History of the Atom

• The theory of atomic structure has changed over time
• 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 J J Thomson concluded from his experiments that atoms weren't solid spheres. His measurements of charge and mass showed that an atom must contain even smaller, negatively charged particles (electrons)
• The plum pudding model showed the atom as a ball of positive charge with electrons embedded in it
• John Dalton's 'solid sphere' idea of the atomic structure had to be changed. The new theory was known as the 'plum pudding model'
• Rutherford showed that the plum pudding model was incorrect
• In 1909 Ernest Rutherford and his student Ernest Marsden conducted the famous alpha particle scattering experiments.
• The alpha particle scattering experiments consisted of firing positively charged alpha particles at an extremely thin sheet of gold
• APSE - From the plum pudding model, they were expecting the particles to pass right through the sheet or be slightly deflected at most. This was because the positive charge of each atom was thought to be very spread out through the plum pudding model
• APSE - While most of the particles did go straight through the gold sheet, some were deflected more than expected, and a small number were deflected backwards
• Rutherford came up with the idea to explain this new evidence - the nuclear model of the atom
• The nuclear model of the atom consists of a tiny, positively charged nucleus at the centre, where most of the mass is concentrated
• In the nuclear model of the atom, a 'cloud' of negative electrons surrounds this nucleus - so most of the atom is empty space
• If the alpha particles were fired directly at the nucleus, they were deflected backwards. Otherwise, they passed through empty space
• 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
• Neils 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 lots of other scientists' observations at the time
• Further experiments after Bohr showed the existence of protons
• Further experiments by Rutherford and others showed that the nucleus can be divided into smaller particles, which have the same charge as a hydrogen nucleus (protons)
• About 20 years after scientists had accepted that atoms have nuclei, James Chadwick carried out an experiment with provided evidence for neutral particles in the nucleus (neutrons)
• The discovery of neutrons resulted in a model of the atom which was pretty close to the modern day accepted version, known as the nuclear model