Timeline of atomic structure

By Chelsea Nolan, Jorgia Leonard and Francesca

Bertolacci

400BC – Democritus

• 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

atom

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.

1864 – Dmitri Mendeleev

• Arranged elements into 7 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.

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

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

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

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.

1927 – Werner Heisenberg

• Proposed Principle of Indeterminancy – you cannot know both the position and velocity of a particle

1930 – Erwin Schrodinger

• Viewed electrons as continuous clouds and introduced “wave machines” as a mathematical model of the atom

• Electron cloud model

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.