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Dalton Proposed His Atomic Theory, Creates a List of Elements and Defines Atoms
Dalton proposed that:
- Elements are composed of tiny particles called atoms
- All atoms of an element have the same physical and chemical properties, but different elements have different physical and chemical properties
- Atoms are not created, destroyed or changed in a chemical reaction. Chemical reactions only involves separation, combination or rearrangement (law of conservation of mass)
- Compounds are made when atoms of one or more elements combine in a specific ration called mass ratio -
Dobereiner Created The Law of Triads
Each triad was a group of three elements with similar appearance and reactions. He discovered that the atomic mass of the middle element was very similar to the ones around it, which lead other scientist to discover that atomic masses could be used to arrange the elements. -
Newlands Creates a Graphical Representation of Elements in Order of Atomic Weights
He sorted the elements into rows of eight and proposed that when they were organised like this they formed columns that would have similar properties, he called this the Law of Octaves -
Mandeleev Published A Table of Elements Organised by Increasing Atomic Masses
Mandeleev realised that the physical and chemical properties of elements were directly related to their atomic mass. Therefore he arranged the elements into a table where groups of similar elements fell into the same vertical column. As he was confident with his work he predicted the physical properties of five elements that were still undiscovered and was correct. -
Thomson Discovered Electrons and Proposed His Atomic Model
Thomson discovered cathode rays and concluded that they consisted of a negatively charged stream of particles with a mass called electrons.
Thomson's atomic model proposed that:
- atoms consisted of tiny negatively charged electrons within a much bigger positively charged sphere
- The sphere was accountable for the majority of the atom's mass -
Rutherford Proposed A Model for The Atomic Structure
He proposed that:
- The atom consisted of nearly all empty space only taken up by electrons
- Electrons orbited a very small region in the centre of the atom called the nucleus
- All of the atom's mass and positive charge was carried in the nucleus
- The atom's positive charge was carried by protons His model was quite consistent with today's observations and has an accurate account for the charge and how the atom carries its mass, however doesn't account for all the atom's mass. -
Moseley Proposes The Use of The Atomic Number For Arranging Elements
As well as this Moseley solved anomalies in Mendeleev's periodic table by predicting the existence of four new elements. He also discovered that the number of protons was equal to the number protons in the atom and could accurately measure the atomic number of an atom using an electron gun. -
Bohr Proposes A Modified Version the Nuclear Atom
Bohr's model suggests that:
- electrons orbit circularly around the nucleus
- only certain radii are allowed
- each electron in each orbit has a different amount of energy with the lowest orbital radii corresponding to the lowest possible energy.
- electrons could orbit without losing an energy -
Chadwick Discovers the Neutron
Chadwick discovers a charge - less particle called the neutron. He identified it as a product of the alpha particles pelting of beryllium. -
Last Changes To The Periodic Table
Glenn Seaborg discovered the last elements and marks the last changes to the periodic table