practice

1. an arrangement of the elements by their physical and chemical properties
6. AKA atomic mass; the sum of the number of protons and neutrons
7. a subatomic particle with a positive charge
8. a horizontal row of elements in the periodic table
12. a subatomic particle with no electric charge
14. an atom of the same element that has a different number of neutrons and, therefore, a different mass
16. a subatomic particle that has a negative charge and almost no mass
17. a region around the nucleus of an atom where electrons are likely to be found
18. the smallest unit of an element that has the properties of that element

2. the number of protons in the nucleus of an atom
3. the center of an atom containing protons and neutrons
4. the weighted average of the masses of all naturally occurring isotopes of an element
5. a vertical column of elements in the periodic table
6. an element that is shiny and conducts heat and electricity well
9. a substance that cannot be separated or broken down into simpler substances by chemical means
10. an atom that has gained or lost electrons; may be positively or negatively charged
11. an element that has properties of both metals and nonmetals
13. an element that is dull, brittle, and conducts heat and electricity poorly
15. explains that all matter is made up of extremely small particles, called atoms