Extension Chemistry (C3)
The History of the Periodic Table, the Modern Periodic Table, the Alkali Metals, the Halogens and the Transition Elements.
History of the Periodic Table
In the early 1800s, they could only go on atomic mass, until quite recently, there were only two ways to catargorize these elements.
Newlands' Law of Octaves was the first go at arranging a table of elements. He noticed that every eighth element had similar properties; so he listed all know elements in rows of seven. Unfortunately, the pattern broke down on the third row, with transition metals like titanium and iron messing it up.
Because he left no gaps, his work was ignored. His work was criticised because:
- His groups contained elements that didn't have similar properties
- He mixed up metals and non-metals
- He didn't leave any gaps for undiscovered elements
In 1869, Dmitri Mendeleev arranged the 50 known elements into a table with various gaps. He put them in the order of atomic mass, just like Newlands but found he had to leave gaps in order to keep the elements with similar properties in the same vertical groups. The gaps he left predicted the properties of elements that hadn't yet been discovered.
The Modern Periodic Table
When the periodic table was released, many scientists didn't take it too seriously as there wasn't much evidence at the time to suggest that the elements really did fit together in that way. When the newly discovered elements fitted into the gaps he left, this was strong evidence in favour of the periodic table. When there was more evidence, scientists believed it could be a useful tool in predicting properties of elements.
When electrons, protons and neutrons were discovered, all the elements were arranged in the order of atomic (proton) numbers and all elements were put into groups.
Using the periodic table, you can work out the detailed arrangement of electrons in an atom of any element, when you know this, you can predict the atom's chemical properties.
Electrons in atoms are set out in shells which correspond to an energy level.
The maximum number of electrons that can occupy each energy level is given by the formula 2 x n2 where n is the number of the energy level.
With the exception of the transition metals, elements in the same group have the same number of electrons in their highest occupied energy level.
The positive charge of the nucleus attracts the electrons and holds them in place. The further away from the nucleus the electron is, the less the attraction.
Electron shielding is when many inner electrons shield the outer electrons from the charge of the nucleus, again, the attraction gets lessened.
The combination of increased distance and increased shielding means that an electron in a…