1									2
									3											4
						5										6
						7
	8
									9		10
11																			12
															13	14					15
									16
	17
			18
							19
									20

Across 3. very reactive nonmetals because their atoms need to gain only one electron 6. in chemistry, a horizontal row of elements in the periodic table 7. three nonmetals, one metalloid, and one metal 10. the sum of the numbers of protons and neutrons in the nucleus of an atom 11. the first row on the periodic table 13. particles of a nucleus that have no electrical charge 16. each vertical column of elements on the periodic table 17. a region around the nucleus of an atom where electrons are likely to be found 18. the smallest unit of an element that maintains the properties of that element 19. a unit of mass that describes the mass of an atom or molecule 20. describes something that occurs or repeats at regular intervals

Down 1. less reactive than alkali metals 2. the mass of an atom expressed in atomic mass units 4. atoms that have the same number of protons and different number of neutrons 5. number of protons in a nucleus 8. are unreactive nonmetals and are in group 18 9. two nonmetals, two metalloids, and one metal 12. positive charged particles in a nucleus 14. a subatomic particle that has a negative charge 15. in physical science, an atom's central region, which is made up of protons and neutrons