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Across 1. Smallest form of matter 3. The horizontal row on a periodic table 5. Contain equal numbers of protons but different numbers of neutrons 8. The mass of an atom of a chemical element expressed in atomic mass units ; Normally a decimal (2 Words) 9. A substance that cannot be chemically interconverted or broken down 11. A subatomic particle occurring in all elements, with a positive electric charge

Down 1. Negative charged ion 2. An subatomic particale with a neutral charge 4. When two elements or substances are put together to form another substance 6. The positively charged central core of an atom 7. Positively charged ion 10. The vertical section of periodic table