History of Periodic Table and Atoms

Dobereiner

• Groups of 3 elements with similar properties (Triad)

• Atomic weight of 1 and 3 is midway of 2

Newland

• List elements in increasing atomic number

• Every 8th element has similar property to the first one (Law of Octaves)

Mendeleev

• List in order of increasing Atomic weight

• Place elements with similar properties under each other

• Left gaps for undiscovered elements

• He reversed some pair to fit into groups with similar properties

Mosely

• Discovered that each elements contained a positive charge of a definite number

• Arranged them in order of increasing atomic number.

• No gaps left, contain noble gases

• Separate block of transition elements

The Greeks

• Believed that matter was infinitely divisible

• Democritus disagreed and insist that matter has small, indivisible particles

Dalton

• All mater is made up of small divisible particles called atoms (assumption)

• All atoms in a given element are identical in mass and properties (assumption)

• These atoms cannot be created/destroyed by chemical reaction

• Atoms combine together to form compounds whose properties are different from the original atoms

Crookes

• Carried out an experiment where he passed electricity through evacuated tube with two opposite charged electrodes. (Cathode to Anode)

• A high voltage of electricity was imposed to produce radiation (cathode rays)

• He made the following observations:

  • They are invisible but they cast a shadow when they are stopped by Maltese Cross

  • The particles travelled in a straight line

  • The particles are negatively charged

Stoney

Named the cathode rays as electrons

Thomson

• Used a more sophisticated vacuum tube to show cathode rays were negatively charged - credited for the discovery of electrons

• He measured the ratio of the charged of the electron to its mass

• Proposed ‘Plum Pudding’ model, where the atom was a positive sphere with negative electrons embedded in it in a random fashion

Milikan

• Measured the size of the charge of electrons using the ‘oil drop’ experiment

• Tiny droplets of oil sprayed between 2 charged metal plate, the charge is calculated

Rutherford

• He carried out ‘Gold Foil’ experiment, bombard alpha particles onto a very thin hold Foil and observe the direction using a detective screen

• He made the following observations:

  • Most of the particles went straight through the foil (undeflected) - atoms made of empty space

  • Some of the particles went through but was deflected - small nucleus that is hard to hit

  • A few were reflected and returned in its path - something positively charged inside the nucleus that cause it to repel (protons)

Chadwick

• Discovered neutrons when he bombarded Berium foil with alpha particles

• Neutrality charged particles were emitted and the mass decreased - Neutrons