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Atomic Structure - Coggle Diagram
Atomic Structure
After discovering the electron in 1897, J J Thomson proposed that the atom looked like a plum pudding.
To explain the two types of static electricity, he suggested that the atom consisted of positive 'dough' with a lot of negative electrons stuck in it. This was consistent with the evidence available at the time:
solids cannot be squashed, therefore the atoms which make them up must be solid throughout
rubbing two solids together often results in static charge so there must be something (electrons) on the outsides of atoms which can be transferred as atoms collide
In 1905, Ernest Rutherford did an experiment to test the plum pudding model. His two students, Hans Geiger and Ernest Marsden, directed a beam of alpha particles at a very thin gold leaf suspended in a vacuum.
Rutherford had discovered the nuclear atom, a small, positively-charged nucleus surrounded by empty space and then a layer of electrons to form the outside of the atom.
The vacuum is important because any deflection of the alpha particles would only be because of collisions with the gold foil and not due to deflections off anything else.
Gold was used because it was the only metal that could be rolled out to be very, very thin without cracking.
The discovery of the make-up of the nucleus (protons and neutrons) came much later, and was not made by Rutherford. The nucleus was calculated to be about 1/10,000th the size of the atom.
Even though Rutherford had proven the existence of the nucleus, scientists were unsure how electrons fitted into this new model.
In 1913, Niels Bohr revised Rutherford's model by suggesting that the electrons orbited the nucleus in different energy levels or at specific distances from the nucleus.
By doing this, he was able to explain that since particular chemicals burn with certain-coloured flames, the pattern of energy released by electrons in the chemical reaction must be the same for every single atom of that element.
Therefore, electrons cannot be arranged at random, but they must have fixed levels of energy within each type of atom.
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When atoms absorb energy, the electrons at a particular level are pushed up to higher levels (at bigger distances from the nucleus). In time, they jump back down to a lower level releasing light of definite frequencies.
The proton had been proposed as being the nuclear particle responsible for the positive charge of the nucleus and for some of the nuclear mass. However, there was still a difference between the atomic number of the atom and the atomic mass.
Bohr and other scientists knew that there also had to be a neutral particle the same size as a proton to keep the nucleus stable and to make up the mass. This was accepted and scientists used models that included the neutron for a further 20 years before it was actually discovered. It wasn't until 1932 that James Chadwick was able to prove its existence.
Chadwick used a version of Rutherford's experiment, using a sheet of beryllium and a paraffin block instead of gold foil. He was able to prove that a proton-sized neutral particle - now known as the neutron - existed.
Protons and neutrons are the heaviest particles in an atom and as a result they make up most of the mass of the atom. The mass of electrons is often not considered to be significant.
The number of protons is what defines the element, ie an atom with six protons in its nucleus will always be carbon, and uranium will always have 92 protons.
In a neutral atom, the number of electrons is always the same as the number of protons. If the atom becomes ionised however, the number of electrons will change. An ion is an atom that has lost or gained one or more electron.
This symbol shows that chlorine has 35 particles in the nucleus (protons and neutrons), 17 of which are protons. It also tells us that chlorine has 18 neutrons (35 - 17) and, as the number of electrons and protons are equal in a neutral atom, chlorine also has 17 electrons.
An element's atomic number defines it. An element with 17 protons will always be chlorine.
However an element's mass numbers can vary, which means that it can have different numbers of neutrons. So although chlorine has a mass number of 35 which means it has 18 neutrons, it can also have a mass number of 37, which means it has 20 neutrons. The different types of chlorine are called isotopes.