Chapter 2

• During the 19th Century, scientists were finding new elements nearly every year
• They were trying to find patterns in the behaviour of the elements, and would try to organise them
• The first suggestion came from John Dalton. He arranged elements in order of their atomic weights, and in 1808, published it in his book.
• John Dalton's periodic table

  
• In 1864, John Newlands built upon Dalton’s ideas. He produced a table called the ‘law of octaves’, where every 8 th element had similar properties. His table did not consider that new elements were still being discovered, and the pattern broke down after calcium. Scientists refused to accept his ideas.
• Whose table is this?

  John Newlands

  
• In 1869, Dmitri Mendeleev solved the problem. He placed the elements in order of their atomic weight. A periodic pattern in their properties could be seen.
• Mendeleev left gaps for undiscovered elements and predicted their properties, which turned out to be correct, so his table was accepted by the scientific community. He also changed the order of some elements so they were grouped with elements with similar properties eg argon and potassium.
• The atomic (proton) number of an element determines its position in the periodic table
• The number of electrons in the outermost shell of an atom determines the chemical properties
• The group number of an element equals the number of electrons in the outermost shell eg sodium in group 1 has 1 electron in its outer shell
• The atoms of metals lose electrons and form positive ions, whereas non-metal atoms gain electrons and form negative ions
• Noble gases in group 0 are unreactive because they have a full outer shell of electrons eg helium & neon
• Metals are located on the left-hand side and central part of the periodic table
• Non-metals are located on the right-hand corner of the periodic table