Across
- 3. a unique (set of) letter(s)
- 4. regions surrounding the nucleus
- 6. average mass of diff forms of the same element
- 7. the number of elements
- 8. vertical columns, families
- 10. was developed based on the physical and chemical properties of well-known elements
Down
- 1. group 18; mostly unreactive
- 2. located in the outermost level
- 5. what an element is known as
- 9. horizontal rows