Compounds:

 A pure substance containing two or more kinds of (a)   .

 The atoms are (a)   combined in some way. Often times (but not always) they come together to form groups of atoms called molecules.

 Compounds (a)   be separated by physical means. Separating a compound requires a chemical reaction.

 An element (a)   be separated into simpler materials .

 Over 100 existing elements are listed and classified on the (a)   .

The centre of the atom is the (a)  

The two types of particle in the centre of the atom are the proton and the (a)  

Niels Bohr suggested particles orbit the centre of the atom. This type of particle is the .

(a)