The history of the atom

It started with a Greek person called Democritus in 5th century BC.He thought that all matter was made of identical lumps that he called atomos. That was it until the 1800's.

In 1804 John Dalton thought that each atom was a different type depending on the element. Then about 100 years later JJ Thomson proved the existance of electrons. Thomson suggested that atoms were balls with a positive charge. He thought that all elcetrons were just small balls inside of the atom, like fruit in a plum pudding, hence the name.

In 1909 Ernest Rutherford and co fired alpha particles at a beam of gold foil. The plum pudding model expected all of the particles to pass straight through or be slightly deflected but that didn't happen. Instead some of the particles went through, some went through at a slight angle, some were deflected massivly and some just bounced straight back at them.They called this the alpha scattering experiment. The plum pudding model couldn't explian the results. Due to the particles being refelcted back the scientists thought that there must be something soild in the centre of the atom. The realised that it must also be a) positive, since it repelled positive alpha particles and, b) dense, since it was strong enough to bounce atoms back. They named this the Nucleus. They also proved that most of the atom must be empty space since most of the particles passed straight through.

In 1911 Niels Bohr theorised that electrons move around and orbit the nucleus like the planets orbit the sun. He also thought it was at certain distances called energy levels. This all agreed with experimental data. Later eveidence proved that neutrons were made up of other particles (protons) which made up the overall charge of the neuclus (positve). In 1932 James Chadwick proved the existance of neutrons which explained the inbalence of the atomic number and the mass number.

The necleus is tiny but it makes up most of the mass of the atom. It contains protons (which are neutral, with a relative charge of 0) - which gives it an overall positive charge. It's radius is about 10,000 times smaller than the raduis of an atom. The rest of the atom is mostly empty space. Negative electrons (relative charge -1) whizz round the outside of the nucleus really fast. They give the atom its overall size - The radius of an atom is about 1x10 to the 10th m. The number of protons = the number of electrons, as protons and electrons have an equal but opposite charge and atoms have no no overall charge. Electrons in energy levels can move within (or sometimes leave) the atom. If they gain energy by absorbing EM radiation they move to a higher energy level, further from the necleu. If they release EM radiation, they move to a lower energy level that's closer to the nucleus. If one or more outer electrons leaves the atom, it becomes a positvly charged ion.