Up and Atoms

2. A pure substance made up of chemically combined atoms in a fixed ratio
4. Has mass and takes up space
7. Columns on the Periodic Table
10. The average mass of atoms of an element, calculated using the relative abundance of isotopes in a naturally-occurring element.
12. Unstable isotopes that decay by releasing radiation
15. Neutral particle in the nucleus of the atom
16. A table containing all the known elements in order of atomic number
19. An impure substance made up of pure substances physically mixed.
20. Elements on the right side of the Periodic Table
21. Radiactive decay made up of gamma rays
22. Levels where electrons are found
23. Negative particle of the atom that orbits the nucleus
24. Atoms with the same atomic number and different mass number
25. The time taken for the radioactivity of a specified isotope to fall to half its original value.

1. The dense centre of an atom containing protons and neutrons
3. Also called semi-metals, have some properties of both metals and non-metals
5. Building blocks of matter
6. The mass of an atom calculated by adding the number of neutrons to the number of protons for an atom
8. Positive particle in the nucleus of an atom
9. Elements on the left side of the Periodic Table
10. Radioactive decay made up of a helium nucleus
11. Radiation released due to the decay of the nucleus of an atom
13. The number of protons in an atom of a particular element
14. Rows on the Periodic Table
17. Radioactive decay made up of an electron
18. An atom that has lost or gained electrons