John Dalton: arranged elements in order of mass.
John Newlands: arranged elements in octaves, lots of elements didn't fit the pattern.
Dmitri Mendeleev: arranged elements in order of atomic mass and started a new row when an element showed properties similar to the one at the start of the previous row. He left gaps for elements that hadn't been discovered yet, so that when they are discovered, elements would still be grouped according to properties.
Modern periodic table: arranges elements in order of atomic number. As you go down the group, you add another shell to the element. This affects the reactivity.
Alkali Metals (group 1)
- Soft - can be cut with a knife.
- Low density - floats on water
- Conducts heat and electricity.
- Low m.p and b.p - decreases going down group because the atoms get larger, meaning that the attraction between the positive metal ions and the delocalised electrons gets weaker. This makes the bonds easier to break when you melt or boil it.
- Reactivity INCREASES as you go down the group - number of shells increases as you go down group, which means they get further away from the nucleus. Therefore the outer shell electron is more weakly attracted to the positive…