1. Timeline of atomic structure
By Chelsea Nolan, Jorgia Leonard and Francesca Bertolacci

2. 400BC – Democritus Ancient Greek philosopher
atom
Ancient Greek philosopher
Little bits called atoms can’t be seen
Atoms are indestructible
Atoms are always in motion
There are an infinite number of atoms differing
Different types of atoms differing in shape and size
“The more any indivisible exceeds, the heavier it is.” Democritus on the mass of atoms
Proposed all matter was made up of tiny, invisible particles
Everything composed of “atoms” which are physically indivisible

3. 1808 – John Dalton Billiard ball model
Element was composed of identical, invisible atoms characteristic of that element and no other
Points of Dalton’s theories: elements are made of extremely small particles called atoms; atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties; atoms cannot be subdivided, created, or destroyed; atoms of different elements combine in simple whole-number ratios to form chemical compounds: in chemical reactions, atoms are combined, separated, or rearranged.

4. 1864 – Dmitri Mendeleev
Arranged elements into groups with similar properties
Discovered that the properties of elements “were periodic functions of their atomic weights”, which became known as Periodic Law
His Periodic Table was compiled on the basis of arranging the elements in ascending order of atomic weight and grouping them by similarity of properties. He predicted the existence and properties of new elements and pointed out accepted atomic weights that were in error.

5. 1898 – Joseph John Thomson (J.J. Thomson)
Identified that atoms were not invisible
Several scientists, such as William Prout and Norman Lockyer, had suggested that atoms were built up from a more fundamental unit, but they envisaged this unit to be the size of the smallest atom, hydrogen. Thomson was the first to suggest that the fundamental unit was over 1000 times smaller than an atom, suggesting the sub-atomic particles now known as electrons. Thomson discovered this through his explorations on the properties of cathode rays.
Atoms consisted of a soft positively charged jelly-like sphere, into which negatively charged electrons were embedded, like plums in a pudding
Plum pudding model

6. 1911 – Ernest Rutherford
Proposed that an atom consisted of mainly empty space
Most of the mass and all the positive charged electrons would be concentrated in the centre- nucleus
Negatively charged electrons would occupy the space around the nucleus

7. 1913 – Niels Bohr
Bohr suggested that an atom could have only certain definite energies, corresponding to its electrons being in certain definite orbits
Electrons could be compared to a solar system with its planets in definite orbits around the sun

8. 1913 – Niels Bohr (continued)
CONTRIBUTED:
The Bohr model of the atom, the theory that electrons travel in discrete orbits around the atom's nucleus.
The shell model of the atom, where the chemical properties of an element are determined by the electrons in the outermost orbit.
The correspondence principle, the basic tool of Old quantum theory.
The liquid drop model of the atomic nucleus.
Much work on the Copenhagen interpretation of quantum mechanics.
The principle of complementarity: that items could be separately analyzed as having several contradictory properties.

9. 1927 – Werner Heisenberg
Proposed Principle of Indeterminancy – you cannot know both the position and velocity of a particle

10. 1930 – Erwin Schrodinger
Viewed electrons as continuous clouds and introduced “wave machines” as a mathematical model of the atom
Electron cloud model

11. 1932 – James Chadwick
Identified a particle in the nucleus that had almost the same mass as a proton but no charge
Called it a neutron, abbreviation for a neutral proton.