms of the reacting substances rearrange to form the products of the reaction. Eg

                                NaOH  + HCl -------------------------------> NaCl  + H²O

   

                Compounds contain atoms of two or more elements combines in fixed proportions. 

Eg  The chemical equation must be balanced; the right side must be equal to the left.

Dalton also thought that the molecule is the smallest part of an element or compound which can exist alone under ordinary conditions (RTP)

                               

Dalton’s theory wasn’t all that accurate but, it aided to form what we know about the atom today. This brings us to the scientific theory. It is the way how theories are improved, disregarded and accepted and how this helps us to understand revolutionizes science.

For a theory to be accepted, it has to meet to following criteria:

                There has to be a fit between the practical and theoretical construct.

                Data has to be reliable and accurate.

                Experiments have to reliable.

                There has to be consensus between the scientific community as to the accuracy of the theory.

                Must be accepted by people.

J.J. Thompson

                He is considered the discoverer of the electron through his experiments on the stream of particles (electrons) emitted by cathode rays. He suggested that atoms consisted of negative charges (electrons) within a sphere with positive charge, the “plum pudding” theory of atomic structure. Late r a scientist named Millikan found the mass of an electron which was 9.10 * 10^-28g and the charge – 1.6022 *10^-19C. Based with this, the relative mass and charge was derived for the electron   ->

Charge ->  -1

Relative Mass - 1/1840

Rutherford

He brought about the idea of the proton. He stated that since the atom is electrically neutral, it must contain the same amount of negative and positive charges. He also stated that these positive charges were concentrated in the centre of the nucleus.

The positive charges (protons) carries an equal but opposite charges to the electron.

Mass- 1

Charge- +1                                       

               

Chadwick

He was able to show that, because of the RMM (Relative Molecular Mass) of hydrogen and helium. That the atom also contains uncharged particles called neutrons, situated in the nucleus and has a similar mass to the proton.

Mass – 1

Charge - none

Henry Moseley

He suggested that the number of protons in an atom of an element determines which element it belongs.

The number of protons is equal to the number of electrons in an atom making it neutral.

The number of protons determines where in the periodic table it is placed.