In the early 1800s, elements in the periodic table were arranged by atomic weight.

Scientists noticed a 'periodic' pattern in the properties of elements, i.e. a pattern that repeats.

Elements were placed in order of atomic weight rather than atomic mass, and didn't take into account the elements chemical properties.

1. He mainly ordered the elements by increasing atomic weight.
2. He arranged elements that had similar properties into rows (periods), and columns (groups).
3. He left gaps.

To ensure that elements with similar properties stayed in the same group, and to leave space for elements that hadn't been discovered yet.

Some isotopes had different masses, but the same chemical properties - so he was correct in putting them in groups with other elements that had the same chemical properties.

1. Increasing atomic number.
2. Patterns of chemical and physical properties (periods/groups).

To group together the elements that have similar properties.

The group number tells you the number of electrons in the outer shell.

E.g. all elements in group 3 will have 3 electrons in their outer shell. 

The period tells us the number of shells an atom has.

E.g. all elements in period 2 have 2 energy shells.

Alkali Metals.

Alkali Earth Metals.

Halogens.

Noble gases.

 If the elements known about at the time were placed consecutively, some elements would be placed into groups where they had different chemical properties from the other elements in that group. This meant they would be in the 'wrong' group, and so Mendeleev thought some undiscovered element must go in that place instead.