atomic theory

2. a colourless, odourless, highly flammable gas
5. an essential or characteristic part of something abstract.
8. a hard silver-grey metal of the transition series
9. a rare radioactive gas belonging to the noble gas series.
11. a colourless, odourless reactive gas
13. a subatomic particle of about the same mass as a proton but without an electric charge
14. a silver-white metal of the alkaline earth series
15. A chemical compound is a chemical substance composed of many identical molecules composed of atoms from more than one element held together by chemical bonds.
16. a radioactive metal occurring in nature only as a product of radioactive decay of uranium.
19. are variants of a particular chemical element which differ in neutron number

1. a non-metal which has two main forms
3. a stable subatomic particle with a charge of negative electricity
4. a stable subatomic particle occurring in all atomic nuclei
6. a hard grey metal of the transition series
7. an inert gaseous element of the noble gas group
9. the emission of ionizing radiation or particles caused by the spontaneous disintegration of atomic nuclei.
10. the smallest unit of ordinary matter that forms a chemical element.
12. relating to an atom or atoms.
17. a substance made by mixing other substances together.
18. appearing or occurring at intervals.