atom history

• the theory of atomic structure has ...


  changed over time
• Democritus


  (460-370 BCE) a Greek philosopher who theorized that all matter could be reduced to particles that could not be divided, which he described as "atomos" (meaning indivisible)
• John Dalton


  developed modern atomic theory, where all elements consist of identical atoms and different elements combine to make compounds
• plum pudding model


  J.J Thomsons model of an atom, in which he thought electrons were randomly distributed within a positively charged cloud
• plum pudding model diagram

  
• plum pudding model explained

  - at the start of the 19th century John Dalton described atoms as solid spheres, and said that different spheres made up the different elements

  - in 1897 JJ Thomson concluded from his experiments that atoms were not solid spheres

  - his measurements of charge and mass showed that an atom must contain even smaller, negatively charged particles (electrons)

  - the solid sphere idea of atomic structure had to be changed, the new theory was known as the plum pudding model

  - the plum pudding model showed the atom as a ball of positive charge with electrons stuck in it
• alpha particle scattering experiment

  an experiment in which alpha particles were fired at gold foil to see if they were deflected - it led to the plum pudding model being abandoned in favour of the nuclear model of the atom
• alpha particle scattering experiment diagram

  
• alpha particle scattering experiment explained

  - in 1909 Ernest Rutherford and his student Marsden conducted the famous alpha particle scattering experiments which involved firing positively charged alpha particles at an extremely thin sheet of gold

  - from the plum pudding model they were expecting the particles to pass straight through the sheet or be slightly deflected at most

  - this was because of the positive charge of each atom was thought to be very spread out through the 'pudding' of the atom

  - most particles did go straight through the thin gold sheet, however some were deflected more than expected and a small number deflected backwards

  - this was happened due most of the mass of the atom being concentrated in the small nucleus, which must have had positive charge since it repelled the positive alpha particles

  - they also realised because nearly all the alpha particles passed straight through, most of the atom is just empty space with positive charge amd concentrated mass in the middle

  - this proved the plum pudding model wrong
• Rutherford model


  discovered that atom is mostly empty space and has a small, dense, positively charged nucleus with electrons travelling around it
• Rutherford model diagram

  
• Rutherford model explained


  - Rutherford came up with an idea to explain the evidence found with the alpha particle scattering experiment, the nuclear model of the atom

  - in this, there's a tiny, positively charged nucleus at the centre, where most of the mass is concentrated

  - a 'cloud' of negatively charged electrons surround this nucleus, meaning most of the atom is empty space

  - when alpha particles came near the concentrated, positive charge of the nucleus, there were deflected

  - if they were fired directly at the nucleus, they were deflected backwards

  - otherwise they passed though the empty space
• Bohr's nuclear model


  the idea that electrons orbit the nucleus in fixed shells.
• Bohr's nuclear model diagram

  
• Bohr's nuclear model explained

  - scientists realised that electrons in a cloud around the nucleus of an atom, as Rutherford described, would be attracted to the nucleus, causing the atom to collapse

  - Niels Bohr's nuclear model of the atom suggested that all the electrons were contained in shells

  - Bohr proposed that electrons orbit the nucleus in fixed shells and aren't anywhere in between, each shell is a fixed distance from the nucleus

  - Bohr's theory of atomic structure was supported by many experiments and it helped to explain lot of other scientists' observations at the time
• discovery of protons


  - further experimentation by Rutherford and others gave the conclusion that the nucleus could be divided into smaller particles

  - one of these smaller particles was a proton

  - each of these smaller particles (protons) has the same charge as a hydrogen nucleus, which is a single proton
• discovery of neutrons


  - about 20 years after scientists had accepted atoms have nuclei, James Chadwick carried out an experiment which provided evidence for neutral particles in the nucleus, known as neutrons

  - the discovery of neutrons resulted in a model of the atom which was similar to the modern day accepted version, known as the nuclear model
• who discovered electrons?

  JJ Thomson in 1897
• who discovered protons?


  Ernest Rutherford in 1909
• who discovered neutrons?


  James Chadwick in 1932
• who came up with the term 'isotope'?

  Frederick Soddy in 1900
• who discovered that electrons orbit the nucleus?

  Niels Bohr in 1913
• current atom model


  - tiny nucleus that makes up most of the mass of the atom consisting of protons and neutrons

  - radius of nucleus is 10000 times smaller than the radius of the atom

  - most of the atom is empty space

  - electrons move around the nucleus in energy shells and give the atom its overall size
• number of protons =

  number of electrons OR atomic number