Chemistry 1

The Early Periodic Table:

In the 19th century scientists tried to classify elements based on their properties and weights. Newlands proposed the law of octaves where similar properties are repeated every eight element. He put 62 elements into 7 groups of similar properties according to their atomic weights. Mendeleev made a table with gaps for unknown elements to be filled in when they were discovered.

The Modern Periodic Table:

Scientists found out about protons and electrons in the 20th century and soon after they developed models of the electron arrangement in atoms. Elements were arranged in order of atomic numbers. The group number is the same as the number of electrons in the outer shell. 

The group 1 elements are called the alkali metals and they react with air and water.

- they have low melting and boiling points

- they are soft solids at room temperature

- they have low densities

- they react with water to produce hydrogen gas and a metal hydroxide

- they have one electron in their outer shell

- they lose elctrons in a reaction to form ionic compounds with a positive charge

- they react with group 7 to form salts

- as you go down group 1 the reactivity of the metals increases

The transition elements are found in the periodic table between groups 2 and 3:

- they have higher melting and boiling points than alkali metals

- they are malleable and ductile

- they are good conductors of heat and electricity

- they are strong and dense good for building materials

- they form positive ions

- they do not act vigorously with oxygen and water

The halogens are non-metallic elements in group 7 of the periodic table:

- they have low melting and boiling points 

- they all have 7 electrons in their outer shell

- they bond covalently with non-metals forming molecules

- halide ions have a 1- charge

- reactivity decreases going down the group