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2. a vertical column of elements in the periodic table.
4. a neutral subatomic particle in the nucleus of an atom.
5. symbol for atomic mass unit.
8. an ion with a positive charge.
9. any of the possible energies an electron may have in an atom.
10. the total number of protons and neutrons in the nucleus of an atom.
13. the elements that are usually poor conductors of heat and electricity.
15. the number of protons in the nucleus of an atom.
19. a negatively charged subatomic particle found in the energy level of an atom.
20. group 2 or 2A

1. a positively charged subatomic particle in the nucleus in an atom.
3. an ion with a negative charge.
4. center of an atom, which is made up of protons and neutrons.
6. group 17 or 7A
7. an electron in the outermost energy level of an atom.
11. the smallest particle that has the properties of an element.
12. a horizontal row of elements in the periodic table.
14. group 16 or 6A
16. group 18 or 8A
17. an element that has properties of both a metal and a non-metal.
18. the elements that are good conductors of heat and electricity.
20. group 1 or 1A
21. an atom or a group of atoms that has lost or gained one or more electrons.