C4 Revision (1/5)

C4 Revision (1/5)

1. The history of the atom

   • Theory of atomic structure
     • In the 19th century John Dalton described atoms as solid spheres, he also said that different spheres made up different elements
     • In 1897 J.J Thomson found they weren't solid spheres. He found that they contain even smaller, negatively charged particles called atoms. This is the plum pudding theory.
   • Rutherford
     • In 1909 him and students, conducted the gold foil experiment. They fired positively charged atoms at a sheet of gold.
     • Most particles passed straight through the gold atoms. Therefore the plum theory was incorrect.
     • He came up with the 'nuclear atoms' theory. A positive nucleus in the centre surrounded by a 'cloud' of negative electrons, most of the atom is empty space
   • Bohr Model
     • Scientists realised that Rutherford was wrong as the nucleus and electrons would attract. So he created a model.
     • He said electrons can only exist in fixed orbits (shells). Each shell has fixed energy.

2. Atoms

   • The nucleus
     • It's in the middle of the atom
     • It contains protons and neutrons
     • It has a positive because of the protons
     • Almost the whole mass of the atom is concentrated here
   • The electrons
     • Move around the nucleus in electron shells
     • They're negatively charged.
     • They're tiny but cover a lot of space.
     • The volume of their orbit determines the size of the atom
     • They have virtually no mass
   • Number of protons equals number of electrons
     • Neutral atoms have no charge overall, as they have the same number of protons as eletrons
     • The charge on the electron is the same size as the charge on the protons, but opposite - so the charges cancel out.
   • Atomic number and mass number
     • Mass number = total mass of protons and neutrons
     • Atomic number = number of protons

3. Elements and isotopes

   • Periodic table
     • there are 100ish elements
     • the modern periodic table has elements in order of ascending atomic number
     • It it laid out with similar properties forming columns
     • Vertical columns are called groups
     • The group to which the element belongs corresponds to the number of electrons in its outershell
     • The rows are called periods, each new period represents another full of electrons
     • The period to which something belongs to corresponds to the number of shells of electrons it has in it
   • Isotopes
     • Isotopes are different forms of the same element, which have the same number of protons but a different number of neutrons
     • Isotopes have the same atomic number but different mass numbers