Science is about ideas..... And trying to disprove them. Then we replace them with a better model. This is how the periodic table was developed - ideas were tested and discarded until the modern periodic table was created. Let's find out about the fathers of the Periodic Table!
By the end of this page you should be able to:
NB: This isn't strictly on the syllabus but it IS useful background information and occassionally comes up in data interpretation questions.
Until around 1750 only 10-15 elements were known and they had little in common. In the next 60 years another 30-40 elements were discovered and some of them had similar chemical properties. So people tried using these properties to group the newly discovered elements.
One of the more successful early efforts was by Johannes Dobereiner - he came up with the Law of Triads in 1817. He tried to relate the atomic masses of elements to their chemical properties(see the diagram to the right).
Scientists at the time rejected this idea because i) there was no reason to order them by mass (protons hadn't been discovered yet) and ii) it only worked for around 15 of the 50 or so known elements. A good scientific theory works in all known situations.
The next person to have a crack at organising the elements was John Newlands. He arranged the elements in order of their atomic mass and stated that every eighth element has similar chemical and physical properties. He's right - at first. Take a look at lithium in the periodic table on the right - it's in the same group as sodium, potassium and rubidium - just like today's periodic table.
But after calcium it all starts to break down because there were undiscovered elements and he didn't realise that the transition metals need their own "group/block".
His ideas were rejected at the time for several reasons:
Mendeleev had the same idea as Newlands and placed the elements in order of atomic mass but he suspected there were undiscovered elements. So whenever an element was in a group where it's chemical and physical properties made it a bad fit he left a gap and moved the elements along until they WERE in a group with similar properties. He didn't leave the gap empty however - he filled it with a prediction of the chemical and physical properties of the missing element.
He was also willing to ignore atomic mass where it made sense. Iodine has a mass of 127 and tellurium has a mass of 128, this would place iodine in group 6 and tellurium in group 7 (find them on the modern periodic table) . Iodine behaves like a group 7 element and tellurium behaves like a group 6 element, so he swapped them around!
He did make one mistake, he placed hydrogen in group 1 which is full of metallic elements. Hydrogen has some odd properties and can behave like a metal AND a non-metal. So these days we place it in the centre on its own.
So they rejected Mendeleev's ideas too..... To be fair they had their reasons: